day20

关联查询

#左连接:表名 left join 表名 以左表为主表，只显示与左表能匹配的行
SELECT s.*,q.* FROM student AS s LEFT JOIN queue_info AS q ON s.q_id = q.id;

#右链接:以右边的表为主表，只显示与右表能匹配的行
SELECT s.*,q.* FROM student AS s RIGHT JOIN queue_info AS q ON s.q_id = q.id;

#内连接：只显示两边都能匹配上的行
SELECT s.*,q.`name` FROM student AS s INNER JOIN queue_info AS q ON s.q_id = q.id;

多表联查

#多表联查:和内连接差不多，都是只显示多表都能匹配上的行，只有查询的语法不同
SELECT s.*,q.* FROM student AS s,queue_info AS q WHERE s.q_id = q.id;