SQL functions Nullif, NULL, ISNULL, coalesce

nullif function Nullif (expression1,expression2): Given two parameters Expression1 and Expression2, if two parameters are equal, NULL is returned, otherwise the first argument is returned. Equivalent to: case when Expression1=Expression2 then NULL ELSE Expression1. For example, select Nullif (1,1) returns Null,select Nullif (1,2) returns 1. There is a practical application, such as preventing release of 0 operations, you can use a/nullif (b,0), it is not afraid of B is 0, of course, in addition to 0 operations can also be judged by other means. Null NULL is a magical thing that represents a null value, an unknown value, and any number with which subtraction returns NULL. ISNULL function ISNULL (expression1,expression2): Given two parameters Expression1 and Expression2, if Expression1 is null, then return Expression2,　　Otherwise, return Expression1. Equivalent to: case when Expression1 isNULL then Expression2 ELSE Expression1. For example, select ISNULL (NULL,1) returns 1,select ISNULL (1,2) returns 1. There is an actual application that can be substituted with a default value for null values, such as select ISNULL (email,'email not filled in') fromTable1, all emails are null, with'email not filled in'to replace. Coalesce function Coalesce (Expression1,expression2,expression3,......)      : Accepts a series of expressions or columns, returning the first non-null value. For example, select COALESCE (Null,null,4, Null,null,5), then return, if the parameters are null, then the error will be.

SQL functions Nullif, NULL, ISNULL, coalesce