时间：2019-06-23 21:10:54
参考：
MySQL 自定义变量¶
不使用自定义变量可以解决大部分查询问题。也可以使用内存计算代替自定义变量。不到万不得已不要使用变量。
用户自定义变量在当前连接中有效。不同的客户端到服务端的链接不能共享使用用户自定义变量。
变量使用语法¶
-
变量赋值。
mysql SET @var_name := expr -
变量递增
mysql SELECT @num := @num + 1 FROM table_name; -
在SQL里面初始化变量。下面这个SQL没有从数据表查询，而是使用SQL构造数据。
```
初始化变量¶
(SELECT @j := 0 as init_j) AS temp
生成三行 1,2,3¶
(SELECT 1 AS id UNION ALL SELECT 2 UNION ALL SELECT 3) AS ids,
全连接 (nnn)¶
(SELECT @j := 0 as init_j) AS temp, (SELECT 1 AS id UNION ALL SELECT 2 UNION ALL SELECT 3) AS ids, (SELECT 'Type-A' AS type UNION ALL select 'Type-B' UNION ALL SELECT 'Type-C') AS types
(SELECT @j := 0 as init_j) AS temp 先执行，执行完之后才会执行外面的 @j := @j + 1¶
```
mysql SELECT temp.init_j, ids.id, types.type, @j := @j + 1 AS uid FROM (SELECT @j := 0 as init_j) AS temp, (SELECT 1 AS id UNION ALL SELECT 2 UNION ALL SELECT 3) AS ids, (SELECT 'Type-A' AS type UNION ALL select 'Type-B' UNION ALL SELECT 'Type-C') AS types