DQL查询语句
11.简单的查询语句(DQL)
语法格式
select 字段名1，字段名2，字段名3，…from 表名;
提示：
1.任何一条sql语句以;结尾。
2.sql语句不区分大小写。
给查询结果的列重命名：
select name,sal12 as yearsal from emp;
别名中含有中文
select ename,sal12 as ‘年薪’ from emp;
mysql> select ename,sal*12 as ‘年薪’ from emp;
+--------+----------+
| ename | 年薪 |
+--------+----------+
| smith | 9600.00 |
| allen | 19200.00 |
| ward | 15000.00 |
| jones | 35700.00 |
| martin | 15000.00 |
| blake | 34200.00 |
| clark | 29400.00 |
| scott | 36000.00 |
| king | 60000.00 |
| turner | 18000.00 |
| adams | 13200.00 |
| james | 11400.00 |
| frod | 36000.00 |
| miller | 15600.00 |
+--------+----------+
注意：标准sql语句中要求字符串用单引号括起来。
as关键字可以省略。
查询所有字段：select * from emp;//实际开发中不建议使用，效率较低。*
12.条件查询：
语法格式：
select
字段，字段…
from
表名;
where
条件;
案例
查询工资等于5000的员工姓名：
select ename from emp where sal=5000;
查询工资大于3000的员工姓名：
select ename,sal from emp where sal>3000;
查询工资不等于3000的：
select ename,sal from emp where sal<>3000;
select ename,sal from emp where sal!=3000;
查询工资在1100-3000的员工：
select ename,sal from emp where sal>=1100 and sal<=3000;
select ename,sal from emp where sal between 1100 and 3000;
mysql> select ename,sal from emp where sal between 1100 and 3000;
+--------+---------+
| ename | sal |
+--------+---------+
| allen | 1600.00 |
| ward | 1250.00 |
| jones | 2975.00 |
| martin | 1250.00 |
| blake | 2850.00 |
| clark | 2450.00 |
| scott | 3000.00 |
| turner | 1500.00 |
| adams | 1100.00 |
| frod | 3000.00 |
| miller | 1300.00 |
+--------+---------+
11 rows in set (0.00 sec)
between and 使用的时候要左小右大。
between and 也可以使用在字符串：//此时左闭右开
select ename from emp where ename between ‘A’ and ‘C’;
mysql> select ename from emp where ename between 'A' and 'C';
+-------+
| ename |
+-------+
| allen |
| blake |
| adams |
+-------+
3 rows in set (0.00 sec)
找出没有津贴的人：
null不能使用等号: is null 或 is not null;
select ename,sal,comm from emp where comm is null;
select ename,sal,comm from emp where comm is null or comm=0;
mysql> select ename,comm from emp where comm is null or comm=0;
+--------+------+
| ename | comm |
+--------+------+
| smith | NULL |
| jones | NULL |
| blake | NULL |
| clark | NULL |
| scott | NULL |
| king | NULL |
| turner | 0.00 |
| adams | NULL |
| james | NULL |
| frod | NULL |
| miller | NULL |
+--------+------+
11 rows in set (0.00 sec)
找出工作岗位是MANAGER和SALESMAN的员工：
select ename,job from emp where job='MANAGER’or job=‘SALESMAN’;
in等同于or:
select ename,job from emp where job in(‘MANAGER’,‘SALESMAN’);
mysql> select ename,job from emp where job in('manager','salesman');
+--------+----------+
| ename | job |
+--------+----------+
| allen | salesman |
| ward | salesman |
| jones | manager |
| martin | salesman |
| blake | manager |
| clark | manager |
| turner | salesman |
+--------+----------+
7 rows in set (0.00 sec)
and和or联合起来用：找出薪资大于1000的且部门编号是20或30部门的员工.
select ename,sal,deptno from emp where sal>100 and (deptno=20 or deptno=30);
注意：当运算符的优先级不确定的时候加小括号。
mysql> select ename,sal,deptno from emp where sal>100 and(deptno=20 or deptno=30);
+--------+---------+--------+
| ename | sal | deptno |
+--------+---------+--------+
| smith | 800.00 | 20 |
| allen | 1600.00 | 30 |
| ward | 1250.00 | 30 |
| jones | 2975.00 | 20 |
| martin | 1250.00 | 30 |
| blake | 2850.00 | 30 |
| scott | 3000.00 | 20 |
| turner | 1500.00 | 30 |
| adams | 1100.00 | 20 |
| james | 950.00 | 30 |
| frod | 3000.00 | 20 |
+--------+---------+--------+
11 rows in set (0.00 sec)
模糊查询like:(特殊符号 % 和 _)
%代表任意多个字符，_代表任意一个字符。
找出名字中含有O的，
select ename from emp where ename like ‘%O%’;
+-------+
| ename |
+-------+
| jones |
| scott |
| frod |
+-------+
找出第二个字母是A的：
select ename from emp where ename like ‘_A%’;
+--------+
| ename |
+--------+
| ward |
| martin |
| james |
+--------+
找出名字中有下划线的：
select name from t_user name like ‘%_%’;
13.排序(升序、降序)
按照工资升序，找出员工名和薪资：
select
ename,sal
from
emp
order by
sal;
默认是升序，asc表示升序，desc表示降序。
select ename,sal from emp order by sal;//升序
select ename,sal from emp order by sal asc;//升序
select ename,sal from emp order by sal desc;//降序
mysql> select ename,sal from emp order by sal;
+--------+---------+
| ename | sal |
+--------+---------+
| smith | 800.00 |
| james | 950.00 |
| adams | 1100.00 |
| ward | 1250.00 |
| martin | 1250.00 |
| miller | 1300.00 |
| turner | 1500.00 |
| allen | 1600.00 |
| clark | 2450.00 |
| blake | 2850.00 |
| jones | 2975.00 |
| scott | 3000.00 |
| frod | 3000.00 |
| king | 5000.00 |
+--------+---------+
按照工资的降序排列，当工资相同的时侯再按照名字的升序排列：
select ename,sal from emp by sal desc,ename asc;
mysql> select ename,sal from emp order by sal desc,ename asc;
+--------+---------+
| ename | sal |
+--------+---------+
| king | 5000.00 |
| frod | 3000.00 |
| scott | 3000.00 |
| jones | 2975.00 |
| blake | 2850.00 |
| clark | 2450.00 |
| allen | 1600.00 |
| turner | 1500.00 |
| miller | 1300.00 |
| martin | 1250.00 |
| ward | 1250.00 |
| adams | 1100.00 |
| james | 950.00 |
| smith | 800.00 |
+--------+---------+
14 rows in set (0.00 sec)
找出工作岗位是SALESMAN的员工，并且要求按照薪资的降序排列。
select 3
ename.job,sal
from 1
emp
where 2
job=‘SALESMAN’
order by 4
sal desc;
mysql> select ename,job,sal from emp where job='salesman' order by sal desc;
+--------+----------+---------+
| ename | job | sal |
+--------+----------+---------+
| allen | salesman | 1600.00 |
| turner | salesman | 1500.00 |
| ward | salesman | 1250.00 |
| martin | salesman | 1250.00 |
+--------+----------+---------+
4 rows in set (0.00 sec)
14.分组函数：都是对某一组数据进行操作的
count 计数
sum 求和
avg 平均值
max 最大值
min 最小值
求出工资总和
select sum(sal) from emp;
找出最高工资
select max(sal) from emp;
求总人数：
select count(*) from emp;
select count(ename)from emp;
分组函数自动忽略NULL。
count()和count(具体的某个字段)的区别：
count():统计总记录条数。
count(comm)：表示统计comm字段中不为NULL的数据总数量。
mysql> select max(sal),sum(sal),count(*) as '总人数' from emp;
+----------+----------+--------+
| max(sal) | sum(sal) | 总人数 |
+----------+----------+--------+
| 5000.00 | 29025.00 | 14 |
+----------+----------+--------+
1 row in set (0.00 sec)
找出工资高于平均工资的员工：
select ename,sal from emp where sal>avg(sal) ;//无效
原因：SQL语句中，分组函数不可以直接使用在where子语句中去。
因为group by实在where执行之后才会执行的。
select 5
…
from 1
…
where 2
…
group by 3
…
having 4
…
order by 6
…
改进：select ename,sal from emp where sal>(select avg(sal) from emp);
mysql> select ename,sal from emp where sal>(select avg(sal) from emp);
+-------+---------+
| ename | sal |
+-------+---------+
| jones | 2975.00 |
| blake | 2850.00 |
| clark | 2450.00 |
| scott | 3000.00 |
| king | 5000.00 |
| frod | 3000.00 |
+-------+---------+
6 rows in set (0.00 sec)
**15.多行处理函数：输入多行，输出一行。
单行处理函数：输入一行，输出一行。
计算员工的年薪：
select ename,(sal+comm)12 as yearsal from emp;
当null参与运算，运算结果一定是NULL
使用ifnull();
select ename,(sal+ifnull(comm,0))12 as yearsal from emp;
ifnull()空处理函数：
ifnull(可能为null的数据，被当作什么处理)：属于单行处理函数
select ename,ifnull(comm,0) as comm from emp;
mysql> select ename,sal+ifnull(comm,0)*12 as yearsal from emp;
+--------+----------+
| ename | yearsal |
+--------+----------+
| smith | 800.00 |
| allen | 5200.00 |
| ward | 7250.00 |
| jones | 2975.00 |
| martin | 18050.00 |
| blake | 2850.00 |
| clark | 2450.00 |
| scott | 3000.00 |
| king | 5000.00 |
| turner | 1500.00 |
| adams | 1100.00 |
| james | 950.00 |
| frod | 3000.00 |
| miller | 1300.00 |
+--------+----------+
14 rows in set (0.00 sec)
16.group by 和 having
group by:按照某个字段或者某些字段进行分组。
having:having是对分组之后的数据进行再次过滤。
案例：找出每个工作岗位的最高薪资。
select max(sal),job from emp group by job;
mysql> select max(sal),job from emp group by job;
+----------+-----------+
| max(sal) | job |
+----------+-----------+
| 1300.00 | clerk |
| 1600.00 | salesman |
| 2975.00 | manager |
| 3000.00 | analyst |
| 5000.00 | persident |
+----------+-----------+
5 rows in set (0.00 sec)
注意：
1.分组函数一般都会和group by联合使用，这也是为什么它被称为分组函数的原因
并且任何一个分组函数(count sum avg max min)都是在group by语句执行结束之后才会
执行的，当一个sql语句没有group by的话，整张表的数据都会自成一组。
2.当一条语句中有group by的话，select后面只能跟分组函数和参与分组的字段。
案例：找出每个部门不同工作岗位的最高薪资。
select
deptno,job,max(sal)
from
emp
group by
deptno,job;
mysql> select deptno,job,max(sal) from emp group by deptno,job;
+--------+-----------+----------+
| deptno | job | max(sal) |
+--------+-----------+----------+
| 20 | clerk | 1100.00 |
| 30 | salesman | 1600.00 |
| 20 | manager | 2975.00 |
| 30 | manager | 2850.00 |
| 10 | manager | 2450.00 |
| 20 | analyst | 3000.00 |
| 10 | persident | 5000.00 |
| 30 | clerk | 950.00 |
| 10 | clerk | 1300.00 |
+--------+-----------+----------+
9 rows in set (0.00 sec)
找出每个部门的最高薪资,要求显示薪资大于2900的数据
select max(sal),deptno from emp group by deptno having max(sal)>2900;//效率低
select max(sal),deptno from emp where sal>2900 group by deptno;//where
mysql> select deptno,max(sal) from emp group by deptno having max(sal)>2900;
+--------+----------+
| deptno | max(sal) |
+--------+----------+
| 20 | 3000.00 |
| 10 | 5000.00 |
+--------+----------+
2 rows in set (0.00 sec)
mysql> select deptno,max(sal) from emp where sal>2900 group by deptno;
+--------+----------+
| deptno | max(sal) |
+--------+----------+
| 20 | 3000.00 |
| 10 | 5000.00 |
+--------+----------+
找出每个部门的平均薪资，要求显示薪资大于2000的薪资;
select deptno,avg(sal) from emp group by deptno having avg(sal)>2000;
where后面不能使用分组函数，只能使用having使用。
mysql> select deptno,avg(sal) from emp group by deptno having avg(sal)>2000;
+--------+-------------+
| deptno | avg(sal) |
+--------+-------------+
| 20 | 2175.000000 |
| 10 | 2916.666667 |
+--------+-------------+
2 rows in set (0.00 sec)
17.完整的DQL语句的运行顺序
select 5
…
from 1
…
where 2
…
group by 3
…
having 4
…
order by 6
…
以上操作使用的案例表
EMP员工表
DEPT部门表
SALGRADE工资等级表
