=
只有在set和update时才是和:=一样，赋值的作用，其它都是等于的作用。鉴于此，用变量实现行号时，必须用:=。
:=
不只在set和update时时赋值的作用，在select也是赋值的作用。
下面是:=和=的在select时作用对比
sql
?| 1 2 |
set @num = 0;
SELECT @num := @num+1 AS rowno, nc as nickname from table_user;
|
结果
sql
?| 1 2 |
set @num = 0;
SELECT @num = @num+1 AS rowno, nc as nickname from table_user;
|
结果
sql
?| 1 2 |
set @num = 0;
SELECT @num = @num AS rowno, nc as nickname from table_user;
|
结果
参考文献：https://www.jb51.net/article/167218.htm