查询及排序

查询

1. 简单查询

1.1 查询一个字段

​ select 字段名 from 表名;

注意：

​ select和from都是关键字。
​ 字段名和表名都是标识符。

强调：

​ 对于SQL语句来说，是通用的，
​ 所有的SQL语句以“;”结尾。
​ 另外SQL语句不区分大小写，都行。

例：查询部门名字

mysql> select dname from dept;
	+------------+
	| dname      |
	+------------+
	| ACCOUNTING |
	| RESEARCH   |
	| SALES      |
	| OPERATIONS |
	+------------+
	
mysql> SELECT DNAME FROM DEPT;
	+------------+
	| DNAME      |
	+------------+
	| ACCOUNTING |
	| RESEARCH   |
	| SALES      |
	| OPERATIONS |
	+------------+

1.2 查询两个字段，或者多个字段

​ 使用逗号隔开“,”

例：查询部门编号和部门名

mysql> select deptno,dname from dept;
	+--------+------------+
	| deptno | dname      |
	+--------+------------+
	|     10 | ACCOUNTING |
	|     20 | RESEARCH   |
	|     30 | SALES      |
	|     40 | OPERATIONS |
	+--------+------------+

1.3 查询所有字段

1. 第一种方式：可以把每个字段都写上
   select a,b,c,d,e,f… from tablename;

2. 第二种方式：可以使用 *

   ​ select * from dept;

   例：查询dept表的所有字段

   mysql> select * from dept;
   	+--------+------------+----------+
   	| DEPTNO | DNAME      | LOC      |
   	+--------+------------+----------+
   	|     10 | ACCOUNTING | NEW YORK |
   	|     20 | RESEARCH   | DALLAS   |
   	|     30 | SALES      | CHICAGO  |
   	|     40 | OPERATIONS | BOSTON   |
   	+--------+------------+----------+
   

   这种方式的缺点：
   1、效率低
   2、可读性差。

1.4 给查询的列起别名

​ 使用as关键字起别名。

例：

mysql> select deptno,dname as deptname from dept;
	+--------+------------+
	| deptno | deptname   |
	+--------+------------+
	|     10 | ACCOUNTING |
	|     20 | RESEARCH   |
	|     30 | SALES      |
	|     40 | OPERATIONS |
	+--------+------------+

注意：只是将显示的查询结果列名显示为deptname，原表列名还是叫：dname
记住：select语句是永远都不会进行修改操作的。（因为只负责查询）

1. as关键字可以省略

   mysql> select deptno,dname deptname from dept;
   

2. 假设起别名的时候，别名里面有空格

   select deptno,dname 'dept name' from dept; //加单引号
   select deptno,dname "dept name" from dept; //加双引号
   	+--------+------------+
   	| deptno | dept name  |
   	+--------+------------+
   	|     10 | ACCOUNTING |
   	|     20 | RESEARCH   |
   	|     30 | SALES      |
   	|     40 | OPERATIONS |
   	+--------+------------+
   

   注意：在所有的数据库当中，字符串统一使用单引号括起来，双引号不标准。

1.5 计算

​ 字段可以使用数学表达式

例：计算员工年薪 sal * 12

mysql> select ename,sal*12 from emp;
	+--------+----------+
	| ename  | sal*12   |
	+--------+----------+
	| 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 |
	| FORD   | 36000.00 |
	| MILLER | 15600.00 |
	+--------+----------+

2. 条件查询

2.1 什么是条件查询

​ 查询出来符合条件的的数据

语法格式：

select
	字段1，字段2，字段3 ...
from
	表名
where
	条件;

2.2 查询条件的类型

2.2.1 = 等于

例：查询薪资等于800的员工姓名和编号

mysql> select * from emp where sal=800;
	+-------+-------+-------+------+------------+--------+------+--------+
	| EMPNO | ENAME | JOB   | MGR  | HIREDATE   | SAL    | COMM | DEPTNO |
	+-------+-------+-------+------+------------+--------+------+--------+
	|  7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL |     20 |
	+-------+-------+-------+------+------------+--------+------+--------+

2.2.2 <> 或 != 不等于

例：查询薪资不等于800的员工姓名和编号

select * from emp where sal != 800;
select * from emp where sal <> 800; // 小于号和大于号组成的不等号

2.2.3 < 小于

例：查询薪资小于2000的员工姓名和编号

mysql> select empno,ename,sal from emp where sal < 2000;
	+-------+--------+---------+
	| empno | ename  | sal     |
	+-------+--------+---------+
	|  7369 | SMITH  |  800.00 |
	|  7499 | ALLEN  | 1600.00 |
	|  7521 | WARD   | 1250.00 |
	|  7654 | MARTIN | 1250.00 |
	|  7844 | TURNER | 1500.00 |
	|  7876 | ADAMS  | 1100.00 |
	|  7900 | JAMES  |  950.00 |
	|  7934 | MILLER | 1300.00 |
	+-------+--------+---------+

2.2.4 <= 小于等于

例：查询薪资小于等于3000的员工姓名和编号

select empno,ename,sal from emp where sal <= 3000;

2.2.5 > 大于

例：查询薪资大于3000的员工姓名和编号

select empno,ename,sal from emp where sal > 3000;

2.2.6 >= 大于等于

例：查询薪资大于等于3000的员工姓名和编号

select empno,ename,sal from emp where sal >= 3000;

2.2.7 between … and …. 两个值之间, 等同于 >= and <=

例：查询薪资在2450和3000之间的员工信息？包括2450和3000

1. 第一种方式：>= and <= （and是并且的意思。）

   mysql> select empno,ename,sal from emp where sal>=2450 and sal<=3000;
   	+-------+-------+---------+
   	| empno | ename | sal     |
   	+-------+-------+---------+
   	|  7566 | JONES | 2975.00 |
   	|  7698 | BLAKE | 2850.00 |
   	|  7782 | CLARK | 2450.00 |
   	|  7788 | SCOTT | 3000.00 |
   	|  7902 | FORD  | 3000.00 |
   	+-------+-------+---------+
   

2. 第二种方式：between … and …

   mysql> select empno,ename,sal
       -> from
       -> emp
       -> where
       -> sal 
       -> between 2450 and 3000;
   	+-------+-------+---------+
   	| empno | ename | sal     |
   	+-------+-------+---------+
   	|  7566 | JONES | 2975.00 |
   	|  7698 | BLAKE | 2850.00 |
   	|  7782 | CLARK | 2450.00 |
   	|  7788 | SCOTT | 3000.00 |
   	|  7902 | FORD  | 3000.00 |
   	+-------+-------+---------+
   

   注意：
   使用between and的时候，必须遵循左小右大。
   between and是闭区间，包括两端的值。

2.2.8 is null 为 null（is not null 不为空）

例：查询哪些员工的津贴/补助为null

mysql> select empno,ename,sal,comm from emp where comm is null;
	+-------+--------+---------+------+
	| empno | ename  | sal     | comm |
	+-------+--------+---------+------+
	|  7369 | SMITH  |  800.00 | NULL |
	|  7566 | JONES  | 2975.00 | NULL |
	|  7698 | BLAKE  | 2850.00 | NULL |
	|  7782 | CLARK  | 2450.00 | NULL |
	|  7788 | SCOTT  | 3000.00 | NULL |
	|  7839 | KING   | 5000.00 | NULL |
	|  7876 | ADAMS  | 1100.00 | NULL |
	|  7900 | JAMES  |  950.00 | NULL |
	|  7902 | FORD   | 3000.00 | NULL |
	|  7934 | MILLER | 1300.00 | NULL |
	+-------+--------+---------+------+

错误写法：

mysql> select empno,ename,sal,comm from emp where comm = null;
	Empty set (0.00 sec)

注意：

在数据库当中null不能使用等号进行衡量。需要使用is null因为数据库中的null代表什么也没有，它不是一个值，所以不能使用等号衡量。

例：查询哪些员工的津贴/补助不为null

select empno,ename,sal,comm from emp where comm is not null;
	+-------+--------+---------+---------+
	| empno | ename  | sal     | comm    |
	+-------+--------+---------+---------+
	|  7499 | ALLEN  | 1600.00 |  300.00 |
	|  7521 | WARD   | 1250.00 |  500.00 |
	|  7654 | MARTIN | 1250.00 | 1400.00 |
	|  7844 | TURNER | 1500.00 |    0.00 |
	+-------+--------+---------+---------+

2.2.9 and 并且

例：查询工作岗位是MANAGER并且工资大于2500的员工信息

mysql> select 
    -> empno,ename,job,sal
    -> from
    -> emp
    -> where
    -> job='MANAGER' and sal>2500;
	+-------+-------+---------+---------+
	| empno | ename | job     | sal     |
	+-------+-------+---------+---------+
	|  7566 | JONES | MANAGER | 2975.00 |
	|  7698 | BLAKE | MANAGER | 2850.00 |
	+-------+-------+---------+---------+

2.2.10 or 或者

例：查询工作岗位是MANAGER和SALESMAN的员工

mysql> select 
    -> empno,ename,job
    -> from 
    -> emp
    -> where
    -> job='MANANGER' or job='SALESMAN';
	+-------+--------+----------+
	| empno | ename  | job      |
	+-------+--------+----------+
	|  7499 | ALLEN  | SALESMAN |
	|  7521 | WARD   | SALESMAN |
	|  7654 | MARTIN | SALESMAN |
	|  7844 | TURNER | SALESMAN |
	+-------+--------+----------+

and和or同时出现的优先级问题

例：查询工资大于2500，并且部门编号为10或20部门的员工

mysql> select * from emp where sal>2500 and deptno=10 or deptno=20;
	+-------+-------+-----------+------+------------+---------+------+--------+
	| EMPNO | ENAME | JOB       | MGR  | HIREDATE   | SAL     | COMM | DEPTNO |
	+-------+-------+-----------+------+------------+---------+------+--------+
	|  7369 | SMITH | CLERK     | 7902 | 1980-12-17 |  800.00 | NULL |     20 |
	|  7566 | JONES | MANAGER   | 7839 | 1981-04-02 | 2975.00 | NULL |     20 |
	|  7788 | SCOTT | ANALYST   | 7566 | 1987-04-19 | 3000.00 | NULL |     20 |
	|  7839 | KING  | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL |     10 |
	|  7876 | ADAMS | CLERK     | 7788 | 1987-05-23 | 1100.00 | NULL |     20 |
	|  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 | 3000.00 | NULL |     20 |
	+-------+-------+-----------+------+------------+---------+------+--------+

分析以上语句：

​ and优先级比or高。
​ 以上语句会先执行and，然后执行or。

以上语句的含义：找出工资大于2500并且部门编号为10的员工，或者20部门所有员工找出来。

and和or同时出现，and优先级较高。如果想让or先执行，需要加“小括号”

select 
	*
from
	emp
where
	sal > 2500 and (deptno = 10 or deptno = 20);	

2.2.11 in 包含

​ 相当于多个 or （not in 不在这个范围中）

例：查询工作岗位是MANAGER和SALESMAN的员工

select empno,ename,job from emp where job = 'MANAGER' or job = 'SALESMAN';
select empno,ename,job from emp where job in('MANAGER', 'SALESMAN');
	+-------+--------+----------+
	| empno | ename  | job      |
	+-------+--------+----------+
	|  7499 | ALLEN  | SALESMAN |
	|  7521 | WARD   | SALESMAN |
	|  7566 | JONES  | MANAGER  |
	|  7654 | MARTIN | SALESMAN |
	|  7698 | BLAKE  | MANAGER  |
	|  7782 | CLARK  | MANAGER  |
	|  7844 | TURNER | SALESMAN |
	+-------+--------+----------+

注意：in不是一个区间。in后面跟的是具体的值。

not in 表示不在这几个值当中的数据。

select ename,sal from emp where sal not in(800, 5000, 3000);
	+--------+---------+
	| ename  | sal     |
	+--------+---------+
	| ALLEN  | 1600.00 |
	| WARD   | 1250.00 |
	| JONES  | 2975.00 |
	| MARTIN | 1250.00 |
	| BLAKE  | 2850.00 |
	| CLARK  | 2450.00 |
	| TURNER | 1500.00 |
	| ADAMS  | 1100.00 |
	| JAMES  |  950.00 |
	| MILLER | 1300.00 |
	+--------+---------+

not 可以取非，主要用在 is 或 in 中
is null
is not null
in
not in

2.2.12 like

​ 称为模糊查询，支持%或下划线匹配

​ %匹配任意多个字符

​ 下划线：任意一个字符。

​ （%是一个特殊的符号，_ 也是一个特殊符号）

例：找出名字中含有O的

mysql> select ename from emp where ename like '%O%';
	+-------+
	| ename |
	+-------+
	| JONES |
	| SCOTT |
	| FORD  |
	+-------+

例：找出名字以T结尾的

mysql> select ename from emp where ename like '%T';
	+-------+
	| ename |
	+-------+
	| SCOTT |
	+-------+

例：找出名字以K开始的

mysql> select ename from emp where ename like 'K%';
	+-------+
	| ename |
	+-------+
	| KING  |
	+-------+

例：找出第二个字每是A的

mysql> select ename from emp where ename like '_A%';
	+--------+
	| ename  |
	+--------+
	| WARD   |
	| MARTIN |
	| JAMES  |
	+--------+

例：找出第三个字母是R的

mysql> select ename from emp where ename like '__R%';
	+--------+
	| ename  |
	+--------+
	| WARD   |
	| MARTIN |
	| TURNER |
	| FORD   |
	+--------+

例：找出名字中有“_”的

mysql> select name from t_student where name like '%\_%'; // \转义字符。
	+----------+
	| name     |
	+----------+
	| jack_son |
	+----------+

3. 排序

3.1 排序

​ order by 默认是升序

例：查询所有员工薪资，排序

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 |
	| FORD   | 3000.00 |
	| KING   | 5000.00 |
	+--------+---------+

3.2 降序

指定降序：desc

mysql> select
    -> ename,sal
    -> from
    -> emp
    -> order by
    -> sal desc;
	+--------+---------+
	| ename  | sal     |
	+--------+---------+
	| KING   | 5000.00 |
	| SCOTT  | 3000.00 |
	| FORD   | 3000.00 |
	| JONES  | 2975.00 |
	| BLAKE  | 2850.00 |
	| CLARK  | 2450.00 |
	| ALLEN  | 1600.00 |
	| TURNER | 1500.00 |
	| MILLER | 1300.00 |
	| WARD   | 1250.00 |
	| MARTIN | 1250.00 |
	| ADAMS  | 1100.00 |
	| JAMES  |  950.00 |
	| SMITH  |  800.00 |
	+--------+---------+

指定升序：asc

mysql> select
    -> ename,sal
    -> from
    -> emp
    -> order by
    -> sal asc;
	+--------+---------+
	| 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 |
	| FORD   | 3000.00 |
	| KING   | 5000.00 |
	+--------+---------+

3.3 两个字段或多个字段排序

例：查询员工名字和薪资，要求按照薪资升序，如果薪资一样的话，再按照名字升序排列。

mysql> select
    -> ename,sal
    -> from
    -> emp
    -> order by
    -> sal asc,ename asc;  // sal在前，起主导，只有sal相等的时候，才会考虑启用ename排序。
	+--------+---------+
	| ename  | sal     |
	+--------+---------+
	| SMITH  |  800.00 |
	| JAMES  |  950.00 |
	| ADAMS  | 1100.00 |
	| MARTIN | 1250.00 |
	| WARD   | 1250.00 |
	| MILLER | 1300.00 |
	| TURNER | 1500.00 |
	| ALLEN  | 1600.00 |
	| CLARK  | 2450.00 |
	| BLAKE  | 2850.00 |
	| JONES  | 2975.00 |
	| FORD   | 3000.00 |
	| SCOTT  | 3000.00 |
	| KING   | 5000.00 |
	+--------+---------+

3.4 （了解）根据字段的位置也可以排序

例：按照查询结果的第2列sal排序。

select ename,sal from emp order by 2; // 2表示第二列。第二列是sal

4. 综合案例

例：找出工资在1250到3000之间的员工信息，要求按照薪资降序排列

mysql> select
    -> ename,sal
    -> from
    -> emp
    -> where
    -> sal between 1250 and 3000
    -> order by
    -> sal desc;
	+--------+---------+
	| ename  | sal     |
	+--------+---------+
	| SCOTT  | 3000.00 |
	| FORD   | 3000.00 |
	| JONES  | 2975.00 |
	| BLAKE  | 2850.00 |
	| CLARK  | 2450.00 |
	| ALLEN  | 1600.00 |
	| TURNER | 1500.00 |
	| MILLER | 1300.00 |
	| WARD   | 1250.00 |
	| MARTIN | 1250.00 |
	+--------+---------+

关键字顺序不能变：

select
	...
from
	...
where
	...
order by
	...

以上语句的执行顺序必须掌握：
第一步：from
第二步：where
第三步：select
第四步：order by（排序总是在最后执行！）