连表查询。比如说需要查询学生表的数据,因为学生表和班级表关联,这个时候需要吧classes里的classname当做学生表里的班级名称显示就需要以下sql语句。
SELECT
s.*,c.`name` className
FROM
student AS s,
classes AS c
WHERE s.`classId` = c.`id`
ORDER BY id
相当于说是吧classes里的className别名成了name字段
c.`name` className
FROM
student AS s,
classes AS c
WHERE s.`classId` = c.`id`