单表多表操作 联表查询
目录
一、单表操作
1. 分组——> group by
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分组指的是：将所有表记录按照某个相同字段进行归类
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用法：
select 聚合函数,选取的字段 from 表名 group by 选择分组的字段 having 条件- 当
group by后的字段为一个表的主键时，那么在select后面就可以查询该主键所在表的所有字段。否则select后查的字段只能是group by后的那个字段。 group by后可以跟多个字段，表示相同这些字段分到一组。如[1,2] [2,3] [,2,3] [3,4] ， [1,2] 分为一组，[2,3] 分为一组 ，[3,4] 分为一组，共三组。
- 当
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聚合函数：
count/sum/max/min/avghaving：表示对分组之后的聚合函数处理的结果，进行再一次的筛选。
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注意：
group by一般要和聚合函数一起使用。- 注意：聚合函数也可单独使用，放在select 之后。但此时select后面不能再跟其他字段。
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where 条件语句和
group by分组语句的先后顺序：
where > group by > having(*********) -
实例：
mysql> select depart_id,avg(age) from employee group by depart_id ; +-----------+----------+ | depart_id | avg(age) | +-----------+----------+ | 1 | 45.2500 | | 2 | 30.0000 | | 3 | 20.0000 | +-----------+----------+ 3 rows in set (0.00 sec) mysql> select depart_id,avg(age) from employee group by depart_id having avg(age) > 35; +-----------+----------+ | depart_id | avg(age) | +-----------+----------+ | 1 | 45.2500 | +-----------+----------+ 1 row in set (0.00 sec)
2. 排序——> order by
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order by对查询的结果进行排序 -
用法：
order by 字段名 asc/desc，其中asc默认的表示升序排序，desc表示降序排序- 如果对多个字段进行排序，如
order by age desc , id asc，则先对age进行降序排序，如果排完序记录中有相同的age时，再把有相同的这些行按id升序排序。
3. 分页——> limit
- 用法：
limit 参数1 , 参数2。参数1表示行索引，从该行开始，表记录第第一行数据的索引是0，往下递增，参数2表示取多少行。
4. 总结（很重要）
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以上高级用法的使用顺序是：
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select * from 表名 where 条件 group by 条件 having 条件 order by 条件 limit 条件; -
where > group by > having > order by > limit -
关键字执行的优先级
- from：找到表
- where：拿着where指定的约束条件，去文件/表中取出一条条记录
- group by：将取出的一条条记录进行分组group by，如果没有group by，则整体作为一组
- select：执行select
- distinct：去重
- having：将分组的结果进行having过滤
- order by：将结果按条件排序：order by
- limit：限制结果的显示条数
二、多表操作
1. 外键
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使用外键的原因：
- 减少占用的空间
- 只需要修改一次原表中的数据 ，其余有对应外键的表中的数据就会相应的修改。
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使用方法：
constraint 外键名 foreign key (被约束的字段) references 约束的表(约束的字段)
2. 一对多（很常见类型）
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实例：
create table department( id int auto_increment primary key, name varchar(32) not null default '' )charset utf8; insert into department (name) values ('研发部'); insert into department (name) values ('运维部'); insert into department (name) values ('前台部'); insert into department (name) values ('小卖部'); create table userinfo ( id int auto_increment primary key, name varchar(32) not null default '', depart_id int not null default 1, constraint fk_user_depart foreign key (depart_id) references department(id), )charset utf8; insert into userinfo (name, depart_id) values ('zekai', 1); insert into userinfo (name, depart_id) values ('xxx', 2); insert into userinfo (name, depart_id) values ('zekai1', 3); insert into userinfo (name, depart_id) values ('zekai2', 4); insert into userinfo (name, depart_id) values ('zekai3', 1); insert into userinfo (name, depart_id) values ('zekai4', 2); insert into userinfo (name, depart_id) values ('zekai4', 5);
3. 多对多（常见类型）
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实例：
create table boy ( id int auto_increment primary key, bname varchar(32) not null default '' )charset utf8; insert into boy (bname) values ('zhangsan'),('lisi'),('zhaoliu'); create table girl ( id int auto_increment primary key, gname varchar(32) not null default '' )charset utf8; insert into girl (gname) values ('cuihua'),('gangdan'),('jianguo'); create table boy2girl ( id int auto_increment primary key, bid int not null default 1, gid int not null default 1, constraint fk_boy2girl_boy foreign key (bid) references boy(id), constraint fk_boy2girl_girl foreign key (gid) references girl(id) )charset utf8; insert into boy2girl (bid, gid) values (1,1),(1,2),(2,3),(3,3),(2,2); select * from boy left join boy2girl on boy.id = boy2girl.bid left join girl on girl.id=boy2girl.gid; mysql> select * from boy left join boy2girl on boy.id = boy2girl.bid left join girl on girl.id=boy2girl.gid; +----+----------+------+------+------+------+---------+ | id | bname | id | bid | gid | id | gname | +----+----------+------+------+------+------+---------+ | 1 | zhangsan | 1 | 1 | 1 | 1 | cuihua | | 1 | zhangsan | 2 | 1 | 2 | 2 | gangdan | | 2 | lisi | 5 | 2 | 2 | 2 | gangdan | | 2 | lisi | 3 | 2 | 3 | 3 | jianguo | | 3 | zhaoliu | 4 | 3 | 3 | 3 | jianguo | +----+----------+------+------+------+------+---------+ 5 rows in set (0.00 sec) mysql> select bname, gname from boy left join boy2girl on boy.id = boy2girl.bid left join girl on girl.id=boy2girl.gid; +----------+---------+ | bname | gname | +----------+---------+ | zhangsan | cuihua | | zhangsan | gangdan | | lisi | gangdan | | lisi | jianguo | | zhaoliu | jianguo | +----------+---------+ 5 rows in set (0.00 sec) mysql> select bname, gname from boy left join boy2girl on boy.id = boy2girl.bid left join girl on girl.id=boy2girl.gid where bname='zhangsan'; +----------+---------+ | bname | gname | +----------+---------+ | zhangsan | cuihua | | zhangsan | gangdan | +----------+---------+ 2 rows in set (0.02 sec)
4. 一对一（不常见类型）
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实例：
user : id name age 1 zekai 18 2 zhangsan 23 3 xxxx 19 由于salary是比较敏感的字段，因此我们需要将此字段单独拆出来， 变成一张独立的表 private： id salary uid (外键 + unique) 1 5000 1 2 6000 2 3 3000 3 create table user ( id int auto_increment primary key, name varchar(32) not null default '' )charset=utf8; insert into user (name) values ('zhangsan'),('zekai'),('kkk'); create table priv( id int auto_increment primary key, salary int not null default 0, uid int not null default 1, constraint fk_priv_user foreign key (uid) references user(id), unique(uid) )charset=utf8; insert into priv (salary, uid) values (2000, 1); insert into priv (salary, uid) values (2800, 2); insert into priv (salary, uid) values (3000, 3); insert into priv (salary, uid) values (6000, 1); ERROR 1062 (23000): Duplicate entry '1' for key 'uid'
三、多表联查
1. 左连接——>left join 表名 on..
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表示已left左边的表为主，会把左边的表中的信息全部显示，被join的表按照左边的表的数据一一对应显示。
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实例
# 原表 mysql> select * from department; +----+--------+ | id | name | +----+--------+ | 1 | 研发部 | | 2 | 运维部 | | 3 | 前台部 | | 4 | 小卖部 | +----+--------+ 4 rows in set (0.07 sec) mysql> select * from userinfo; +----+--------+-----------+ | id | name | depart_id | +----+--------+-----------+ | 1 | zekai | 1 | | 2 | xxx | 2 | | 3 | zekai1 | 3 | | 4 | zekai2 | 4 | | 5 | zekai3 | 1 | | 6 | zekai4 | 2 | +----+--------+-----------+ 6 rows in set (0.00 sec) # 联表查询： # 错误写法 mysql> select name from userinfo left join department on depart_id = department.id; ERROR 1052 (23000): Column 'name' in field list is ambiguous # 正确写法 mysql> select userinfo.name as uname, department.name as dname from userinfo left join department on depart_id = department.id; # 查询结果 +--------+--------+ | uname | dname | +--------+--------+ | zekai | 研发部 | | zekai3 | 研发部 | | xxx | 运维部 | | zekai4 | 运维部 | | zekai1 | 前台部 | | zekai2 | 小卖部 | +--------+--------+ 6 rows in set (0.00 sec)
2. 右连接——>right join 表名 on..
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表示已right右边的表为主，会把右边的表中的信息全部显示，被join的表按照右边的表的数据一一对应显示。
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实例：
mysql> insert into department (name) values ('财务部'); Query OK, 1 row affected (0.04 sec) mysql> mysql> select * from department; ); +----+--------+ | id | name | +----+--------+ | 1 | 研发部 | | 2 | 运维部 | | 3 | 前台部 | | 4 | 小卖部 | | 5 | 财务部 | +----+--------+ 5 rows in set (0.00 sec) mysql> select * from userinfo; +----+--------+-----------+ | id | name | depart_id | +----+--------+-----------+ | 1 | zekai | 1 | | 2 | xxx | 2 | | 3 | zekai1 | 3 | | 4 | zekai2 | 4 | | 5 | zekai3 | 1 | | 6 | zekai4 | 2 | +----+--------+-----------+ 6 rows in set (0.00 sec) mysql> select userinfo.name as uname, department.name as dname from userinfo left join department on depart_id = department.id; +--------+--------+ | uname | dname | +--------+--------+ | zekai | 研发部 | | zekai3 | 研发部 | | xxx | 运维部 | | zekai4 | 运维部 | | zekai1 | 前台部 | | zekai2 | 小卖部 | +--------+--------+ 6 rows in set (0.00 sec) mysql> select userinfo.name as uname, department.name as dname from userinfo right join department on depart_id = department.id; +--------+--------+ | uname | dname | +--------+--------+ | zekai | 研发部 | | zekai3 | 研发部 | | xxx | 运维部 | | zekai4 | 运维部 | | zekai1 | 前台部 | | zekai2 | 小卖部 | | NULL | 财务部 | +--------+--------+ 7 rows in set (0.00 sec)
3. 内连接——>inner join 表名 on..
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把两个表中共同的部分取出来连接。
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实例：
mysql> select * from department inner join userinfo on department.id=userinfo.depart_id; +----+--------+----+--------+-----------+ | id | name | id | name | depart_id | +----+--------+----+--------+-----------+ | 1 | 研发部 | 1 | zekai | 1 | | 1 | 研发部 | 5 | zekai3 | 1 | | 2 | 运维部 | 2 | xxx | 2 | | 2 | 运维部 | 6 | zekai4 | 2 | | 3 | 前台部 | 3 | zekai1 | 3 | | 4 | 小卖部 | 4 | zekai2 | 4 | +----+--------+----+--------+-----------+ 6 rows in set (0.00 sec)
4. 交叉连接
- 语法：
from 表1,表2
