Ifnull, Nullif and isnull usage differences in MYSQL

The usage of isnull,ifnull,nullif in MySQL is as follows:

Usage of IsNull (expr):
If expr is null, then the return value of IsNull () is 1, otherwise the return value is 0.
Mysql> Select IsNull (+);
0
Mysql> Select IsNull (1/0);
1
The comparison of NULL values using = is usually wrong.

The IsNull () function has some of the same characteristics as the IS null comparison operator. See the description for is null.

Usage of ifnull (EXPR1,EXPR2):

If EXPR1 is not NULL, the return value of Ifnull () is expr1;
Otherwise its return value is EXPR2. The return value of Ifnull () is either a number or a string, 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 for

Ifnull (EXPR1,EXPR2) is one of the more "common" in two expressions, in the order of string,    real, or    
INTEGER. Suppose a case of an expression-based table is,     or MySQL must store the return value of Ifnull () in a temporary table in the internal memory:   
create   table   tmp   select   ifnull (1, ' Test ')    as    test;   
In this example, the type of the test column is    CHAR (4).       
nullif (EXPR1,EXPR2)    usage:   
if expr1 
=   expr2     is established, the return value is NULL , otherwise the return value is    Expr1. This and case   when   expr1   =   expr2   
THEN     NULL    else   expr1   end are the same.      
MySQL >   select   
Nullif ( );    

, NULL
mysql> SELECT nullif;
1
If the arguments are not equal, the MySQL two-time value is expr1.

Http://www.cnblogs.com/JuneZhang/archive/2010/08/26/1809306.html

Ifnull, Nullif and isnull usage differences in MYSQL