#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）
#[]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
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
#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 |+----------------------+----------------------------+----------------------+
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. inmysql> 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 notmysql> select *from stu where math=90 and english=60;+----+--------+------+---------+| id | name | math | english |+----+--------+------+---------+| 2 | 小乔 | 90 | 60 || 3 | 小乔 | 90 | 60 |+----+--------+------+---------+
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 |+--------+-------------------+
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 |+--------+--------+
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 |+------+-----------+------+--------+--------+--------+
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 |+------+--------+------+--------+--------+--------+
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");
#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 |+------+--------+------+---------+------+--------+
#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
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NULLmysql> 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 |+------+--------+------+---------+------+--------+
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 |+------+--------+------+---------+------+--------+
#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 |+------+--------+------+---------+------+--------+
#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yyhmysql> 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 |+--------------+--------------+
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 depton emp.dept_id = dept.id group by dept.namehaving 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));
