MySQL基础篇：SELECT几种子句

文章目录

1、概述
2、where 条件查询
3、group by 分组查询
- 常用的聚合函数
- group by + 聚合函数
4、having筛选
5、order by 排序
6、limit 分页

1、概述

在MySQL的查询中，select主要有5中子句类型，主要包括

where 条件查询
group by 分组查询
having 筛选
order by 排序
limit 分页

准备演示的表结构及示例数据

CREATE TABLE "department" ("did" int(11) NOT NULL AUTO_INCREMENT,"dname" varchar(100) NOT NULL,"description" varchar(200) DEFAULT NULL,"manager_id" int(11) DEFAULT NULL,PRIMARY KEY ("did")
) ENGINE=InnoDB DEFAULT  CHARSET=utf8mb4;CREATE TABLE "employee" ("eid" int(11) NOT NULL AUTO_INCREMENT,"ename" varchar(100) NOT NULL,"gender" char(1) NOT NULL DEFAULT '男',"card_id" char(18) DEFAULT NULL,"‘mid‘" int(11) DEFAULT NULL,"dept_id" int(11) DEFAULT NULL,PRIMARY KEY ("eid"),UNIQUE KEY "card_id" ("card_id"),KEY "dept_id" ("dept_id"),CONSTRAINT "employee_ibfk_1" FOREIGN KEY ("dept_id") REFERENCES "department" ("did")
) ENGINE=InnoDB DEFAULT  CHARSET=utf8mb4;CREATE TABLE "salary" ("eid" int(11) NOT NULL,"basic_salary" decimal(10,2) DEFAULT NULL,"performance_salary" decimal(10,2) DEFAULT NULL,PRIMARY KEY ("eid"),CONSTRAINT "salary_ibfk_1" FOREIGN KEY ("eid") REFERENCES "employee" ("eid")
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;INSERT INTO `department`(dname,description) VALUES ( '研发部', '业务平台研发');
INSERT INTO `department` (dname,description) VALUES ( '市场部', '市场推广');
INSERT INTO `department` (dname,description) VALUES ( '财务部', '财务管理');INSERT INTO `employee` (ename,gender,card_id,‘mid‘,dept_id) VALUES ('张三', '男', '123456789012345678', null, '1');
INSERT INTO `employee` (ename,gender,card_id,‘mid‘,dept_id) VALUES ('李四', '女', '123456789012345665', null, '2');
INSERT INTO `employee` (ename,gender,card_id,‘mid‘,dept_id) VALUES ('赵柳', '男', '123456789012235678', '2', '2');
INSERT INTO `employee` (ename,gender,card_id,‘mid‘,dept_id) VALUES ('王五', '男', '123456789012115678', '1', '1');
INSERT INTO `employee` (ename,gender,card_id,‘mid‘,dept_id) VALUES ('谷雨', '男', '123456789012115978', '1', '1');INSERT INTO `salary` VALUES ('1', '12000.00', '6000.00');
INSERT INTO `salary` VALUES ('2', '9000.00', '8000.00');
INSERT INTO `salary` VALUES ('3', '11000.00', '5500.00');
INSERT INTO `salary` VALUES ('4', '5800.00', '7800.00');

2、where 条件查询

where ：从原表的记录中进行筛选

3、group by 分组查询

在实际的应用中，需要进行一些汇总操作，例如：统计整个公司的人数或者统计每一个部门的人数等。

常用的聚合函数

函数名称	函数说明
AVG([DISTINCT] expr)	返回expr的平均值
COUNT([DISTINCT] expr)	返回expr的非NULL值的数目
MIN([DISTINCT] expr)	返回expr的最小值
MAX([DISTINCT] expr)	返回expr的最大值
SUM([DISTINCT] expr)	返回expr的平均值

#聚合函数
#AVG(【DISTINCT】 expr) 返回 expr 的平均值
mysql> select avg(basic_salary) from salary;
+-------------------+
| avg(basic_salary) |
+-------------------+
| 9450.000000       |
+-------------------+
1 row in set#COUNT（【DISTINCT】 expr）返回 expr 的非 NULL 值的数目
#统计员工总人数
#count(*)统计的是记录数
mysql> select count(*) from employee;
+----------+
| count(*) |
+----------+
|        5 |
+----------+
1 row in setmysql>
#统计员工表的员工所在部门数
#统计的是非NULL值
mysql> select count(dept_id) from employee;
+----------------+
| count(dept_id) |
+----------------+
|              4 |
+----------------+
1 row in set
#统计的是非 NULL 值，并且去重
mysql> select count( distinct dept_id) from employee;
+--------------------------+
| count( distinct dept_id) |
+--------------------------+
|                        2 |
+--------------------------+
1 row in set#MIN(【DISTINCT】 expr)返回 expr 的最小值
#查询最低基本工资值
mysql> select min(basic_salary) from salary;
+-------------------+
| min(basic_salary) |
+-------------------+
| 5800              |
+-------------------+
1 row in set#MAX(【DISTINCT】 expr)返回 expr 的最大值
#查询最高基本工资值
mysql> select max(basic_salary) from salary;
+-------------------+
| max(basic_salary) |
+-------------------+
| 12000             |
+-------------------+
1 row in set#查询最高基本工资与最低基本工资的差值
mysql> select max(basic_salary)-min(basic_salary) from salary;
+-------------------------------------+
| max(basic_salary)-min(basic_salary) |
+-------------------------------------+
| 6200                                |
+-------------------------------------+
1 row in set#SUM（【DISTINCT】 expr）返回 expr 的总和
#查询基本工资总和
mysql> select sum(basic_salary) from salary;
+-------------------+
| sum(basic_salary) |
+-------------------+
| 37800.00          |
+-------------------+
1 row in set

group by + 聚合函数

#group by + 聚合函数
#统计每个部门的人数
mysql> select dept_id, count(*) from employee group by dept_id;
+---------+----------+
| dept_id | count(*) |
+---------+----------+
|       1 |        2 |
|       2 |        2 |
+---------+----------+
2 rows in set#统计每个部门的平均基本工资
mysql> select emp.dept_id,avg(s.basic_salary) from employee emp,salary s where emp.eid=s.eid group by emp.dept_id;
+---------+---------------------+
| dept_id | avg(s.basic_salary) |
+---------+---------------------+
|       1 | 8900.000000         |
|       2 | 10000.000000        |
+---------+---------------------+
2 rows in set
#统计每个部门基本工资最高者
mysql> select emp.dept_id, max(s.basic_salary) from employee emp,salary s where emp.eid=s.eid group by emp.dept_id;
+---------+---------------------+
| dept_id | max(s.basic_salary) |
+---------+---------------------+
|       1 | 12000               |
|       2 | 11000               |
+---------+---------------------+
2 rows in set
#统计每个部门基本工资之和
mysql> select emp.dept_id, sum(s.basic_salary) from employee emp,salary s where emp.eid=s.eid group by emp.dept_id;
+---------+---------------------+
| dept_id | sum(s.basic_salary) |
+---------+---------------------+
|       1 | 17800.00            |
|       2 | 20000.00            |
+---------+---------------------+
2 rows in set

注意事项

1、用count(*)、count(1)，那个好呢？

对于myisam引擎的表来说，没有区别，因为myisam引擎内部有一个计数器在维护着行数
Innodb的表，用count(*) 直接读行数，效率很低，因为Innodb真的要去数一遍所有的数据。

2、关于MySQL的group by的特殊说明

在SELECT列表中所有未包含在组函数中的列都应该是包含在group by 子句中的，也就是说，SELECT列表中最好不要出现group by子句中没有的列。

4、having筛选

having与where类似，可以筛选数据

having与where的不同点

where针对表中的列发挥作用，查询数据；having针对查询结果中的列发挥作用，筛选数据
where后面不能写分组函数；having后面可以使用分组函数
having只用于group by 分组统计语句

# 按照部门统计员工人数，仅显示部门人数少于3人的
mysql> SELECT dept_id,COUNT(*) AS c
FROM employee
WHERE dept_id IS NOT NULL
GROUP BY dept_id
HAVING c <3;
+---------+---+
| dept_id | c |
+---------+---+
|       2 | 2 |
+---------+---+
1 row in set#查询每个部门的平均工资，并且仅显示平均工资高于8000
mysql> select emp.dept_id,avg(s.basic_salary) as avg_salary-> from employee emp,salary s -> where emp.eid=s.eid and dept_id is not null-> group by emp.dept_id-> having avg_salary>8000;
+---------+--------------+
| dept_id | avg_salary   |
+---------+--------------+
|       1 | 8900.000000  |
|       2 | 10000.000000 |
+---------+--------------+
2 rows in setmysql>

5、order by 排序

（1）按一个或多个字段对查询结果进行排序

用法：order by col1,col2,col3,……

说明：先按col1排序，如果col1相同就按照col2排序，以此类推

col1,col2,col3可以是select后面的字段也可以不是

（2）默认是升序，也可以在字段后面加asc显示说明是升序，desc为降序

如果两个字段排序不一样，例如

order by 字段1 asc, 字段2 desc

（3）order by 后面除了跟1个或多个字段，还可以写表达式、函数、别名等

#排序
# 查询员工基本工资，按照基本工资升序排序，如果工资相同，按照eid升序排序
mysql> select employee.eid,basic_salary from employee inner join salary on employee.eid=salary.eid order by basic_salary,eid;
+-----+--------------+
| eid | basic_salary |
+-----+--------------+
|   4 | 5800         |
|   2 | 9000         |
|   3 | 11000        |
|   1 | 12000        |
+-----+--------------+
4 rows in set
#查询员工基本工资，按照基本工资降序排序，如果工资相同，按照eid排列
mysql> select employee.eid,basic_salary from employee inner join salary on employee.eid=salary.eid order by basic_salary desc,eid asc;
+-----+--------------+
| eid | basic_salary |
+-----+--------------+
|   1 | 12000        |
|   3 | 11000        |
|   2 | 9000         |
|   4 | 5800         |
+-----+--------------+
4 rows in setmysql>
#统计每个部门的平均基本工资，并按照平均工资降序排列
mysql> select emp.dept_id,avg(s.basic_salary) from employee emp,salary s where emp.eid=s.eid group by emp.dept_id order by avg(s.basic_salary) desc;
+---------+---------------------+
| dept_id | avg(s.basic_salary) |
+---------+---------------------+
|       2 | 10000.000000        |
|       1 | 8900.000000         |
+---------+---------------------+
2 rows in setmysql>

6、limit 分页

limit m,n

m表示从下标为m的记录开始查询，第一条记录下标为0
n表示取出n条出来，如果从m开始不够n条记录，就有几条取几条
m = (page-1)*n，其中，page是页码，n表示每页显示的条目

如果第一页：limit 0,n

如果第二页：limit n,n

依次类推，得出公式 limit (page-1)*n,n

#分页
#查询员工信息，每页显示 3 条，第二页
mysql> select * from employee limit 3,3;
+-----+-------+--------+--------------------+-------+---------+
| eid | ename | gender | card_id            | ‘mid‘ | dept_id |
+-----+-------+--------+--------------------+-------+---------+
|   4 | 王五  | 男     | 123456789012115678 |     1 |       1 |
|   5 | 谷雨  | 男     | 123456789012115978 |     1 |       1 |
+-----+-------+--------+--------------------+-------+---------+
2 rows in set
#统计每个部门的平均基本工资，并显示前三名
mysql> select emp.dept_id,avg(s.basic_salary) from employee emp,salary s where emp.eid=s.eid group by emp.dept_id order by avg(s.basic_salary) desc limit 0,3;
+---------+---------------------+
| dept_id | avg(s.basic_salary) |
+---------+---------------------+
|       2 | 10000.000000        |
|       1 | 8900.000000         |
+---------+---------------------+
2 rows in setmysql>