MySQL 标量子查询

标量子查询是指子查询返回的是单一值的标量，如一个数字或一个字符串，也是子查询中最简单的返回形式。

一个标量子查询的例子如下：

SELECT * FROM article WHERE uid = (SELECT uid FROM user WHERE status=1 ORDER BY uid DESC LIMIT 1)

在该例子中，子查询语句：

SELECT uid FROM user WHERE status = 1 ORDER BY uid DESC LIMIT 1

返回的是单一的数字（如 2），实际的查询语句为：

SELECT * FROM article WHERE uid = 2