Use of mysql ifnull Functions

Mysql ifnull function uses mysql ifnull (expr1, expr2) If expr1 is not NULL, IFNULL () returns expr1, otherwise it returns expr2. IFNULL () returns a number or string value, depending on the context in which it is used.

[sql] mysql> select IFNULL(1,0);                             -> 1        mysql> select IFNULL(0,10);                             -> 0        mysql> select IFNULL(1/0,10);                             -> 10        mysql> select IFNULL(1/0,yes);                             -> yes     

 

IF (expr1, expr2, expr3) IF expr1 is TRUE (expr1 <> 0 and expr1 <> NULL), IF () returns expr2, otherwise it returns expr3. IF () returns a number or string value, depending on the context in which it is used.

[sql] mysql> select IF(1>2,2,3);                             -> 3        mysql> select IF(1<2,yes,no);                             -> yes       mysql> select IF(strcmp(test,test1),yes,no);                             -> no     

 

Expr1 is calculated as an integer, which means that if you are testing a floating point or string value, you should use a comparison operation.

[sql] mysql> select IF(0.1,1,0);                             -> 0        mysql> select IF(0.1<>0,1,0);                             -> 1    

 

In the first case above, IF (0.1) returns 0, because 0.1 is converted to an integer, resulting in testing IF (0 ). This may not be what you expected. In the second case, compare and test the original floating point value to see if it is non-zero. The comparison result is used as an integer.

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 version returns result, where value = compare-value. In the second version, if the first condition is true, result is returned. If no matching result value exists, the result after ELSE is returned. If no ELSE part exists, NULL is returned.

[sql] 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