mysql variable declaration and assignment

Variables in sql server must be declared first and then assigned a value:

Local variables are marked with one @, and global variables are marked with two @s (commonly used global variables are generally already defined);

The syntax for declaring local variables: declare @variable Name data type; for example: declare @num int;

Assignment: There are two methods (@num is the variable name, value is the value)

set @num=value; or select @num=value;

If you want To obtain a field value in a query statement, you can use select to assign a value to a variable, as follows:

select @num=Field name from table name where...

Variables in MySQL do not need to be declared in advance. When using them, use "@variable name" directly. "Just use it.

First usage: set @num=1; or set @num:=1; //Variables are used to save data here, use @num variables directly

Second usage: select @num:=1; Or select @num:=field name from table name where...

Pay attention to the above two assignment symbols. You can use "=" or ":=" when using set, but you must use ":= assignment" when using select

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