Mysql函数：IFNULL,NULLIF,CASE

今天才用到了这几个函数
因为AVG(column)得出了null，又不想写在java代码就编码到sql中了。
下面是mysql官方文档
http://dev.mysql.com/doc/refman/5.1/en/control-flow-functions.html

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 version returns the result where value=compare_value. 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.

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
The return type of a CASE expression is the compatible aggregated type of all return values, but also depends on the context in which it is used. If used in a string context, the result is returned as a string. If used in a numeric context, the result is returned as a decimal, real, or integer value.

Note
The syntax of the CASE expression shown here differs slightly from that of the SQL CASE statement described in Section 12.7.6.2, “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.

IF(expr1,expr2,expr3)

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

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'
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.

Expression Return Value
expr2 or expr3 returns a string string
expr2 or expr3 returns a floating-point value floating-point
expr2 or expr3 returns an integer integer
If expr2 and expr3 are both strings, the result is case sensitive if either string is case sensitive.

Note
There is also an IF statement, which differs from the IF() function described here. See Section 12.7.6.1, “IF Statement”.

IFNULL(expr1,expr2)

If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns expr2. IFNULL() returns a numeric or string value, depending on the context in which it is used.

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 result value 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).

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.

mysql> SELECT NULLIF(1,1);
        -> NULL
mysql> SELECT NULLIF(1,2);
        -> 1
Note that MySQL evaluates expr1 twice if the arguments are not equal.