CASE OPERATOR

Implement conditional logic in SQL queries. This operator evaluates conditions and returns a specific value when the first true condition is met.

Syntax

CASE value WHEN [compare_value] THEN result [WHEN [compare_value] THEN
result ...] [ELSE result] END

CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...]
[ELSE result] END

The BNF documents the two CASE-operator forms (simple and searched); each gets its own diagram.

Railroad diagram of the simple CASE form

Railroad diagram of the searched CASE form

Description

The first version returns the result for the first value=compare_value comparison that is true. The second version returns the result for the first condition that is true. If there was no matching result value, the result after ELSE is returned, or NULL if there is no ELSE part.

There is also a CASE statement, which differs from the CASE operator described here.

Examples

SELECT CASE 1 WHEN 1 THEN 'one' WHEN 2 THEN 'two' ELSE 'more' END;
+------------------------------------------------------------+
| CASE 1 WHEN 1 THEN 'one' WHEN 2 THEN 'two' ELSE 'more' END |
+------------------------------------------------------------+
| one                                                        |
+------------------------------------------------------------+

SELECT CASE WHEN 1>0 THEN 'true' ELSE 'false' END;
+--------------------------------------------+
| CASE WHEN 1>0 THEN 'true' ELSE 'false' END |
+--------------------------------------------+
| true                                       |
+--------------------------------------------+


SELECT CASE BINARY 'B' WHEN 'a' THEN 1 WHEN 'b' THEN 2 END;
+-----------------------------------------------------+
| CASE BINARY 'B' WHEN 'a' THEN 1 WHEN 'b' THEN 2 END |
+-----------------------------------------------------+
|                                                NULL |
+-----------------------------------------------------+

Only the first matching condition is processed:

SELECT 
  CASE true 
     WHEN (1=1) THEN '1=1' -- result is returned 
     WHEN (1=1 OR 2=2) THEN '1=1 OR 2=2' -- condition not processed
     ELSE 'else'
  END 
;
+-------------------------------------------------------------------------------------+
| CASE true WHEN (1=1) THEN '1=1' WHEN (1=1 OR 2=2) THEN '1=1 OR 2=2' ELSE 'else' END |
+-------------------------------------------------------------------------------------+
+ 1=1                                                                                 +
+-------------------------------------------------------------------------------------+

See Also

• The CASE statement, which differs from the CASE operator described above.

• The IF() function.

• Operator Precedence

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