单表操作
分组
分组含义
分组指的是：将所有记录按照某个相同字段进行归类
分组语法
select 聚合函数, 选取的字段 from 表名 group by 分组的字段;group by : 是分组的关键词
group by 必须和 聚合函数(count) 出现
例子
# 1. 以性别为例， 进行分组， 统计一下男生和女生的人数是多少个：
mysql> select count(id), gender from employee group by gender;
+-----------+--------+
| count(id) | gender |
+-----------+--------+
| 10 | male |
| 8 | female |
+-----------+--------+
mysql> select gender, count(id) as total from employee group by gender;
+--------+-------+
| gender | total |
+--------+-------+
| male | 10 |
| female | 8 |
+--------+-------+
# 2. 对部门进行分组， 求出每个部门年龄最大的那个人？
mysql> select depart_id,max(age) from employee group by depart_id;
+-----------+----------+
| depart_id | max(age) |
+-----------+----------+
| 1 | 81 |
| 2 | 48 |
| 3 | 28 |
+-----------+----------+聚合函数
'''
max()：最大值
min()：最小值
avg()：平均值
sum()：和
count()：记数'''having
having语法
表示对group by 之后的数据， 进行再一次的二次筛选
直接写在分组之后
having例子
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)
mysql> select depart_id,avg(age) as pj from employee group by depart_id having pj > 35;
+-----------+---------+
| depart_id | pj |
+-----------+---------+
| 1 | 45.2500 |
+-----------+---------+
1 row in set (0.00 sec)排序
排序语法
`select * from 表名 order by 字段名 desc;不写默认为asc升序 , desc为降序
如果对多个字段进行排序，如：age desc, id asc;
表示： 先对age进行降序， 如果age有相同的行， 则对id进行升序
例子
select * from employee order by age desc, id desc;分页
分页语法
select * from 表名 limit 索引值,记录数;limit offset， size
offset： 行数据索引 , 索引从0开始 size: 取多少条数据
例子
mysql> select * from employee limit 0,10;
mysql> select * from employee limit 10,10;单表操作小结
使用的顺序：
select * from 表名 where 条件 group by 条件 having 条件 order by 条件 limit 条件;
优先级 : where > group by > having > order by > limit
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多表操作
外键
使用的原因：
a. 减少占用的空间
b. 只需要修改department表中一次， 其余的表中的数据就会相应的修改
一对多
语法
constraint 外键名 foreign key (被约束的字段) references 约束的表(约束的字段)例子
# 建department表 create table department( id int auto_increment primary key, name varchar(32) not null default '' )charset utf8; # 插入记录 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'); # 建userinfo表 create table userinfo ( id int auto_increment primary key, name varchar(32) not null default '', depart_id int not null default 1, # 将userinfo表的depart_id和department的id建立关系 constraint fk_user_depart foreign key (depart_id) references department(id) )charset utf8; # 插入信息 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);
多对多
语法
constraint 外键名 foreign key (被约束的字段) references 约束的表(约束的字段)constraint 外键名 foreign key (被约束的字段) references 约束的表(约束的字段)例子
# 建boy表 create table boy ( id int auto_increment primary key, bname varchar(32) not null default '' )charset utf8; # 插入记录 insert into boy (bname) values ('zhangsan'),('lisi'),('zhaoliu'); # 建girl表 create table girl ( id int auto_increment primary key, gname varchar(32) not null default '' )charset utf8; # 插入信息 insert into girl (gname) values ('cuihua'),('gangdan'),('jianguo'); # 建boy2girl表 create table boy2girl ( id int auto_increment primary key, bid int not null default 1, gid int not null default 1, # 建立多对多关系 constraint fk_boy2girl_boy foreign key (bid) references boy(id), constraint fk_boy2girl_girl foreign key (gid) references girl(id) )charset utf8; # 插入信息 insert into boy2girl (bid, gid) values (1,1),(1,2),(2,3),(3,3),(2,2);
一对一
语法
constraint 外键名 foreign key (被约束的字段) references 约束的表(约束的字段)例子
# 建user表 create table user ( id int auto_increment primary key, name varchar(32) not null default '' )charset=utf8; # 插入信息 insert into user (name) values ('zhangsan'),('zekai'),('kkk'); # 建priv表 create table priv( id int auto_increment primary key, salary int not null default 0, uid int not null default 1, # 创建关系 constraint fk_priv_user foreign key (uid) references user(id), # 唯一性 不能重复 unique(uid) )charset=utf8; # 插入信息 insert into priv (salary, uid) values (2000, 1); insert into priv (salary, uid) values (2800, 2); insert into priv (salary, uid) values (3000, 3); ''' 一旦重复就会报错 insert into priv (salary, uid) values (6000, 1); ERROR 1062 (23000): Duplicate entry '1' for key 'uid' '''
多表联查
两表联查
left join....on
语法
select * from 左表 left join 右表 on 两表有关联的字段的条件left join....on 保留左边表的所有字段记录
例子
mysql> select * from userinfo left join department on depart_id =department.id 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)
right join...on
语法
select * from 左表 right join 右表 on 两表有关联的字段的条件right join...on 保留右边表的所有字段记录
例子
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 | +--------+--------+ 7 rows in set (0.00 sec)
inner join
三表联查
三表联查就是在两表的基础上再加一句左连接或者右连接
例子:
mysql> select * from boy left join boy2girl on boy.id = boy2girl.bid left join girl on girl.id = boy2girl.gid;