The use of @ symbol in MySql

MySql中@符号的使用

@符号的作用

• @后接变量名,to define a variable,The variable is valid at the statement level,That is, it is always valid in another execution,基本示例如下：

-- 由于通常情况下=Considered an equals comparison operator,Therefore the assignment operator is generally used:=
SELECT @lt:=1, @lt:=@lt+1, @lt:=@lt+1;

实现rownum

-- 由于tmp只有一行,When the two tables are linked by a Cartesian product,The result set is effectively equivalent to an additional column,而由于r变量的特性,Each row is incremented based on the original value
SELECT a.BATCHNO, a.YEAR, @r:=@r+1 FROM m1 a, (SELECT @r:=0) tmp;

实现窗口函数

-- base变量的初值,Cannot be used with window functionspartition byThe grouped enumeration values ​​are the same
-- 实现此功能 row_number() (partition by base order by batchno) rn
-- case when 和 @base:= 的顺序不能颠倒,否则会有问题
SELECT a.BATCHNO, a.YEAR, a.BUDITEM, case when @base=a.BUDITEM then @r:=@r+1 ELSE
@r:=1 END rn, @base:=a.BUDITEM FROM m1 a, (SELECT @r:=1, @base:='') tmp ORDER BY a.buditem