MySQL judges null and empty string

1. Null in MySQL cannot be used with any operator with other fields or variables (functions, stored procedures). There may be a problem if you use arithmetic data.

2. The judgment of NULL:

Create a user table: ID Key name can be empty

select * from user;

Insert into user values (' + ', null); # #创建一条name为空的数据

Insert into user values (' 222 ', '); # #创建一条为空字符的数据

　　

Use ISNULL to determine if null: Only name is null when the return value of the ISNULL (exp) function is 1, the empty string and the data are 0;

The SQL statement filtered to NULL can also be used with the SELECT * from user, where name is not null;

or select * from user where ISNULL (name) = 0;

3. Simultaneous culling of null and empty strings

SELECT * from user where ISNULL (name) =0 and LENGTH (Trim (name)) >0;

4 determining whether null or an empty string in a function or stored procedure

SELECT Id,name,casewhen (ISNULL (name) =1) | | (LENGTH (Trim (NAME)) =0) Then  ' AAA ' Endfromuser

　

MySQL judges null and empty string