MYSQL学习笔记(二)：基本的SELECT语句使用(基本、条件、聚合函数查询)

前言：

学习和使用数据库可以说是程序员必须具备能力，这里将更新关于MYSQL的使用讲解，大概应该会更新30篇+，涵盖入门、进阶、高级(一些原理分析);
这一篇是讲解SELECT语句使用，包括基本、条件、聚合函数查询，并且结合案例进行讲解；
虽然MYSQL命令很多，但是自己去多敲一点，到后面忘记了，查一下就可以回忆起来使用了；
这一系列也是本人学习MYSQL做的笔记，也是为了方便后面忘记查询；
参考资料：尚硅谷、黑马、csdn和知乎博客；
欢迎收藏 + 关注，本人将会持续更新。

文章目录

课程使用数据文件: 后台私信获取。

基本查询

MySQL 数据库使用SELECT语句来查询数据。

1. 查询多个字段

以下为在MySQL数据库中查询数据通用的 SELECT 语法：

SELECT 字段名,字段名... FROM 表名;
SELECT * FROM 表名;					#查询所有字段

查询语句中可以使用一个或者多个表，表之间使用逗号(,)分割；
SELECT 命令可以读取一条或者多条记录。
你可以使用星号（*）来代替其他字段，SELECT语句会返回表的所有字段数据
注意：星号少用

2.设置别名

SELECT 字段1 [AS 别名],字段2 [AS 别名]... FROM 表名;

3.去除重复记录

SELECT DISTINCT 字段列表 FROM 表名;

4.四则运算查询

运算符	描述
+	加法
-	减法
*	乘法
/（DIV）	除法
%（MOD）	求余

查询需求

查询指定字段 ename，job，sal的数据

select ename,job,sal from emp;

/*
mysql> select ename, job, sal from emp;
+--------+-----------+------+
| ename  | job       | sal  |
+--------+-----------+------+
| SMITH  | CLERK     |  800 |
| ALLEN  | SALESMAN  | 1600 |
| WARD   | SALESMAN  | 1250 |
| JONES  | MANAGER   | 2975 |
| MARTIN | SALESMAN  | 1250 |
| BLAKE  | MANAGER   | 2850 |
| CLARK  | MANAGER   | 2450 |
| SCOTT  | ANALYST   | 3000 |
| KING   | PRESIDENT | 5000 |
| TURNER | SALESMAN  | 1500 |
| ADAMS  | CLERK     | 1100 |
| JAMES  | CLERK     |  950 |
| FORD   | ANALYST   | 3000 |
| MILLER | CLERK     | 1300 |
+--------+-----------+------+
*/

查询所有字段

# 方法一，将所有字段名写出
select empno,ename,job,mgr,hiredate,sal,comm,deptno from emp;

# 方法二：select * from 表名;  注意：少用*
select * from emp;			#注意：不直观 影响效率 遵循开发规范最好不要写* 吧所有字段写出了，一目了然

/*
mysql> select * from emp;
+-------+--------+-----------+------+------------+------+------+--------+
| empno | ename  | job       | mgr  | hiredate   | sal  | comm | deptno |
+-------+--------+-----------+------+------------+------+------+--------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800 | NULL |     20 |
|  7499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 | 1600 |  300 |     30 |
|  7521 | WARD   | SALESMAN  | 7698 | 1981-02-22 | 1250 |  500 |     30 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975 | NULL |     20 |
|  7654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 | 1250 | 1400 |     30 |
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850 | NULL |     30 |
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450 | NULL |     10 |
|  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000 | NULL |     20 |
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000 | NULL |     10 |
|  7844 | TURNER | SALESMAN  | 7698 | 1981-09-08 | 1500 |    0 |     30 |
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100 | NULL |     20 |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950 | NULL |     30 |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000 | NULL |     20 |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300 | NULL |     10 |
+-------+--------+-----------+------+------------+------+------+--------+
14 rows in set (0.00 sec)
*/

查询所有员工的职位，并起别名

# 方法一：在字段名后面加上 as + 别名
select job as '工作' from emp;

# 方法二：在字段名后直接加上别名
select job '工作' from emp;		#as可以省略

/*
mysql> select job as '工作' from emp;
+-----------+
| 工作      |
+-----------+
| CLERK     |
| SALESMAN  |
| SALESMAN  |
| MANAGER   |
| SALESMAN  |
| MANAGER   |
| MANAGER   |
| ANALYST   |
| PRESIDENT |
| SALESMAN  |
| CLERK     |
| CLERK     |
| ANALYST   |
| CLERK     |
+-----------+
14 rows in set (0.00 sec)
*/

查询员工的的职位有哪些(不要重复)

# 去重，关键字：DISTINCT，只需要在最前面加一个即可
SELECT DISTINCT job FROM emp;

/*
mysql> select distinct job from emp;
+-----------+
| job       |
+-----------+
| CLERK     |
| SALESMAN  |
| MANAGER   |
| ANALYST   |
| PRESIDENT |
+-----------+
5 rows in set (0.00 sec)
*/

查询员工年薪即 sal* 12

select ename, sal, sal*10 '年终奖' from emp;
 
 /*
 mysql> select ename, sal, sal*10 '年终奖' from emp;
+--------+------+--------+
| ename  | sal  | 年终奖 |
+--------+------+--------+
| SMITH  |  800 |   8000 |
| ALLEN  | 1600 |  16000 |
| WARD   | 1250 |  12500 |
| JONES  | 2975 |  29750 |
| MARTIN | 1250 |  12500 |
| BLAKE  | 2850 |  28500 |
| CLARK  | 2450 |  24500 |
| SCOTT  | 3000 |  30000 |
| KING   | 5000 |  50000 |
| TURNER | 1500 |  15000 |
| ADAMS  | 1100 |  11000 |
| JAMES  |  950 |   9500 |
| FORD   | 3000 |  30000 |
| MILLER | 1300 |  13000 |
+--------+------+--------+
14 rows in set (0.00 sec)
 */

条件查询

我们知道从 MySQL 表中使用 SELECT 语句来查询数据，如需有条件地从表中选取数据，可将 WHERE 子句添加到 SELECT 语句中。

语法

SELECT 字段名 FROM 表名 WHERE 条件;

运算符表

关系运算符	功能
>	大于
>=	大于等于
<	小于
<=	小于等于
=	等于
<> 或 !=	不等于

逻辑运算符	功能
AND 或 &&	并且（多个条件同时成立）
OR 或 \|\|	或者（多个条件任意成立一个）
NOT 或！	非，不是

其他	功能
BETWEEN…AND…	在某个范围之间（含最小、最大值）
IN(…)	在in之后的列表中的值，多选一
LIKE	模糊匹配（_匹配单个字符，%匹配任意个字符）
IS [NOT] NULL	是 NULL
_	通配符：占位符
%	通配符：占位，如%S

查询列表里面的元素如果遇到冲突，要加 ' '，

查询需求(案例)

结合运算符

查询工资等于3000的员工

SELECT * FROM emp WHERE sal=3000;

/*
mysql> select * from emp where sal=3000;
+-------+-------+---------+------+------------+------+------+--------+
| empno | ename | job     | mgr  | hiredate   | sal  | comm | deptno |
+-------+-------+---------+------+------------+------+------+--------+
|  7788 | SCOTT | ANALYST | 7566 | 1987-04-19 | 3000 | NULL |     20 |
|  7902 | FORD  | ANALYST | 7566 | 1981-12-03 | 3000 | NULL |     20 |
+-------+-------+---------+------+------------+------+------+--------+
2 rows in set (0.00 sec)
*/

查询工资小于1000的员工

SELECT * FROM emp WHERE sal<1000;   

/*
mysql> select * from emp where sal < 1000;
+-------+-------+-------+------+------------+------+------+--------+
| empno | ename | job   | mgr  | hiredate   | sal  | comm | deptno |
+-------+-------+-------+------+------------+------+------+--------+
|  7369 | SMITH | CLERK | 7902 | 1980-12-17 |  800 | NULL |     20 |
|  7900 | JAMES | CLERK | 7698 | 1981-12-03 |  950 | NULL |     30 |
+-------+-------+-------+------+------------+------+------+--------+
2 rows in set (0.00 sec)
*/

查询工资小于等于1000的员工

SELECT * FROM emp WHERE sal<=1000;

/*
mysql> select * from emp where sal <= 1000;
+-------+-------+-------+------+------------+------+------+--------+
| empno | ename | job   | mgr  | hiredate   | sal  | comm | deptno |
+-------+-------+-------+------+------------+------+------+--------+
|  7369 | SMITH | CLERK | 7902 | 1980-12-17 |  800 | NULL |     20 |
|  7900 | JAMES | CLERK | 7698 | 1981-12-03 |  950 | NULL |     30 |
+-------+-------+-------+------+------------+------+------+--------+
2 rows in set (0.00 sec)

*/

结合空/非空运算符

查询没有奖金的员工

SELECT * FROM emp WHERE comm IS NULL;

/*
mysql> select * from emp where comm is null;
+-------+--------+-----------+------+------------+------+------+--------+
| empno | ename  | job       | mgr  | hiredate   | sal  | comm | deptno |
+-------+--------+-----------+------+------------+------+------+--------+
|  7369 | SMITH  | CLERK     | 7902 | 1980-12-17 |  800 | NULL |     20 |
|  7566 | JONES  | MANAGER   | 7839 | 1981-04-02 | 2975 | NULL |     20 |
|  7698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 | 2850 | NULL |     30 |
|  7782 | CLARK  | MANAGER   | 7839 | 1981-06-09 | 2450 | NULL |     10 |
|  7788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 | 3000 | NULL |     20 |
|  7839 | KING   | PRESIDENT | NULL | 1981-11-17 | 5000 | NULL |     10 |
|  7876 | ADAMS  | CLERK     | 7788 | 1987-05-23 | 1100 | NULL |     20 |
|  7900 | JAMES  | CLERK     | 7698 | 1981-12-03 |  950 | NULL |     30 |
|  7902 | FORD   | ANALYST   | 7566 | 1981-12-03 | 3000 | NULL |     20 |
|  7934 | MILLER | CLERK     | 7782 | 1982-01-23 | 1300 | NULL |     10 |
+-------+--------+-----------+------+------------+------+------+--------+
10 rows in set (0.00 sec)
*/

查询有奖金的员工

# 语法 is not
SELECT * FROM emp WHERE comm IS NOT NULL;

/*
mysql> select * from emp where comm is not null;
+-------+--------+----------+------+------------+------+------+--------+
| empno | ename  | job      | mgr  | hiredate   | sal  | comm | deptno |
+-------+--------+----------+------+------------+------+------+--------+
|  7499 | ALLEN  | SALESMAN | 7698 | 1981-02-20 | 1600 |  300 |     30 |
|  7521 | WARD   | SALESMAN | 7698 | 1981-02-22 | 1250 |  500 |     30 |
|  7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250 | 1400 |     30 |
|  7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500 |    0 |     30 |
+-------+--------+----------+------+------------+------+------+--------+
4 rows in set (0.00 sec)

*/

结合逻辑运算符

查询工资在1200到1800之间的员工(包含1200和1800岁)

# 并且三种写法：
-- &&
-- and
-- between(用于两个条件的并且)
SELECT * FROM emp WHERE sal>=1200 && sal<=1800;
SELECT * FROM emp WHERE sal>=1200 AND sal<=1800;
SELECT * FROM emp WHERE sal BETWEEN 1200 AND 1800;

/*
mysql> select * from emp where sal>=1200 and sal <= 1800;
+-------+--------+----------+------+------------+------+------+--------+
| empno | ename  | job      | mgr  | hiredate   | sal  | comm | deptno |
+-------+--------+----------+------+------------+------+------+--------+
|  7499 | ALLEN  | SALESMAN | 7698 | 1981-02-20 | 1600 |  300 |     30 |
|  7521 | WARD   | SALESMAN | 7698 | 1981-02-22 | 1250 |  500 |     30 |
|  7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250 | 1400 |     30 |
|  7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500 |    0 |     30 |
|  7934 | MILLER | CLERK    | 7782 | 1982-01-23 | 1300 | NULL |     10 |
+-------+--------+----------+------+------------+------+------+--------+
5 rows in set (0.00 sec)
*/

查询职位为推销员，且工资小于1500的员工

SELECT * FROM emp WHERE job='salesman' AND sal < 1500;

/*
mysql> SELECT * FROM emp WHERE job='salesman' AND sal < 1500;
+-------+--------+----------+------+------------+------+------+--------+
| empno | ename  | job      | mgr  | hiredate   | sal  | comm | deptno |
+-------+--------+----------+------+------------+------+------+--------+
|  7521 | WARD   | SALESMAN | 7698 | 1981-02-22 | 1250 |  500 |     30 |
|  7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250 | 1400 |     30 |
+-------+--------+----------+------+------------+------+------+--------+
2 rows in set (0.00 sec)
*/

查询工资为800 或 3000 或 5000的员工

# 或：
-- or
-- in(…………)
SELECT * FROM emp WHERE sal=800 OR sal=3000 OR sal=5000;
SELECT * FROM emp WHERE sal IN(800,3000,5000);

/*
mysql> select * from emp where sal=800 or sal=3000 or sal=5000;
+-------+-------+-----------+------+------------+------+------+--------+
| empno | ename | job       | mgr  | hiredate   | sal  | comm | deptno |
+-------+-------+-----------+------+------------+------+------+--------+
|  7369 | SMITH | CLERK     | 7902 | 1980-12-17 |  800 | NULL |     20 |
|  7788 | SCOTT | ANALYST   | 7566 | 1987-04-19 | 3000 | NULL |     20 |
|  7839 | KING  | PRESIDENT | NULL | 1981-11-17 | 5000 | NULL |     10 |
|  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 | 3000 | NULL |     20 |
+-------+-------+-----------+------+------------+------+------+--------+
4 rows in set (0.00 sec)
*/

正则表达式

这个现用现查即可。

查询姓名为四个字的员工，

SELECT * FROM emp WHERE ename LIKE '____';

/*
mysql> select * from emp where ename like '____';
+-------+-------+-----------+------+------------+------+------+--------+
| empno | ename | job       | mgr  | hiredate   | sal  | comm | deptno |
+-------+-------+-----------+------+------------+------+------+--------+
|  7521 | WARD  | SALESMAN  | 7698 | 1981-02-22 | 1250 |  500 |     30 |
|  7839 | KING  | PRESIDENT | NULL | 1981-11-17 | 5000 | NULL |     10 |
|  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 | 3000 | NULL |     20 |
+-------+-------+-----------+------+------------+------+------+--------+
3 rows in set (0.00 sec)
*/

查询姓名最后一位是S的员工

SELECT * FROM emp WHERE ename LIKE '%S';

/*
mysql> select * from emp where ename like '%s';
+-------+-------+---------+------+------------+------+------+--------+
| empno | ename | job     | mgr  | hiredate   | sal  | comm | deptno |
+-------+-------+---------+------+------------+------+------+--------+
|  7566 | JONES | MANAGER | 7839 | 1981-04-02 | 2975 | NULL |     20 |
|  7876 | ADAMS | CLERK   | 7788 | 1987-05-23 | 1100 | NULL |     20 |
|  7900 | JAMES | CLERK   | 7698 | 1981-12-03 |  950 | NULL |     30 |
+-------+-------+---------+------+------------+------+------+--------+
3 rows in set (0.00 sec)
*/

聚合函数(统计函数)

SQL 允许对表中的数据进行计算，将一列数据作为一个整体，进行纵向计算。

函数名	作用
MAX(column)	返回某列的最低值（没有则返回NULL）
MIN(column)	返回某列的最高值（没有则返回NULL）
COUNT(column)	返回某列的行数（不包括 NULL 值）
COUNT(*)	返回被选列行数（包括NULL）
SUM(column)	求和
AVG(column)	求平均值

注意：

其中 COUNT 函数可用于任何数据类型 (因为它只是计数)
而 SUM 、AVG 函数都只能对数值类型做计算
MAX 和 MIN 可用于数值、字符串或是日期时间数据类型

统计该企业员工数量

select count(ename) 人数 from emp;                                                
/*
+------+                                                                                                                
| 人数 |                                                                                                                
+------+                                                                                                                
|   14 |                                                                                                                
+------+                                                                                                                
1 row in set (0.01 sec)     
*/

统计该企业员工的平均工资

select avg(sal) '平均工资' from emp;                                             /*                    
+-----------+                                                                                                           
| 平均工资  |                                                                                                           
+-----------+                                                                                                           
| 2073.2143 |                                                                                                           
+-----------+                                                                                                           
1 row in set (0.00 sec)   
*/

查询该企业员工的最高工资

select max(sal) '最高工资' from emp;
/*
+----------+
| 最高工资 |
+----------+
|     5000 |
+----------+
1 row in set (0.00 sec)
*/

查询该企业员工的最低工资

select min(sal) '最低工资' from emp;
/*
+----------+
| 最低工资 |
+----------+
|      800 |
+----------+
1 row in set (0.00 sec)
*/

计算所有销售的工资之和

select sum(sal) '总工资' from emp;
/*
+--------+
| 总工资 |
+--------+
|  29025 |
+--------+
1 row in set (0.00 sec)
*/

MYSQL学习笔记(二)：基本的SELECT语句使用(基本、条件、聚合函数查询)

文章目录

基本查询

查询需求

条件查询

运算符表

查询需求(案例)

结合运算符

结合空/非空运算符

正则表达式

聚合函数(统计函数)

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