CASE Expressions

CASE expressions let you use IF ... THEN ... ELSE logic in SQL statements without having to invoke procedures. The syntax is:

Description of the illustration case_expression.gif

simple_case_expression::=

Description of the illustration simple_case_expression.gif

searched_case_expression::=

Description of the illustration searched_case_expression.gif

else_clause::=

Description of the illustration else_clause.gif

In a simple CASE expression, Oracle Database searches for the first WHEN ... THEN pair for which expr is equal to comparison_expr and returns return_expr. If none of the WHEN ... THEN pairs meet this condition, and an ELSE clause exists, then Oracle returns else_expr. Otherwise, Oracle returns null. You cannot specify the literal NULL for every return_expr and the else_expr.

In a searched CASE expression, Oracle searches from left to right until it finds an occurrence of condition that is true, and then returns return_expr. If no condition is found to be true, and an ELSE clause exists, then Oracle returns else_expr. Otherwise, Oracle returns null.

Oracle Database uses short-circuit evaluation. For a simple CASE expression, the database evaluates each comparison_expr value only before comparing it to expr, rather than evaluating all comparison_expr values before comparing any of them with expr. Consequently, Oracle never evaluates a comparison_expr if a previous comparison_expr is equal to expr. For a searched CASE expression, the database evaluates each condition to determine whether it is true, and never evaluates a condition if the previous condition was true.

For a simple CASE expression, the expr and all comparison_expr values must either have the same datatype (CHAR, VARCHAR2, NCHAR, or NVARCHAR2, NUMBER, BINARY_FLOAT, or BINARY_DOUBLE) or must all have a numeric datatype. If all expressions have a numeric datatype, then Oracle determines the argument with the highest numeric precedence, implicitly converts the remaining arguments to that datatype, and returns that datatype.

For both simple and searched CASE expressions, all of the return_exprs must either have the same datatype (CHAR, VARCHAR2, NCHAR, or NVARCHAR2, NUMBER, BINARY_FLOAT, or BINARY_DOUBLE) or must all have a numeric datatype. If all return expressions have a numeric datatype, then Oracle determines the argument with the highest numeric precedence, implicitly converts the remaining arguments to that datatype, and returns that datatype.

The maximum number of arguments in a CASE expression is 255. All expressions count toward this limit, including the initial expression of a simple CASE expression and the optional ELSE expression. Each WHEN ... THEN pair counts as two arguments. To avoid exceeding this limit, you can nest CASE expressions so that the return_expr itself is a CASE expression.

Simple CASE Example For each customer in the sample oe.customers table, the following statement lists the credit limit as "Low" if it equals $100, "High" if it equals $5000, and "Medium" if it equals anything else.

SELECT cust_last_name,
   CASE credit_limit WHEN 100 THEN 'Low'
   WHEN 5000 THEN 'High'
   ELSE 'Medium' END AS credit
   FROM customers
   ORDER BY cust_last_name, credit;

CUST_LAST_NAME       CREDIT
-------------------- ------
Adjani               Medium
Adjani               Medium
Alexander            Medium
Alexander            Medium
Altman               High
Altman               Medium
. . .

Searched CASE Example The following statement finds the average salary of the employees in the sample table oe.employees, using $2000 as the lowest salary possible:

SELECT AVG(CASE WHEN e.salary > 2000 THEN e.salary
   ELSE 2000 END) "Average Salary" FROM employees e;

Average Salary
--------------
    6461.68224