mysql的join查询分类
1.语句分类
2.join语句分类
join的查询速度快于子查询，主要是因为子查询过程中需要创建中间表
内连接：inner join on = join on
左外连接：left join on = LEFT OUTER JOIN on
右外连接：right join on = right OUTER JOIN on
全外连接： full join on 但mysql不支持，可使用union 连接左外连接和右外连接数据
交叉连接：CROSS JOIN 相当于a*b表的结果，没有on从句
3.sql实例
a.建表
/* 老师表 */
CREATE TABLE teachers (
id int(4) not null auto_increment,
name varchar(20) not null default '' comment '老师名称',
subject varchar(20) not null default '' comment '学科',
primary key(id)
)engine=INNODB auto_increment=1 DEFAULT charset=utf8 comment '老师表';insert into teachers (name,subject) values ('张三','数学'),('李四','语文'),('王五','英语'),('张柳','历史'),('三七','地理');
/* 父亲表 */
DROP table father;
create table father(
id int(4) not null auto_increment,
name varchar(20) not null default '' comment '名称',
children varchar(20) not null default '' comment '孩子的名称',
primary key (id)
)engine=innodb auto_increment=1 default charset=utf8 comment '父亲表';insert into father(name,children) values ('张三','小孩1'),('李四','小孩2'),('王五','小孩3'),('大大','小孩4'),('小杰','小孩5');
create table teacherDate(
id int(4) not null auto_increment,
name varchar(20) not null default '' comment '老师名称',
date date not null comment '上课日期',
hour int(4) not null default 0 comment '当天的上课时间',
primary key(id)
)engine=innodb auto_increment=1 default charset=utf8 comment '老师的上课时间表';insert into teacherDate (name,date,hour) values ('张三','2019-06-11',3),('张三','2019-06-10',4),('张三','2019-06-09',5),('张三','2019-06-08',6)
,('李四','2019-06-11',3),('李四','2019-06-10',4),('李四','2019-06-09',5),('李四','2019-06-08',6)
,('王五','2019-06-11',3),('王五','2019-06-10',4),('王五','2019-06-09',5),('张柳','2019-06-08',6)
,('三七','2019-06-10',4),('三七','2019-06-09',5),('张柳','2019-06-11',6);
b.内连接
----属于老师且是父亲的信息 ：两表求并集 inner join = join----
SELECT a.name,a.subject,b.children from teachers a JOIN father b on a.name=b.name;
SELECT a.name,a.subject,b.children from teachers a INNER JOIN father b on a.name=b.name;
c.左连接
----是老师但不是父亲的 left join = LEFT OUTER JOIN ----
SELECT a.name,a.subject,b.children from teachers a left join father b on a.name=b.name where b.name is null ;
SELECT a.name,a.subject,b.children from teachers a left outer join father b on a.name=b.name where b.name is null ;
d.右连接
----是父亲但不是老师 right join = right OUTER JOIN ----
select b.name,a.subject,b.children from teachers a right join father b on a.name=b.name where a.name is null;
e.全外连接
----选出老师以及父亲的全部信息 ----
SELECT a.name ,a.subject,b.children from teachers a FULL JOIN father b on a.name=b.name ;/* mysql不支持full join on *//* 将左关联和右关联查询结果使用union合并起来 */
SELECT a.name,a.subject,b.children from teachers a left join father b on a.name=b.name
union
select b.name,a.subject,b.children from teachers a right join father b on a.name=b.name;
f.交叉连接
---- 交叉连接：相当于A表*B表 没有on从句 ----
SELECT a.name,a.subject,b.children from teachers a CROSS JOIN father b ;
4.join查询的优化场景
a.优化子查询
子查询： SELECT COUNT(a.id) from shops a where a.sid in (SELECT b.sid from consignee b where b.static=1);
join优化：SELECT COUNT(a.id) from shops a LEFT JOIN consignee b ON a.sid=b.sid where b.static=1;
join优化后的查询相当于子查询时间的一半
b.update不能操作from中的表
需求：更新 同时是老师和父亲的人员的学科信息为 家庭学
update teachers set subject='家庭学' where name in(/* 不能更新from从句中的表 */
SELECT a.name from teachers a inner join father b on a.name=b.name
); //[Err] 1093 - You can't specify target table 'teachers' for update in FROM clause可使用如下语句:
UPDATE teachers d join (SELECT a.name from teachers a inner join father b on a.name=b.name) c on d.name=c.name set d.subject='家庭学';
update teachers a inner join father b on a.name=b.name set a.subject='家庭学3';
UPDATE teachers a , father b set a.subject='家庭学2' where a.name=b.name ;
c.join优化聚合子查询
查询出各个老师上课最多的日期
--查询特定老师上课最多的日期--
select c.id,c.name,max(c.hour) from teacherDate c where c.name='三七' ;
--查询老师的上课信息--
SELECT a.name,a.`subject`,b.date,b.hour from teachers a join teacherDate b on a.name=b.name;通过将1的查询结果作为2的子查询的方式进行处理
SELECT a.name,a.`subject`,b.date,b.hour from teachers a join teacherDate b on a.name=b.name
where b.hour in (select max(c.hour) from teacherDate c where c.name=b.name );
d.选出每个人打怪做多的两天的记录
