12.5 Flow Control Functions

Table 12.7 Flow Control Operators

Name	Description
CASE	Case operator
IF()	If/else construct
IFNULL()	Null if/else construct
NULLIF()	Return NULL if expr1 = expr2

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

  Note

  The syntax of the CASE operator described here differs slightly from that of the SQL CASE statement described in Section 13.6.5.1, “CASE Statement”, for use inside stored programs. The CASE statement cannot have an ELSE NULL clause, and it is terminated with END CASE instead of END.

  The return type of a CASE expression result is the aggregated type of all result values.

  mysql> SELECT CASE 1 WHEN 1 THEN 'one'
      ->     WHEN 2 THEN 'two' ELSE 'more' END;
          -> 'one'
  mysql> SELECT CASE WHEN 1>0 THEN 'true' ELSE 'false' END;
          -> 'true'
  mysql> SELECT CASE BINARY 'B'
      ->     WHEN 'a' THEN 1 WHEN 'b' THEN 2 END;
          -> NULL

• IF(expr1,expr2,expr3)

  If expr1 is TRUE (expr1 <> 0 and expr1 IS NOT NULL), IF() returns expr2. Otherwise, it returns expr3.

  Note

  There is also an IF statement, which differs from the IF() function described here. See Section 13.6.5.2, “IF Statement”.

  If only one of expr2 or expr3 is explicitly NULL, the result type of the IF() function is the type of the non-NULL expression.

  The default return type of IF() (which may matter when it is stored into a temporary table) is calculated as follows:

  • If expr2 or expr3 produce a string, the result is a string.

    If expr2 and expr3 are both strings, the result is case-sensitive if either string is case-sensitive.

  • If expr2 or expr3 produce a floating-point value, the result is a floating-point value.

  • If expr2 or expr3 produce an integer, the result is an integer.

  mysql> SELECT IF(1>2,2,3);
          -> 3
  mysql> SELECT IF(1<2,'yes','no');
          -> 'yes'
  mysql> SELECT IF(STRCMP('test','test1'),'no','yes');
          -> 'no'

• IFNULL(expr1,expr2)

  If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns expr2.

  mysql> SELECT IFNULL(1,0);
          -> 1
  mysql> SELECT IFNULL(NULL,10);
          -> 10
  mysql> SELECT IFNULL(1/0,10);
          -> 10
  mysql> SELECT IFNULL(1/0,'yes');
          -> 'yes'

  The default return type of IFNULL(expr1,expr2) is the more “general” of the two expressions, in the order STRING, REAL, or INTEGER. Consider the case of a table based on expressions or where MySQL must internally store a value returned by IFNULL() in a temporary table:

  mysql> CREATE TABLE tmp SELECT IFNULL(1,'test') AS test;
  mysql> DESCRIBE tmp;
  +-------+--------------+------+-----+---------+-------+
  | Field | Type         | Null | Key | Default | Extra |
  +-------+--------------+------+-----+---------+-------+
  | test  | varbinary(4) | NO   |     |         |       |
  +-------+--------------+------+-----+---------+-------+

  In this example, the type of the test column is VARBINARY(4) (a string type).

• NULLIF(expr1,expr2)

  Returns NULL if expr1 = expr2 is true, otherwise returns expr1. This is the same as CASE WHEN expr1 = expr2 THEN NULL ELSE expr1 END.

  The return value has the same type as the first argument.

  mysql> SELECT NULLIF(1,1);
          -> NULL
  mysql> SELECT NULLIF(1,2);
          -> 1

  Note

  MySQL evaluates expr1 twice if the arguments are not equal.