constraint

Purpose

Use a constraint to define an integrity constraint--a rule that restricts the values in a database. Oracle Database lets you create six types of constraints and lets you declare them in two ways.

The six types of integrity constraint are described briefly here and more fully in "Semantics":

• A NOT NULL constraint prohibits a database value from being null.

• A unique constraint prohibits multiple rows from having the same value in the same column or combination of columns but allows some values to be null.

• A primary key constraint combines a NOT NULL constraint and a unique constraint in a single declaration. It prohibits multiple rows from having the same value in the same column or combination of columns and prohibits values from being null.

• A foreign key constraint requires values in one table to match values in another table.

• A check constraint requires a value in the database to comply with a specified condition.

• A REF column by definition references an object in another object type or in a relational table. A REF constraint lets you further describe the relationship between the REF column and the object it references.

You can define constraints syntactically in two ways:

• As part of the definition of an individual column or attribute. This is called inline specification.

• As part of the table definition. This is called out-of-line specification.

NOT NULL constraints must be declared inline. All other constraints can be declared either inline or out of line.

Constraint clauses can appear in the following statements:

• CREATE TABLE (see CREATE TABLE)

• ALTER TABLE (see ALTER TABLE)

• CREATE VIEW (see CREATE VIEW)

• ALTER VIEW (see ALTER VIEW)

View Constraints Oracle Database does not enforce view constraints. However, you can enforce constraints on views through constraints on base tables.

You can specify only unique, primary key, and foreign key constraints on views, and they are supported only in DISABLE NOVALIDATE mode. You cannot define view constraints on attributes of an object column.

Prerequisites

You must have the privileges necessary to issue the statement in which you are defining the constraint.

To create a foreign key constraint, in addition, the parent table or view must be in your own schema or you must have the REFERENCES privilege on the columns of the referenced key in the parent table or view.

Syntax

constraint::=

Description of the illustration constraint.gif

(inline_constraint::=, out_of_line_constraint::=, inline_ref_constraint::=, out_of_line_ref_constraint::=)

inline_constraint::=

Description of the illustration inline_constraint.gif

(references_clause::=)

out_of_line_constraint::=

Description of the illustration out_of_line_constraint.gif

(references_clause::=, constraint_state::=)

inline_ref_constraint::=

Description of the illustration inline_ref_constraint.gif

(references_clause::=, constraint_state::=)

out_of_line_ref_constraint::=

Description of the illustration out_of_line_ref_constraint.gif

(references_clause::=, constraint_state::=)

references_clause::=

Description of the illustration references_clause.gif

constraint_state::=

Description of the illustration constraint_state.gif

(using_index_clause::=, exceptions_clause::=)

using_index_clause::=

Description of the illustration using_index_clause.gif

(create_index::=, index_properties::=)

index_properties::=

Description of the illustration index_properties.gif

(global_partitioned_index::=, local_partitioned_index ::=--part of CREATE INDEX, index_attributes::=. The INDEXTYPE IS ... clause is not valid when defining a constraint.)

index_attributes::=

Description of the illustration index_attributes.gif

(physical_attributes_clause::=, logging_clause::=, key_compression::=--all part of CREATE INDEX, parallel_clause: not supported in using_index_clause)

exceptions_clause::=

Description of the illustration exceptions_clause.gif

Semantics

This section describes the semantics of constraint. For additional information, refer to the SQL statement in which you define or redefine a constraint for a table or view.

Oracle Database does not support constraints on columns or attributes whose type is a user-defined object, nested table, VARRAY, REF, or LOB, with two exceptions:

• NOT NULL constraints are supported for a column or attribute whose type is user-defined object, VARRAY, REF, or LOB.

• NOT NULL, foreign key, and REF constraints are supported on a column of type REF.

CONSTRAINT constraint_name  Specify a name for the constraint. If you omit this identifier, then Oracle Database generates a name with the form SYS_Cn. Oracle stores the name and the definition of the integrity constraint in the USER_, ALL_, and DBA_CONSTRAINTS data dictionary views (in the CONSTRAINT_NAME and SEARCH_CONDITION columns, respectively).

NOT NULL Constraints

A NOT NULL constraint prohibits a column from containing nulls. The NULL keyword by itself does not actually define an integrity constraint, but you can specify it to explicitly permit a column to contain nulls. You must define NOT NULL and NULL using inline specification. If you specify neither NOT NULL nor NULL, then the default is NULL.

NOT NULL constraints are the only constraints you can specify inline on XMLType and VARRAY columns.

To satisfy a NOT NULL constraint, every row in the table must contain a value for the column.

Restrictions on NOT NULL Constraints NOT NULL constraints are subject to the following restrictions:

• You cannot specify NULL or NOT NULL in a view constraint.

• You cannot specify NULL or NOT NULL for an attribute of an object. Instead, use a CHECK constraint with the IS [NOT] NULL condition.

Unique Constraints

A unique constraint designates a column as a unique key. A composite unique key designates a combination of columns as the unique key. When you define a unique constraint inline, you need only the UNIQUE keyword. When you define a unique constraint out of line, you must also specify one or more columns. You must define a composite unique key out of line.

To satisfy a unique constraint, no two rows in the table can have the same value for the unique key. However, the unique key made up of a single column can contain nulls. To satisfy a composite unique key, no two rows in the table or view can have the same combination of values in the key columns. Any row that contains nulls in all key columns automatically satisfies the constraint. However, two rows that contain nulls for one or more key columns and the same combination of values for the other key columns violate the constraint.

When you specify a unique constraint on one or more columns, Oracle implicitly creates an index on the unique key. If you are defining uniqueness for purposes of query performance, then Oracle recommends that you instead create the unique index explicitly using a CREATE UNIQUE INDEX statement. You can also use the CREATE UNIQUE INDEX statement to create a unique function-based index that defines a conditional unique constraint. See "Using a Function-based Index to Define Conditional Uniqueness: Example" for more information.

Restrictions on Unique Constraints Unique constraints are subject to the following restrictions:

• None of the columns in the unique key can be of LOB, LONG, LONG RAW, VARRAY, NESTED TABLE, OBJECT, REF, TIMESTAMP WITH TIME ZONE, or user-defined type. However, the unique key can contain a column of TIMESTAMP WITH LOCAL TIME ZONE.

• A composite unique key cannot have more than 32 columns.

• You cannot designate the same column or combination of columns as both a primary key and a unique key.

• You cannot specify a unique key when creating a subview in an inheritance hierarchy. The unique key can be specified only for the top-level (root) view.

Primary Key Constraints

A primary key constraint designates a column as the primary key of a table or view. A composite primary key designates a combination of columns as the primary key. When you define a primary key constraint inline, you need only the PRIMARY KEY keywords. When you define a primary key constraint out of line, you must also specify one or more columns. You must define a composite primary key out of line.

A primary key constraint combines a NOT NULL and unique constraint in one declaration. Therefore, to satisfy a primary key constraint:

• No primary key value can appear in more than one row in the table.

• No column that is part of the primary key can contain a null.

Restrictions on Primary Key Constraints Primary constraints are subject to the following restrictions:

• A table or view can have only one primary key.

• None of the columns in the primary key can be LOB, LONG, LONG RAW, VARRAY, NESTED TABLE, BFILE, REF, TIMESTAMP WITH TIME ZONE, or user-defined type. However, the primary key can contain a column of TIMESTAMP WITH LOCAL TIME ZONE.

• The size of the primary key cannot exceed approximately one database block.

• A composite primary key cannot have more than 32 columns.

• You cannot designate the same column or combination of columns as both a primary key and a unique key.

• You cannot specify a primary key when creating a subview in an inheritance hierarchy. The primary key can be specified only for the top-level (root) view.

Foreign Key Constraints

A foreign key constraint (also called a referential integrity constraint) designates a column as the foreign key and establishes a relationship between that foreign key and a specified primary or unique key, called the referenced key. A composite foreign key designates a combination of columns as the foreign key.

The table or view containing the foreign key is called the child object, and the table or view containing the referenced key is called the parent object. The foreign key and the referenced key can be in the same table or view. In this case, the parent and child tables are the same. If you identify only the parent table or view and omit the column name, then the foreign key automatically references the primary key of the parent table or view. The corresponding column or columns of the foreign key and the referenced key must match in order and datatype.

You can define a foreign key constraint on a single key column either inline or out of line. You must specify a composite foreign key and a foreign key on an attribute out of line.

To satisfy a composite foreign key constraint, the composite foreign key must refer to a composite unique key or a composite primary key in the parent table or view, or the value of at least one of the columns of the foreign key must be null.

You can designate the same column or combination of columns as both a foreign key and a primary or unique key. You can also designate the same column or combination of columns as both a foreign key and a cluster key.

You can define multiple foreign keys in a table or view. Also, a single column can be part of more than one foreign key.

Restrictions on Foreign Key Constraints Foreign key constraints are subject to the following restrictions:

• None of the columns in the foreign key can be of LOB, LONG, LONG RAW, VARRAY, NESTED TABLE, BFILE, REF, TIMESTAMP WITH TIME ZONE, or user-defined type. However, the primary key can contain a column of TIMESTAMP WITH LOCAL TIME ZONE.

• The referenced unique or primary key constraint on the parent table or view must already be defined.

• A composite foreign key cannot have more than 32 columns.

• The child and parent tables must be on the same database. To enable referential integrity constraints across nodes of a distributed database, you must use database triggers. See CREATE TRIGGER.

• If either the child or parent object is a view, then the constraint is subject to all restrictions on view constraints. See "View Constraints".

• You cannot define a foreign key constraint in a CREATE TABLE statement that contains an AS subquery clause. Instead, you must create the table without the constraint and then add it later with an ALTER TABLE statement.

references_clause Foreign key constraints use the references_clause syntax. When you specify a foreign key constraint inline, you need only the references_clause. When you specify a foreign key constraint out of line, you must also specify the FOREIGN KEY keywords and one or more columns.

ON DELETE Clause The ON DELETE clause lets you determine how Oracle Database automatically maintains referential integrity if you remove a referenced primary or unique key value. If you omit this clause, then Oracle does not allow you to delete referenced key values in the parent table that have dependent rows in the child table.

• Specify CASCADE if you want Oracle to remove dependent foreign key values.

• Specify SET NULL if you want Oracle to convert dependent foreign key values to NULL. You cannot specify this clause for a virtual column, because the values in a virtual column cannot be updated directly. Rather, the values from which the virtual column are derived must be updated.

Restriction on ON DELETE You cannot specify this clause for a view constraint.

Check Constraints

A check constraint lets you specify a condition that each row in the table must satisfy. To satisfy the constraint, each row in the table must make the condition either TRUE or unknown (due to a null). When Oracle evaluates a check constraint condition for a particular row, any column names in the condition refer to the column values in that row.

The syntax for inline and out-of-line specification of check constraints is the same. However, inline specification can refer only to the column (or the attributes of the column if it is an object column) currently being defined, whereas out-of-line specification can refer to multiple columns or attributes.

Oracle does not verify that conditions of check constraints are not mutually exclusive. Therefore, if you create multiple check constraints for a column, design them carefully so their purposes do not conflict. Do not assume any particular order of evaluation of the conditions.

Restrictions on Check Constraints Check constraints are subject to the following restrictions:

• You cannot specify a check constraint for a view. However, you can define the view using the WITH CHECK OPTION clause, which is equivalent to specifying a check constraint for the view.

• The condition of a check constraint can refer to any column in the table, but it cannot refer to columns of other tables.

• Conditions of check constraints cannot contain the following constructs:

  • Subqueries and scalar subquery expressions

  • Calls to the functions that are not deterministic (CURRENT_DATE, CURRENT_TIMESTAMP, DBTIMEZONE, LOCALTIMESTAMP, SESSIONTIMEZONE, SYSDATE, SYSTIMESTAMP, UID, USER, and USERENV)

  • Calls to user-defined functions

  • Dereferencing of REF columns (for example, using the DEREF function)

  • Nested table columns or attributes

  • The pseudocolumns CURRVAL, NEXTVAL, LEVEL, or ROWNUM

  • Date constants that are not fully specified

REF Constraints

REF constraints let you describe the relationship between a column of type REF and the object it references.

ref_constraint REF constraints use the ref_constraint syntax. You define a REF constraint either inline or out of line. Out-of-line specification requires you to specify the REF column or attribute you are further describing.

• For ref_column, specify the name of a REF column of an object or relational table.

• For ref_attribute, specify an embedded REF attribute within an object column of a relational table.

Both inline and out-of-line specification let you define a scope constraint, a rowid constraint, or a referential integrity constraint on a REF column.

If the scope table or referenced table of the REF column has a primary-key-based object identifier, then the REF column is a user-defined REF column.

SCOPE REF Constraints

In a table with a REF column, each REF value in the column can conceivably reference a row in a different object table. The SCOPE clause restricts the scope of references to a single table, scope_table. The values in the REF column or attribute point to objects in scope_table, in which object instances of the same type as the REF column are stored.

Specify the SCOPE clause to restrict the scope of references in the REF column to a single table. For you to specify this clause, scope_table must be in your own schema or you must have SELECT privileges on scope_table or SELECT ANY TABLE system privileges. You can specify only one scope table for each REF column.

Restrictions on Scope Constraints Scope constraints are subject to the following restrictions:

• You cannot add a scope constraint to an existing column unless the table is empty.

• You cannot specify a scope constraint for the REF elements of a VARRAY column.

• You must specify this clause if you specify AS subquery and the subquery returns user-defined REF datatypes.

• You cannot subsequently drop a scope constraint from a REF column.

Rowid REF Constraints

Specify WITH ROWID to store the rowid along with the REF value in ref_column or ref_attribute. Storing the rowid with the REF value can improve the performance of dereferencing operations, but will also use more space. Default storage of REF values is without rowids.

Restrictions on Rowid Constraints Rowid constraints are subject to the following restrictions:

• You cannot define a rowid constraint for the REF elements of a VARRAY column.

• You cannot subsequently drop a rowid constraint from a REF column.

• If the REF column or attribute is scoped, then this clause is ignored and the rowid is not stored with the REF value.

Referential Integrity Constraints on REF Columns

The references_clause of the ref_constraint syntax lets you define a foreign key constraint on the REF column. This clause also implicitly restricts the scope of the REF column or attribute to the referenced table. However, whereas a foreign key constraint on a non-REF column references an actual column in the parent table, a foreign key constraint on a REF column references the implicit object identifier column of the parent table.

If you do not specify a constraint name, then Oracle generates a system name for the constraint of the form SYS_Cn.

If you add a referential integrity constraint to an existing REF column that is already scoped, then the referenced table must be the same as the scope table of the REF column. If you later drop the referential integrity constraint, then the REF column will remain scoped to the referenced table.

As is the case for foreign key constraints on other types of columns, you can use the references_clause alone for inline declaration. For out-of-line declaration you must also specify the FOREIGN KEY keywords plus one or more REF columns or attributes.

Restrictions on Foreign Key Constraints on REF Columns Foreign key constraints on REF columns have the following additional restrictions:

• Oracle implicitly adds a scope constraint when you add a referential integrity constraint to an existing unscoped REF column. Therefore, all the restrictions that apply for scope constraints also apply in this case.

• You cannot specify a column after the object name in the references_clause.

Specifying Constraint State

As part of constraint definition, you can specify how and when Oracle should enforce the constraint.

constraint_state You can use the constraint_state with both inline and out-of-line specification. Specify the clauses of constraint_state in the order shown, from top to bottom, and do not specify any clause more than once.

DEFERRABLE Clause The DEFERRABLE and NOT DEFERRABLE parameters indicate whether or not, in subsequent transactions, constraint checking can be deferred until the end of the transaction using the SET CONSTRAINT(S) statement. If you omit this clause, then the default is NOT DEFERRABLE.

• Specify NOT DEFERRABLE to indicate that in subsequent transactions you cannot use the SET CONSTRAINT[S] clause to defer checking of this constraint until the transaction is committed. The checking of a NOT DEFERRABLE constraint can never be deferred to the end of the transaction.

  If you declare a new constraint NOT DEFERRABLE, then it must be valid at the time the CREATE TABLE or ALTER TABLE statement is committed or the statement will fail.

• Specify DEFERRABLE to indicate that in subsequent transactions you can use the SET CONSTRAINT[S] clause to defer checking of this constraint until after the transaction is committed. This setting in effect lets you disable the constraint temporarily while making changes to the database that might violate the constraint until all the changes are complete.

You cannot alter the deferrability of a constraint. Whether you specify either of these parameters, or make the constraint NOT DEFERRABLE implicitly by specifying neither of them, you cannot specify this clause in an ALTER TABLE statement. You must drop the constraint and re-create it.

Restriction on [NOT] DEFERRABLE You cannot specify either of these parameters for a view constraint.

INITIALLY Clause The INITIALLY clause establishes the default checking behavior for constraints that are DEFERRABLE. The INITIALLY setting can be overridden by a SET CONSTRAINT(S) statement in a subsequent transaction.

• Specify INITIALLY IMMEDIATE to indicate that Oracle should check this constraint at the end of each subsequent SQL statement. If you do not specify INITIALLY at all, then the default is INITIALLY IMMEDIATE.

  If you declare a new constraint INITIALLY IMMEDIATE, then it must be valid at the time the CREATE TABLE or ALTER TABLE statement is committed or the statement will fail.

• Specify INITIALLY DEFERRED to indicate that Oracle should check this constraint at the end of subsequent transactions.

This clause is not valid if you have declared the constraint to be NOT DEFERRABLE, because a NOT DEFERRABLE constraint is automatically INITIALLY IMMEDIATE and cannot ever be INITIALLY DEFERRED.

VALIDATE | NOVALIDATE The behavior of VALIDATE and NOVALIDATE always depends on whether the constraint is enabled or disabled, either explicitly or by default. Therefore they are described in the context of "ENABLE Clause" and "DISABLE Clause".

ENABLE Clause Specify ENABLE if you want the constraint to be applied to the data in the table.

If you enable a unique or primary key constraint, and if no index exists on the key, then Oracle Database creates a unique index. Unless you specify KEEP INDEX when subsequently disabling the constraint, this index is dropped and the database rebuilds the index every time the constraint is reenabled.

You can also avoid rebuilding the index and eliminate redundant indexes by creating new primary key and unique constraints initially disabled. Then create (or use existing) nonunique indexes to enforce the constraint. Oracle does not drop a nonunique index when the constraint is disabled, so subsequent ENABLE operations are facilitated.

• ENABLE VALIDATE specifies that all old and new data also complies with the constraint. An enabled validated constraint guarantees that all data is and will continue to be valid.

  If any row in the table violates the integrity constraint, then the constraint remains disabled and Oracle returns an error. If all rows comply with the constraint, then Oracle enables the constraint. Subsequently, if new data violates the constraint, then Oracle does not execute the statement and returns an error indicating the integrity constraint violation.

  If you place a primary key constraint in ENABLE VALIDATE mode, then the validation process will verify that the primary key columns contain no nulls. To avoid this overhead, mark each column in the primary key NOT NULL before entering data into the column and before enabling the primary key constraint of the table.

• ENABLE NOVALIDATE ensures that all new DML operations on the constrained data comply with the constraint. This clause does not ensure that existing data in the table complies with the constraint.

If you specify neither VALIDATE nor NOVALIDATE, then the default is VALIDATE.

If you change the state of any single constraint from ENABLE NOVALIDATE to ENABLE VALIDATE, then the operation can be performed in parallel, and does not block reads, writes, or other DDL operations.

Restriction on the ENABLE Clause You cannot enable a foreign key that references a disabled unique or primary key.

DISABLE Clause Specify DISABLE to disable the integrity constraint. Disabled integrity constraints appear in the data dictionary along with enabled constraints. If you do not specify this clause when creating a constraint, then Oracle automatically enables the constraint.

• DISABLE VALIDATE disables the constraint and drops the index on the constraint, but keeps the constraint valid. This feature is most useful in data warehousing situations, because it lets you load large amounts of data while also saving space by not having an index. This setting lets you load data from a nonpartitioned table into a partitioned table using the exchange_partition_clause of the ALTER TABLE statement or using SQL*Loader. All other modifications to the table (inserts, updates, and deletes) by other SQL statements are disallowed.

  See Also:

  Oracle Database Data Warehousing Guide for more information on using this setting

• DISABLE NOVALIDATE signifies that Oracle makes no effort to maintain the constraint (because it is disabled) and cannot guarantee that the constraint is true (because it is not being validated).

  You cannot drop a table whose primary key is being referenced by a foreign key even if the foreign key constraint is in DISABLE NOVALIDATE state. Further, the optimizer can use constraints in DISABLE NOVALIDATE state.

  See Also:

  Oracle Database Performance Tuning Guide for information on when to use this setting

If you specify neither VALIDATE nor NOVALIDATE, then the default is NOVALIDATE.

If you disable a unique or primary key constraint that is using a unique index, then Oracle drops the unique index. Refer to the CREATE TABLE enable_disable_clause for additional notes and restrictions.

RELY Clause  RELY and NORELY are valid only when you are modifying an existing constraint (in the ALTER TABLE ... MODIFY constraint syntax). These parameters specify whether a constraint in NOVALIDATE mode is to be taken into account for query rewrite. Specify RELY to activate an existing constraint in NOVALIDATE mode for query rewrite in an unenforced query rewrite integrity mode. The constraint is in NOVALIDATE mode, so Oracle does not enforce it. The default is NORELY.

Unenforced constraints are generally useful only with materialized views and query rewrite. Depending on the QUERY_REWRITE_INTEGRITY mode, query rewrite can use only constraints that are in VALIDATE mode, or that are in NOVALIDATE mode with the RELY parameter set, to determine join information.

Restriction on the RELY Clause You cannot set a nondeferrable NOT NULL constraint to RELY.

Using Indexes to Enforce Constraints

When defining the state of a unique or primary key constraint, you can specify an index for Oracle to use to enforce the constraint, or you can instruct Oracle to create the index used to enforce the constraint.

using_index_clause You can specify the using_index_clause only when enabling unique or primary key constraints. You can specify the clauses of the using_index_clause in any order, but you can specify each clause only once.

• If you specify schema.index, then Oracle attempts to enforce the constraint using the specified index. If Oracle cannot find the index or cannot use the index to enforce the constraint, then Oracle returns an error.

• If you specify the create_index_statement, then Oracle attempts to create the index and use it to enforce the constraint. If Oracle cannot create the index or cannot use the index to enforce the constraint, then Oracle returns an error.

• If you neither specify an existing index nor create a new index, then Oracle creates the index. In this case:

  • The index receives the same name as the constraint.

  • If table is partitioned, then you can specify a locally or globally partitioned index for the unique or primary key constraint.

Restrictions on the using_index_clause The following restrictions apply to the using_index_clause:

• You cannot specify this clause for a view constraint.

• You cannot specify this clause for a NOT NULL, foreign key, or check constraint.

• You cannot specify an index (schema.index) or create an index (create_index_statement) when enabling the primary key of an index-organized table.

• You cannot specify the parallel_clause of index_attributes.

• The INDEXTYPE IS ... clause of index_properties is not valid in the definition of a constraint.

Handling Constraint Exceptions

When defining the state of a constraint, you can specify a table into which Oracle places the rowids of all rows violating the constraint.

exceptions_clause Use the exceptions_clause syntax to define exception handling. If you omit schema, then Oracle assumes the exceptions table is in your own schema. If you omit this clause altogether, then Oracle assumes that the table is named EXCEPTIONS. The EXCEPTIONS table or the table you specify must exist on your local database.

You can create the EXCEPTIONS table using one of these scripts:

• UTLEXCPT.SQL uses physical rowids. Therefore it can accommodate rows from conventional tables but not from index-organized tables. (See the Note that follows.)

• UTLEXPT1.SQL uses universal rowids, so it can accommodate rows from both conventional and index-organized tables.

If you create your own exceptions table, then it must follow the format prescribed by one of these two scripts.

If you are collecting exceptions from index-organized tables based on primary keys (rather than universal rowids), then you must create a separate exceptions table for each index-organized table to accommodate its primary-key storage. You create multiple exceptions tables with different names by modifying and resubmitting the script.

Restrictions on the exceptions_clause The following restrictions apply to the exceptions_clause:

• You cannot specify this clause for a view constraint.

• You cannot specify this clause in a CREATE TABLE statement, because no rowids exist until after the successful completion of the statement.

  See Also:

  • The DBMS_IOT package in Oracle Database PL/SQL Packages and Types Reference for information on the SQL scripts

  • Oracle Database Performance Tuning Guide for information on eliminating migrated and chained rows

View Constraints

Oracle does not enforce view constraints. However, operations on views are subject to the integrity constraints defined on the underlying base tables. This means that you can enforce constraints on views through constraints on base tables.

Notes on View Constraints View constraints are a subset of table constraints and are subject to the following restrictions:

• You can specify only unique, primary key, and foreign key constraints on views. However, you can define the view using the WITH CHECK OPTION clause, which is equivalent to specifying a check constraint for the view.

• View constraints are supported only in DISABLE NOVALIDATE mode. You cannot specify any other mode. You must specify the keyword DISABLE when you declare the view constraint. You need not specify NOVALIDATE explicitly, as it is the default.

• The RELY and NORELY parameters are optional. View constraints, because they are unenforced, are usually specified with the RELY parameter to make them more useful. The RELY or NORELY keyword must precede the DISABLE keyword. Refer to "RELY Clause" for more information.

• Because view constraints are not enforced directly, you cannot specify INITIALLY DEFERRED or DEFERRABLE.

• You cannot specify the using_index_clause, the exceptions_clause clause, or the ON DELETE clause of the references_clause.

• You cannot define view constraints on attributes of an object column.

Examples

Unique Key Example The following statement is a variation of the statement that created the sample table sh.promotions. It defines inline and implicitly enables a unique key on the promo_id column (other constraints are not shown):

CREATE TABLE promotions_var1
    ( promo_id         NUMBER(6)
                       CONSTRAINT promo_id_u  UNIQUE
    , promo_name       VARCHAR2(20)
    , promo_category   VARCHAR2(15)
    , promo_cost       NUMBER(10,2)
    , promo_begin_date DATE
    , promo_end_date   DATE
    ) ;

The constraint promo_id_u identifies the promo_id column as a unique key. This constraint ensures that no two promotions in the table have the same ID. However, the constraint does allow promotions without identifiers.

Alternatively, you can define and enable this constraint out of line:

CREATE TABLE promotions_var2
    ( promo_id         NUMBER(6)
    , promo_name       VARCHAR2(20)
    , promo_category   VARCHAR2(15)
    , promo_cost       NUMBER(10,2)
    , promo_begin_date DATE
    , promo_end_date   DATE
    , CONSTRAINT promo_id_u UNIQUE (promo_id)
   USING INDEX PCTFREE 20
      TABLESPACE stocks
      STORAGE (INITIAL 8K  NEXT 6K) ); 

The preceding statement also contains the using_index_clause, which specifies storage characteristics for the index that Oracle creates to enable the constraint.

Composite Unique Key Example The following statement defines and enables a composite unique key on the combination of the warehouse_id and warehouse_name columns of the oe.warehouses table:

ALTER TABLE warehouses
   ADD CONSTRAINT wh_unq UNIQUE (warehouse_id, warehouse_name)
   USING INDEX PCTFREE 5
   EXCEPTIONS INTO wrong_id;

The wh_unq constraint ensures that the same combination of warehouse_id and warehouse_name values does not appear in the table more than once.

The ADD CONSTRAINT clause also specifies other properties of the constraint:

• The USING INDEX clause specifies storage characteristics for the index Oracle creates to enable the constraint.

• The EXCEPTIONS INTO clause causes Oracle to write to the wrong_id table information about any rows currently in the warehouses table that violate the constraint. If the wrong_id exceptions table does not already exist, then this statement will fail.

Primary Key Example The following statement is a variation of the statement that created the sample table hr.locations. It creates the locations_demo table and defines and enables a primary key on the location_id column (other constraints from the hr.locations table are omitted):

CREATE TABLE locations_demo
    ( location_id    NUMBER(4) CONSTRAINT loc_id_pk PRIMARY KEY
    , street_address VARCHAR2(40)
    , postal_code    VARCHAR2(12)
    , city           VARCHAR2(30)
    , state_province VARCHAR2(25)
    , country_id     CHAR(2)
    ) ;

The loc_id_pk constraint, specified inline, identifies the location_id column as the primary key of the locations_demo table. This constraint ensures that no two locations in the table have the same location number and that no location identifier is NULL.

Alternatively, you can define and enable this constraint out of line:

CREATE TABLE locations_demo
    ( location_id    NUMBER(4) 
    , street_address VARCHAR2(40)
    , postal_code    VARCHAR2(12)
    , city           VARCHAR2(30)
    , state_province VARCHAR2(25)
    , country_id     CHAR(2)
    , CONSTRAINT loc_id_pk PRIMARY KEY (location_id));

NOT NULL Example The following statement alters the locations_demo table (created in "Primary Key Example") to define and enable a NOT NULL constraint on the country_id column:

ALTER TABLE locations_demo
   MODIFY (country_id CONSTRAINT country_nn NOT NULL); 

The constraint country_nn ensures that no location in the table has a null country_id.

Composite Primary Key Example The following statement defines a composite primary key on the combination of the prod_id and cust_id columns of the sample table sh.sales:

ALTER TABLE sales 
    ADD CONSTRAINT sales_pk PRIMARY KEY (prod_id, cust_id) DISABLE; 

This constraint identifies the combination of the prod_id and cust_id columns as the primary key of the sales table. The constraint ensures that no two rows in the table have the same combination of values for the prod_id column and cust_id columns.

The constraint clause (PRIMARY KEY) also specifies the following properties of the constraint:

• The constraint definition does not include a constraint name, so Oracle generates a name for the constraint.

• The DISABLE clause causes Oracle to define the constraint but not enable it.

Foreign Key Constraint Example The following statement creates the dept_20 table and defines and enables a foreign key on the department_id column that references the primary key on the department_id column of the departments table:

CREATE TABLE dept_20 
   (employee_id     NUMBER(4), 
    last_name       VARCHAR2(10), 
    job_id          VARCHAR2(9), 
    manager_id      NUMBER(4), 
    hire_date       DATE, 
    salary          NUMBER(7,2), 
    commission_pct  NUMBER(7,2), 
    department_id   CONSTRAINT fk_deptno 
                    REFERENCES departments(department_id) ); 

The constraint fk_deptno ensures that all departments given for employees in the dept_20 table are present in the departments table. However, employees can have null department numbers, meaning they are not assigned to any department. To ensure that all employees are assigned to a department, you could create a NOT NULL constraint on the department_id column in the dept_20 table in addition to the REFERENCES constraint.

Before you define and enable this constraint, you must define and enable a constraint that designates the department_id column of the departments table as a primary or unique key.

The foreign key constraint definition does not use the FOREIGN KEY clause, because the constraint is defined inline. The datatype of the department_id column is not needed, because Oracle automatically assigns to this column the datatype of the referenced key.

The constraint definition identifies both the parent table and the columns of the referenced key. Because the referenced key is the primary key of the parent table, the referenced key column names are optional.

Alternatively, you can define this foreign key constraint out of line:

CREATE TABLE dept_20 
   (employee_id     NUMBER(4), 
    last_name       VARCHAR2(10), 
    job_id          VARCHAR2(9), 
    manager_id      NUMBER(4), 
    hire_date       DATE, 
    salary          NUMBER(7,2), 
    commission_pct  NUMBER(7,2), 
    department_id, 
   CONSTRAINT fk_deptno 
      FOREIGN  KEY (department_id) 
      REFERENCES  departments(department_id) ); 

The foreign key definitions in both variations of this statement omit the ON DELETE clause, causing Oracle to prevent the deletion of a department if any employee works in that department.

ON DELETE Example This statement creates the dept_20 table, defines and enables two referential integrity constraints, and uses the ON DELETE clause:

CREATE TABLE dept_20 
   (employee_id     NUMBER(4) PRIMARY KEY, 
    last_name       VARCHAR2(10), 
    job_id          VARCHAR2(9), 
    manager_id      NUMBER(4) CONSTRAINT fk_mgr
                    REFERENCES employees ON DELETE SET NULL, 
    hire_date       DATE, 
    salary          NUMBER(7,2), 
    commission_pct  NUMBER(7,2), 
    department_id   NUMBER(2)   CONSTRAINT fk_deptno 
                    REFERENCES departments(department_id) 
                    ON DELETE CASCADE ); 

Because of the first ON DELETE clause, if manager number 2332 is deleted from the employees table, then Oracle sets to null the value of manager_id for all employees in the dept_20 table who previously had manager 2332.

Because of the second ON DELETE clause, Oracle cascades any deletion of a department_id value in the departments table to the department_id values of its dependent rows of the dept_20 table. For example, if Department 20 is deleted from the departments table, then Oracle deletes all of the employees in Department 20 from the dept_20 table.

Composite Foreign Key Constraint Example The following statement defines and enables a foreign key on the combination of the employee_id and hire_date columns of the dept_20 table:

ALTER TABLE dept_20
   ADD CONSTRAINT fk_empid_hiredate
   FOREIGN KEY (employee_id, hire_date)
   REFERENCES hr.job_history(employee_id, start_date)
   EXCEPTIONS INTO wrong_emp;

The constraint fk_empid_hiredate ensures that all the employees in the dept_20 table have employee_id and hire_date combinations that exist in the employees table. Before you define and enable this constraint, you must define and enable a constraint that designates the combination of the employee_id and hire_date columns of the employees table as a primary or unique key.

The EXCEPTIONS INTO clause causes Oracle to write information to the wrong_emp table about any rows in the dept_20 table that violate the constraint. If the wrong_emp exceptions table does not already exist, then this statement will fail.

Check Constraint Examples The following statement creates a divisions table and defines a check constraint in each column of the table:

CREATE TABLE divisions  
   (div_no    NUMBER  CONSTRAINT check_divno
              CHECK (div_no BETWEEN 10 AND 99) 
              DISABLE, 
    div_name  VARCHAR2(9)  CONSTRAINT check_divname
              CHECK (div_name = UPPER(div_name)) 
              DISABLE, 
    office    VARCHAR2(10)  CONSTRAINT check_office
              CHECK (office IN ('DALLAS','BOSTON',
              'PARIS','TOKYO')) 
              DISABLE); 

Each constraint restricts the values of the column in which it is defined:

• check_divno ensures that no division numbers are less than 10 or greater than 99.

• check_divname ensures that all division names are in uppercase.

• check_office restricts office locations to Dallas, Boston, Paris, or Tokyo.

Because each CONSTRAINT clause contains the DISABLE clause, Oracle only defines the constraints and does not enable them.

The following statement creates the dept_20 table, defining out of line and implicitly enabling a check constraint:

CREATE TABLE dept_20
   (employee_id     NUMBER(4) PRIMARY KEY, 
    last_name       VARCHAR2(10), 
    job_id          VARCHAR2(9), 
    manager_id      NUMBER(4), 
    salary          NUMBER(7,2), 
    commission_pct  NUMBER(7,2), 
    department_id   NUMBER(2),
    CONSTRAINT check_sal CHECK (salary * commission_pct <= 5000));

This constraint uses an inequality condition to limit an employee's total commission, the product of salary and commission_pct, to $5000:

• If an employee has non-null values for both salary and commission, then the product of these values must not exceed $5000 to satisfy the constraint.

• If an employee has a null salary or commission, then the result of the condition is unknown and the employee automatically satisfies the constraint.

Because the constraint clause in this example does not supply a constraint name, Oracle generates a name for the constraint.

The following statement defines and enables a primary key constraint, two foreign key constraints, a NOT NULL constraint, and two check constraints:

CREATE TABLE order_detail 
  (CONSTRAINT pk_od PRIMARY KEY (order_id, part_no), 
   order_id    NUMBER 
      CONSTRAINT fk_oid 
         REFERENCES oe.orders(order_id), 
   part_no     NUMBER 
      CONSTRAINT fk_pno 
         REFERENCES oe.product_information(product_id), 
   quantity    NUMBER 
      CONSTRAINT nn_qty NOT NULL 
      CONSTRAINT check_qty CHECK (quantity > 0), 
   cost        NUMBER 
      CONSTRAINT check_cost CHECK (cost > 0) ); 

The constraints enable the following rules on table data:

• pk_od identifies the combination of the order_id and part_no columns as the primary key of the table. To satisfy this constraint, no two rows in the table can contain the same combination of values in the order_id and the part_no columns, and no row in the table can have a null in either the order_id or the part_no column.

• fk_oid identifies the order_id column as a foreign key that references the order_id column in the orders table in the sample schema oe. All new values added to the column order_detail.order_id must already appear in the column oe.orders.order_id.

• fk_pno identifies the product_id column as a foreign key that references the product_id column in the product_information table owned by oe. All new values added to the column order_detail.product_id must already appear in the column oe.product_information.product_id.

• nn_qty forbids nulls in the quantity column.

• check_qty ensures that values in the quantity column are always greater than zero.

• check_cost ensures the values in the cost column are always greater than zero.

This example also illustrates the following points about constraint clauses and column definitions:

• Out-of-line constraint definition can appear before or after the column definitions. In this example, the out-of-line definition of the pk_od constraint precedes the column definitions.

• A column definition can contain multiple inline constraint definitions. In this example, the definition of the quantity column contains the definitions of both the nn_qty and check_qty constraints.

• A table can have multiple CHECK constraints. Multiple CHECK constraints, each with a simple condition enforcing a single business rule, are preferable to a single CHECK constraint with a complicated condition enforcing multiple business rules. When a constraint is violated, Oracle returns an error identifying the constraint. Such an error more precisely identifies the violated business rule if the identified constraint enables a single business rule.

Attribute-Level Constraints Example The following example guarantees that a value exists for both the first_name and last_name attributes of the name column in the students table:

CREATE TYPE person_name AS OBJECT
   (first_name VARCHAR2(30), last_name VARCHAR2(30));
/

CREATE TABLE students (name person_name, age INTEGER,
   CHECK (name.first_name IS NOT NULL AND 
          name.last_name IS NOT NULL));

REF Constraint Examples The following example creates a duplicate of the sample schema object type cust_address_typ, and then creates a table containing a REF column with a SCOPE constraint:

CREATE TYPE cust_address_typ_new AS OBJECT
    ( street_address     VARCHAR2(40)
    , postal_code        VARCHAR2(10)
    , city               VARCHAR2(30)
    , state_province     VARCHAR2(10)
    , country_id         CHAR(2)
    );
/
CREATE TABLE address_table OF cust_address_typ_new;

CREATE TABLE customer_addresses (
   add_id NUMBER, 
   address REF cust_address_typ_new
   SCOPE IS address_table);

The following example creates the same table but with a referential integrity constraint on the REF column that references the object identifier column of the parent table:

CREATE TABLE customer_addresses (
   add_id NUMBER,
   address REF cust_address_typ REFERENCES address_table);

The following example uses the type department_typ and the table departments_obj_t, created in "Creating Object Tables: Examples". A table with a scoped REF is then created.

CREATE TABLE employees_obj
   ( e_name   VARCHAR2(100),
     e_number NUMBER,
     e_dept   REF department_typ SCOPE IS departments_obj_t );

The following statement creates a table with a REF column which has a referential integrity constraint defined on it:

CREATE TABLE employees_obj
   ( e_name   VARCHAR2(100),
     e_number NUMBER,
     e_dept   REF department_typ REFERENCES departments_obj_t);

Explicit Index Control Example The following statement shows another way to create a unique (or primary key) constraint that gives you explicit control over the index (or indexes) Oracle uses to enforce the constraint:

CREATE TABLE promotions_var3
    ( promo_id         NUMBER(6)
    , promo_name       VARCHAR2(20)
    , promo_category   VARCHAR2(15)
    , promo_cost       NUMBER(10,2)
    , promo_begin_date DATE
    , promo_end_date   DATE
    , CONSTRAINT promo_id_u UNIQUE (promo_id, promo_cost)
         USING INDEX (CREATE UNIQUE INDEX promo_ix1
            ON promotions_var3 (promo_id, promo_cost))
    , CONSTRAINT promo_id_u2 UNIQUE (promo_cost, promo_id) 
         USING INDEX promo_ix1);

This example also shows that you can create an index for one constraint and use that index to create and enable another constraint in the same statement.

DEFERRABLE Constraint Examples The following statement creates table games with a NOT DEFERRABLE INITIALLY IMMEDIATE constraint check (by default) on the scores column:

CREATE TABLE games (scores NUMBER CHECK (scores >= 0));

To define a unique constraint on a column as INITIALLY DEFERRED DEFERRABLE, issue the following statement:

CREATE TABLE games
  (scores NUMBER, CONSTRAINT unq_num UNIQUE (scores)
   INITIALLY DEFERRED DEFERRABLE);