Day41-数据库入门学习-单表查询、多表查询、子查询
1.复制表
#创建了一张测试表 mysql>create table test (id int primary key auto_increment,name char(10)); #显示原表结构 mysql> desc test; +-------+----------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | char(10) | YES | | NULL | | +-------+----------+------+-----+---------+----------------+ #显示原表数据 mysql> select *from test; +----+------+ | id | name | +----+------+ | 1 | a | +----+------+ #复制表 mysql> create table copy select *from test; #显示复制表的结构 mysql> desc copy; +-------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+-------+ | id | int(11) | NO | | 0 | | | name | char(10) | YES | | NULL | | +-------+----------+------+-----+---------+-------+ #显示复制表的数据 mysql> select *from copy; +----+------+ | id | name | +----+------+ | 1 | a | +----+------+
总结：复制表，只是拷贝结构与数据，但是索引、 描述不能拷贝（自增）
2.单表查询
2.1完整的查询语句
#[]可选 {}必选 |或 select [distinct] {* | 字段名 | 聚合函数 | 表达式} from 表名 [where 条件 group by 字段名 having 条件 order by 字段名 limit 显示的条数] #注意:关键字的顺序必须与上述语法一致
2.2执行顺序
1.from,先找到文件/表
2.where,拿着where的约束条件，与文件/表中的记录依次比较，正确的数据取出来
3.group，对取出来的数据进行分组
4.having,对分组的数据进行过滤
5.distinct,去重
6.order by,对筛选后的数据排序
7.limit,限制结果的显示条数
2.3简单查询
#准备表 create table stu(id int primary key auto_increment,name char(10),math float,english float);
insert into stu values(null,"赵云",90,30);
insert into stu values(null,"小乔",90,60);
insert into stu values(null,"小乔",90,60);
insert into stu values(null,"大乔",10,70);
insert into stu values(null,"李清照",100,100);
insert into stu values(null,"铁拐李",20,55);
insert into stu values(null,"小李子",20,55); mysql> select *from stu; +----+-----------+------+---------+ | id | name | math | english | +----+-----------+------+---------+ | 1 | 赵云 | 90 | 30 | | 2 | 小乔 | 90 | 60 | | 3 | 小乔 | 90 | 60 | | 4 | 大乔 | 10 | 70 | | 5 | 李清照 | 100 | 100 | | 6 | 铁拐李 | 20 | 55 | | 7 | 小李子 | 20 | 55 | +----+-----------+------+---------+ #避免重复 mysql> select distinct name from stu; +-----------+ | name | +-----------+ | 赵云 | | 小乔 | | 大乔 | | 李清照 | | 铁拐李 | | 小李子 | +-----------+ #四则运算 mysql> select name,math+10 as math from stu; +-----------+------+ | name | math | +-----------+------+ | 赵云 | 100 | | 小乔 | 100 | | 小乔 | 100 | | 大乔 | 20 | | 李清照 | 110 | | 铁拐李 | 30 | | 小李子 | 30 | +-----------+------+ #显示格式 mysql> select -> concat("name:",name), -> concat("english:",english), -> concat("math:",math) -> from stu; +----------------------+----------------------------+----------------------+ | concat("name:",name) | concat("english:",english) | concat("math:",math) | +----------------------+----------------------------+----------------------+ | name:赵云 | english:30 | math:90 | | name:小乔 | english:60 | math:90 | | name:小乔 | english:60 | math:90 | | name:大乔 | english:70 | math:10 | | name:李清照 | english:100 | math:100 | | name:铁拐李 | english:55 | math:20 | | name:小李子 | english:55 | math:20 | +----------------------+----------------------------+----------------------+
2.4 where
1. 比较运算符：> < >= <= <> !=
2. between 80 and 100 值在80到100之间
3. in(80,90,100) 值是80或90或100
4. like 'egon[%|_]'
%表示任意多字符
_表示一个字符
5. 逻辑运算符：在多个条件直接可以使用逻辑运算符 and or not
+----+-----------+------+---------+ | id | name | math | english | +----+-----------+------+---------+ | 1 | 赵云 | 90 | 30 | | 2 | 小乔 | 90 | 60 | | 3 | 小乔 | 90 | 60 | | 4 | 大乔 | 10 | 70 | | 5 | 李清照 | 100 | 100 | | 6 | 铁拐李 | 20 | 55 | | 7 | 小李子 | 20 | 55 | +----+-----------+------+---------+ #1. 比较运算符：> < >= <= <> != mysql> select *from stu where math>70; +----+-----------+------+---------+ | id | name | math | english | +----+-----------+------+---------+ | 1 | 赵云 | 90 | 30 | | 2 | 小乔 | 90 | 60 | | 3 | 小乔 | 90 | 60 | | 5 | 李清照 | 100 | 100 | +----+-----------+------+---------+ #2. between and mysql> select *from stu where math between 80 and 100 ; +----+-----------+------+---------+ | id | name | math | english | +----+-----------+------+---------+ | 1 | 赵云 | 90 | 30 | | 2 | 小乔 | 90 | 60 | | 3 | 小乔 | 90 | 60 | | 5 | 李清照 | 100 | 100 | +----+-----------+------+---------+ #3. in mysql> select *from stu where math in (80,90,100) ; +----+-----------+------+---------+ | id | name | math | english | +----+-----------+------+---------+ | 1 | 赵云 | 90 | 30 | | 2 | 小乔 | 90 | 60 | | 3 | 小乔 | 90 | 60 | | 5 | 李清照 | 100 | 100 | +----+-----------+------+---------+ #4. like %或_ mysql> select *from stu where name like '小_' ; +----+--------+------+---------+ | id | name | math | english | +----+--------+------+---------+ | 2 | 小乔 | 90 | 60 | | 3 | 小乔 | 90 | 60 | +----+--------+------+---------+ mysql> select *from stu where name like '李%' ; +----+-----------+------+---------+ | id | name | math | english | +----+-----------+------+---------+ | 5 | 李清照 | 100 | 100 | +----+-----------+------+---------+ #5. and or not mysql> select *from stu where math=90 and english=60; +----+--------+------+---------+ | id | name | math | english | +----+--------+------+---------+ | 2 | 小乔 | 90 | 60 | | 3 | 小乔 | 90 | 60 | +----+--------+------+---------+
2.5 group by
用于给数据分组
1.在生活中是为了方便管理
2.在数据库中是为了 方便统计
可以按照任意字段分组，但是分组完毕后，只能查看分组的字段，如果想查看组内信息，需要借助于聚合函数
聚合函数：
将一堆数据经过计算，得到一个数据
sum() 求和
avg() 求平均数
max()/min() 求最大值 / 最小值
count() 个数
#由于没有设置ONLY_FULL_GROUP_BY,于是也可以有结果，默认都是组内的第一条记录，但其实这是没有意义的 mysql> set global sql_mode='ONLY_FULL_GROUP_BY'; Query OK, 0 rows affected (0.00 sec) mysql> quit #设置成功后，一定要退出，然后重新登录方可生效 #准备数据 create table emp (id int,name char(10),sex char,dept char(10),job char(10),salary double); insert into emp values (1,"刘备","男","市场","总监",5800), (2,"张飞","男","市场","员工",3000), (3,"关羽","男","市场","员工",4000), (4,"孙权","男","行政","总监",6000), (5,"周瑜","男","行政","员工",5000), (6,"小乔","女","行政","员工",4000), (7,"曹操","男","财务","总监",10000), (8,"司马懿","男","财务","员工",6000); #查询每个部门有几个人 mysql> select dept,count(*)as 人数 from emp group by dept; +--------+--------+ | dept | 人数 | +--------+--------+ | 市场 | 3 | | 行政 | 3 | | 财务 | 2 | +--------+--------+ #计算每个部门的平均工资 mysql> select dept,avg(salary)as 平均工资 from emp group by dept; +--------+-------------------+ | dept | 平均工资 | +--------+-------------------+ | 市场 | 4266.666666666667 | | 行政 | 5000 | | 财务 | 8000 | +--------+-------------------+
2.6 having
用于对分组后的数据进行过滤
1. having不会单独出现 都是和group by 一起出现
2. where 发生在分组group by之前，因而Where中可以有任意字段，但是绝对不能使用聚合函数。
3. having发生在分组group by之后，因而Having中可以使用分组的字段，无法直接取到其他字段,可以使用聚合函数
#准备数据 mysql> create table emp (id int,name char(10),sex char,dept char(10),job char(10),salary double); mysql> insert into emp values (1,"刘备","男","市场","总监",5800), -> (2,"张飞","男","市场","员工",3000), -> (3,"关羽","男","市场","员工",4000), -> (4,"孙权","男","行政","总监",6000), -> (5,"周瑜","男","行政","员工",5000), -> (6,"小乔","女","行政","员工",4000), -> (7,"曹操","男","财务","总监",10000), -> (8,"司马懿","男","财务","员工",6000); mysql> select *from emp; +------+-----------+------+--------+--------+--------+ | id | name | sex | dept | job | salary | +------+-----------+------+--------+--------+--------+ | 1 | 刘备 | 男 | 市场 | 总监 | 5800 | | 2 | 张飞 | 男 | 市场 | 员工 | 3000 | | 3 | 关羽 | 男 | 市场 | 员工 | 4000 | | 4 | 孙权 | 男 | 行政 | 总监 | 6000 | | 5 | 周瑜 | 男 | 行政 | 员工 | 5000 | | 6 | 小乔 | 女 | 行政 | 员工 | 4000 | | 7 | 曹操 | 男 | 财务 | 总监 | 10000 | | 8 | 司马懿 | 男 | 财务 | 员工 | 6000 | +------+-----------+------+--------+--------+--------+ #查询平均工资大于5000的部门 mysql> select dept,avg(salary)as 平均工资 from emp group by dept having avg(salary)>5000; +--------+--------------+ | dept | 平均工资 | +--------+--------------+ | 财务 | 8000 | +--------+--------------+ #查询每个职位有多少人 mysql> select job,count(*)as 人数 from emp group by job; +--------+--------+ | job | 人数 | +--------+--------+ | 员工 | 5 | | 总监 | 3 | +--------+--------+
2.7 order by
用于对记录进行 排序
desc为降序
asc为升序
#准备数据 mysql> create table emp (id int,name char(10),sex char,dept char(10),job char(10),salary double); mysql> insert into emp values (1,"刘备","男","市场","总监",5800), -> (2,"张飞","男","市场","员工",3000), -> (3,"关羽","男","市场","员工",4000), -> (4,"孙权","男","行政","总监",6000), -> (5,"周瑜","男","行政","员工",5000), -> (6,"小乔","女","行政","员工",4000), -> (7,"曹操","男","财务","总监",10000), -> (8,"司马懿","男","财务","员工",6000); mysql> select *from emp; +------+-----------+------+--------+--------+--------+ | id | name | sex | dept | job | salary | +------+-----------+------+--------+--------+--------+ | 1 | 刘备 | 男 | 市场 | 总监 | 5800 | | 2 | 张飞 | 男 | 市场 | 员工 | 3000 | | 3 | 关羽 | 男 | 市场 | 员工 | 4000 | | 4 | 孙权 | 男 | 行政 | 总监 | 6000 | | 5 | 周瑜 | 男 | 行政 | 员工 | 5000 | | 6 | 小乔 | 女 | 行政 | 员工 | 4000 | | 7 | 曹操 | 男 | 财务 | 总监 | 10000 | | 8 | 司马懿 | 男 | 财务 | 员工 | 6000 | #按单列排序 #默认是ASC升序 mysql> SELECT * FROM emp ORDER BY salary; +------+-----------+------+--------+--------+--------+ | id | name | sex | dept | job | salary | +------+-----------+------+--------+--------+--------+ | 2 | 张飞 | 男 | 市场 | 员工 | 3000 | | 3 | 关羽 | 男 | 市场 | 员工 | 4000 | | 6 | 小乔 | 女 | 行政 | 员工 | 4000 | | 5 | 周瑜 | 男 | 行政 | 员工 | 5000 | | 1 | 刘备 | 男 | 市场 | 总监 | 5800 | | 4 | 孙权 | 男 | 行政 | 总监 | 6000 | | 8 | 司马懿 | 男 | 财务 | 员工 | 6000 | | 7 | 曹操 | 男 | 财务 | 总监 | 10000 | +------+-----------+------+--------+--------+--------+ #升序 mysql> SELECT * FROM emp ORDER BY salary ASC; +------+-----------+------+--------+--------+--------+ | id | name | sex | dept | job | salary | +------+-----------+------+--------+--------+--------+ | 2 | 张飞 | 男 | 市场 | 员工 | 3000 | | 3 | 关羽 | 男 | 市场 | 员工 | 4000 | | 6 | 小乔 | 女 | 行政 | 员工 | 4000 | | 5 | 周瑜 | 男 | 行政 | 员工 | 5000 | | 1 | 刘备 | 男 | 市场 | 总监 | 5800 | | 4 | 孙权 | 男 | 行政 | 总监 | 6000 | | 8 | 司马懿 | 男 | 财务 | 员工 | 6000 | | 7 | 曹操 | 男 | 财务 | 总监 | 10000 | +------+-----------+------+--------+--------+--------+ #降序 mysql> SELECT * FROM emp ORDER BY salary DESC; +------+-----------+------+--------+--------+--------+ | id | name | sex | dept | job | salary | +------+-----------+------+--------+--------+--------+ | 7 | 曹操 | 男 | 财务 | 总监 | 10000 | | 4 | 孙权 | 男 | 行政 | 总监 | 6000 | | 8 | 司马懿 | 男 | 财务 | 员工 | 6000 | | 1 | 刘备 | 男 | 市场 | 总监 | 5800 | | 5 | 周瑜 | 男 | 行政 | 员工 | 5000 | | 3 | 关羽 | 男 | 市场 | 员工 | 4000 | | 6 | 小乔 | 女 | 行政 | 员工 | 4000 | | 2 | 张飞 | 男 | 市场 | 员工 | 3000 | +------+-----------+------+--------+--------+--------+ 按多列排序: #先按照职位排序，如果职位相同，则按照薪资排序 mysql> SELECT * from emp ORDER BY job, salary DESC; +------+-----------+------+--------+--------+--------+ | id | name | sex | dept | job | salary | +------+-----------+------+--------+--------+--------+ | 8 | 司马懿 | 男 | 财务 | 员工 | 6000 | | 5 | 周瑜 | 男 | 行政 | 员工 | 5000 | | 3 | 关羽 | 男 | 市场 | 员工 | 4000 | | 6 | 小乔 | 女 | 行政 | 员工 | 4000 | | 2 | 张飞 | 男 | 市场 | 员工 | 3000 | | 7 | 曹操 | 男 | 财务 | 总监 | 10000 | | 4 | 孙权 | 男 | 行政 | 总监 | 6000 | | 1 | 刘备 | 男 | 市场 | 总监 | 5800 | +------+-----------+------+--------+--------+--------+
2.8 limit
用于限制显示的条数
limit [start,]count
#准备数据 mysql> create table emp (id int,name char(10),sex char,dept char(10),job char(10),salary double); mysql> insert into emp values (1,"刘备","男","市场","总监",5800), -> (2,"张飞","男","市场","员工",3000), -> (3,"关羽","男","市场","员工",4000), -> (4,"孙权","男","行政","总监",6000), -> (5,"周瑜","男","行政","员工",5000), -> (6,"小乔","女","行政","员工",4000), -> (7,"曹操","男","财务","总监",10000), -> (8,"司马懿","男","财务","员工",6000); mysql> select *from emp; +------+-----------+------+--------+--------+--------+ | id | name | sex | dept | job | salary | +------+-----------+------+--------+--------+--------+ | 1 | 刘备 | 男 | 市场 | 总监 | 5800 | | 2 | 张飞 | 男 | 市场 | 员工 | 3000 | | 3 | 关羽 | 男 | 市场 | 员工 | 4000 | | 4 | 孙权 | 男 | 行政 | 总监 | 6000 | | 5 | 周瑜 | 男 | 行政 | 员工 | 5000 | | 6 | 小乔 | 女 | 行政 | 员工 | 4000 | | 7 | 曹操 | 男 | 财务 | 总监 | 10000 | | 8 | 司马懿 | 男 | 财务 | 员工 | 6000 | +------+-----------+------+--------+--------+--------+ # 看看表里前三条数据 mysql> select *from emp limit 3; +------+--------+------+--------+--------+--------+ | id | name | sex | dept | job | salary | +------+--------+------+--------+--------+--------+ | 1 | 刘备 | 男 | 市场 | 总监 | 5800 | | 2 | 张飞 | 男 | 市场 | 员工 | 3000 | | 3 | 关羽 | 男 | 市场 | 员工 | 4000 | +------+--------+------+--------+--------+--------+ # 看看表里的3-5条 mysql> select *from emp limit 2,3; +------+--------+------+--------+--------+--------+ | id | name | sex | dept | job | salary | +------+--------+------+--------+--------+--------+ | 3 | 关羽 | 男 | 市场 | 员工 | 4000 | | 4 | 孙权 | 男 | 行政 | 总监 | 6000 | | 5 | 周瑜 | 男 | 行政 | 员工 | 5000 | +------+--------+------+--------+--------+--------+ # 查看工资最高的那个人的信息 mysql> select *from emp order by salary desc limit 1; +------+--------+------+--------+--------+--------+ | id | name | sex | dept | job | salary | +------+--------+------+--------+--------+--------+ | 7 | 曹操 | 男 | 财务 | 总监 | 10000 | +------+--------+------+--------+--------+--------+
3.多表查询
在多个表中查询需要的数据
例如：有班级表和学生表
给你一个班级名称， 请查询所有的学员数据
先查班级表 得到一个班级的id， 再根据id去学院表查询对应的学员
#准备数据： create table emp (id int,name char(10),sex char,dept_id int); insert emp values(1,"大黄","m",1); insert emp values(2,"老王","m",2); insert emp values(3,"老李","w",30); mysql> select *from emp; +------+--------+------+---------+ | id | name | sex | dept_id | +------+--------+------+---------+ | 1 | 大黄 | m | 1 | | 2 | 老王 | m | 2 | | 3 | 老李 | w | 30 | +------+--------+------+---------+ create table dept (id int,name char(10)); insert dept values(1,"市场"); insert dept values(2,"财务"); mysql> select *from dept; +------+--------+ | id | name | +------+--------+ | 1 | 市场 | | 2 | 财务 | | 3 | 行政 | +------+--------+ insert dept values(3,"行政");
多表查询的方式
1.笛卡尔积查询
#什么是笛卡尔积,用坐标中的一条记录，去链接另一张表的所有记录 #就像是把两张表的数据做了一个乘法 #这将导致产生大量的无用重复数据 #我们要的效果是：员工表中的部门id与部门表中的id相同，就拼接在一起 #用 where 筛选出正确的数据 mysql> select *from emp,dept where emp.dept_id=dept.id; +------+--------+------+---------+------+--------+ | id | name | sex | dept_id | id | name | +------+--------+------+---------+------+--------+ | 1 | 大黄 | m | 1 | 1 | 市场 | | 2 | 老王 | m | 2 | 2 | 财务 | +------+--------+------+---------+------+--------+
2.内连接查询
#inner join...on... mysql> select *from emp inner join dept on emp.dept_id=dept.id; +------+--------+------+---------+------+--------+ | id | name | sex | dept_id | id | name | +------+--------+------+---------+------+--------+ | 1 | 大黄 | m | 1 | 1 | 市场 | | 2 | 老王 | m | 2 | 2 | 财务 | +------+--------+------+---------+------+--------+ #与from...where...一样，只不过inner join...on...是专门用于多表连接的
3.左外连接
left join #左边员工表中的数据完全显示，右边表中的数据匹配上才显示，没有则NULL mysql> select *from emp left join dept on emp.dept_id=dept.id; +------+--------+------+---------+------+--------+ | id | name | sex | dept_id | id | name | +------+--------+------+---------+------+--------+ | 1 | 大黄 | m | 1 | 1 | 市场 | | 2 | 老王 | m | 2 | 2 | 财务 | | 3 | 老李 | w | 30 | NULL | NULL | +------+--------+------+---------+------+--------+
4.右外连接
right join #右边表中的数据完全显示，左边表中的数据匹配上才显示 mysql> select *from emp right join dept on emp.dept_id=dept.id; +------+--------+------+---------+------+--------+ | id | name | sex | dept_id | id | name | +------+--------+------+---------+------+--------+ | 1 | 大黄 | m | 1 | 1 | 市场 | | 2 | 老王 | m | 2 | 2 | 财务 | | NULL | NULL | NULL | NULL | 3 | 行政 | +------+--------+------+---------+------+--------+
5. 全外链接
#全外连接：在内连接的基础上增加左边有右边没有的和右边有左边没有的结果 #mysql不支持 full join，但是可以用union间接实现 union：表示合并查询，意思是把多个查询结果合并在一起显示，要求是被合并的表结构必须相同，默认去除重复 union all：合并但是不去除重复 sql> select *from emp right join dept on emp.dept_id = dept.id ->union ->select *from emp left join dept on emp.dept_id = dept.id; +------+--------+------+---------+------+--------+ | id | name | sex | dept_id | id | name | +------+--------+------+---------+------+--------+ | 1 | 大黄 | m | 1 | 1 | 市场 | | 2 | 老王 | m | 2 | 2 | 财务 | | NULL | NULL | NULL | NULL | 3 | 行政 | | 3 | 老李 | w | 30 | NULL | NULL | +------+--------+------+---------+------+--------+
6.三表查询
#准备数据 #老师表 create table tec(id int,name char(10)); insert into tec value(1,"egon"); insert into tec value(2,"yyh"); mysql> select *from tec; +------+------+ | id | name | +------+------+ | 1 | egon | | 2 | yyh | +------+------+ #学生表 create table stu(id int,name char(10)); insert into stu value(1,"大傻"); insert into stu value(2,"中傻"); insert into stu value(3,"小傻"); mysql> select *from stu; +------+--------+ | id | name | +------+--------+ | 1 | 大傻 | | 2 | 中傻 | | 3 | 小傻 | #关联表 create table s_t(s_id int,t_id int); insert into s_t value(1,2); insert into s_t value(2,2); insert into s_t value(3,1); mysql> select *from s_t; +------+------+ | s_id | t_id | +------+------+ | 1 | 2 | | 2 | 2 | | 3 | 1 | +------+------+ #需求 找出 yyh 这个老师 教过的学生信息 #思路： # 第一步，到关系表中去查询，哪些老师教过哪些学生(学生的id)，形成了一个临时表 mysql> select *from tec inner join s_t on tec.id=s_t.t_id; +------+------+------+------+ | id | name | s_id | t_id | +------+------+------+------+ | 2 | yyh | 1 | 2 | | 2 | yyh | 2 | 2 | | 1 | egon | 3 | 1 | +------+------+------+------+ # 第二步， 将上一步得到临时表与学生表进行连接 mysql> select *from tec inner join s_t on tec.id=s_t.t_id -> inner join stu on s_id=stu.id; +------+------+------+------+------+--------+ | id | name | s_id | t_id | id | name | +------+------+------+------+------+--------+ | 2 | yyh | 1 | 2 | 1 | 大傻 | | 2 | yyh | 2 | 2 | 2 | 中傻 | | 1 | egon | 3 | 1 | 3 | 小傻 | +------+------+------+------+------+--------+ # 第三步，加上额外的筛选条件，老师的name是yyh mysql> select tec.name as 老师名字 ,stu.name as 学生名字 from tec inner join s_t on tec.id=s_t.t_id -> inner join stu on s_id=stu.id -> where tec.name='yyh'; +--------------+--------------+ | 老师名字 | 学生名字 | +--------------+--------------+ | yyh | 大傻 | | yyh | 中傻 | +--------------+--------------+
4.子查询
子查询：将上一次查询的结果作为本次查询的原始数据（或是查询条件）
#准备数据 create table emp (id int,name char(10),sex char,dept char(10),job char(10),salary double); insert into emp values (1,"刘备","男","市场","总监",5800), (2,"张飞","男","市场","员工",3000), (3,"关羽","男","市场","员工",4000), (4,"孙权","男","行政","总监",6000), (5,"周瑜","男","行政","员工",5000), (6,"小乔","女","行政","员工",4000), (7,"曹操","男","财务","总监",10000), (8,"司马懿","男","财务","员工",6000); #需求：查询出工资最高的人的信息 #先查询出最高工资是多少 #拿着最高工资去表中看，谁的工资和最高工资匹配 mysql> select *from emp where salary=(select max(salary) from emp); +------+--------+------+--------+--------+--------+ | id | name | sex | dept | job | salary | +------+--------+------+--------+--------+--------+ | 7 | 曹操 | 男 | 财务 | 总监 | 10000 | +------+--------+------+--------+--------+--------+ #in 关键字子查询 #查询平均年龄大于25的部门名称 #1.求每个部门的平均年龄 select avg(age) from emp group by dept_id; #2.筛选结果 select dept_id,avg(age) as a from emp group by dept_id having a > 25; #3.最后 select name from dept where id in (select dept_id from emp group by dept_id having avg(age) > 25); #子查询方式: #平均年龄大于25的部门有哪些? #先要求出每个部门的平年龄!每个表示什么? 分组 select name from dept where id in (select dept_id from emp group by dept_id having avg(age) > 25); #多表查询方式: #先把数据拼接到一起 在加以筛选 select dept.name from emp inner join dept on emp.dept_id = dept.id group by dept.name having avg(age) >25; #exists关键字子查询 #exists 后跟子查询 子查询有结果是为True 没有结果时为False #为true时外层执行 为false外层不执行 select *from emp where exists (select *from emp where salary > 1000); select (exists (select *from emp where salary > 10000));
