【MySQL】MySQL表的增删查改以及聚合函数/group by句子的使用

文章目录

一、创建--Create
- 1.单行数据 + 全列插入
- 2.多行数据 + 指定列插入
- 3.插入否则更新
- 4.替换 -- replace
二、读取--Retrieve
- 1.SELECT列
- - 1.1全列查询
  - 1.2指定列查询
  - 1.3查询字段为表达式
  - 1.4为查询结果指定别名
  - 1.5 结果去重 -- distinct
- 2.WHERE 条件
- 3.结果排序
- 4.筛选分页结果
三、更新--Update
四、删除--Delete
- 1.删除数据
- 2.截断表
五、插入查询结果
六、聚合函数
七、group by子句的使用

一、创建–Create

语法：

INSERT [INTO] table_name[(column [, column] ...)]VALUES (value_list) [, (value_list)] ...
value_list: value, [, value] ...

1.单行数据 + 全列插入

-- 创建一张学生表
CREATE TABLE students (id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,sn INT NOT NULL UNIQUE COMMENT '学号',name VARCHAR(20) NOT NULL,qq VARCHAR(20)
);

插入两条记录，value_list 数量必须和定义表的列的数量及顺序一致

注意，这里在插入的时候，也可以不用指定id(当然，那时候就需要明确插入数据到那些列了)，那么mysql会使用默认的值进行自增。

INSERT INTO students VALUES (100, 10000, '唐三藏', NULL);
INSERT INTO students VALUES (101, 10001, '孙悟空', '11111');

查看插入结果
SELECT * FROM students;
+-----+-------+-----------+-------+
| id | sn | name | qq |
+-----+-------+-----------+-------+
| 100 | 10000 | 唐三藏 | NULL |
| 101 | 10001 | 孙悟空 | 11111 |
+-----+-------+-----------+-------+
2 rows in set (0.00 sec)

2.多行数据 + 指定列插入

插入两条记录，value_list 数量必须和指定列数量及顺序一致

INSERT INTO students (id, sn, name) VALUES
(102, 20001, '曹孟德'),
(103, 20002, '孙仲谋');

-- 查看插入结果
SELECT * FROM students;
+-----+-------+-----------+-------+
| id | sn | name | qq |
+-----+-------+-----------+-------+
| 100 | 10000 | 唐三藏 | NULL |
| 101 | 10001 | 孙悟空 | 11111 |
| 102 | 20001 | 曹孟德 | NULL |
| 103 | 20002 | 孙仲谋 | NULL |
+-----+-------+-----------+-------+
4 rows in set (0.00 sec)

3.插入否则更新

由于主键或者 唯一键 对应的值已经存在而导致插入失败

-- 主键冲突
INSERT INTO students (id, sn, name) VALUES (100, 10010, '唐大师');
ERROR 1062 (23000): Duplicate entry '100' for key 'PRIMARY'-- 唯一键冲突
INSERT INTO students (sn, name) VALUES (20001, '曹阿瞒');
ERROR 1062 (23000): Duplicate entry '20001' for key 'sn'

可以选择性的进行同步更新操作语法

INSERT ... ON DUPLICATE KEY UPDATEcolumn = value [, column = value] ...

INSERT INTO students (id, sn, name) VALUES (100, 10010, '唐大师')
ON DUPLICATE KEY UPDATE sn = 10010, name = '唐大师';
Query OK, 2 rows affected (0.47 sec)

– 0 row affected: 表中有冲突数据，但冲突数据的值和 update 的值相等

– 1 row affected: 表中没有冲突数据，数据被插入

– 2 row affected: 表中有冲突数据，并且数据已经被更新

通过 MySQL 函数获取受到影响的数据行数

SELECT ROW_COUNT();
+-------------+
| ROW_COUNT() |
+-------------+
| 2 |
+-------------+
1 row in set (0.00 sec)
-- ON DUPLICATE KEY 当发生重复key的时候

4.替换 – replace

– 主键或者唯一键没有冲突，则直接插入；

– 主键或者唯一键如果冲突，则删除后再插入

REPLACE INTO students (sn, name) VALUES (20001, '曹阿瞒');

– 1 row affected: 表中没有冲突数据，数据被插入

– 2 row affected: 表中有冲突数据，删除后重新插入

二、读取–Retrieve

语法：

SELECT[DISTINCT] {* | {column [, column] ...}[FROM table_name][WHERE ...][ORDER BY column [ASC | DESC], ...]LIMIT ...

案例：

-- 创建表结构
CREATE TABLE exam_result (id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,name VARCHAR(20) NOT NULL COMMENT '同学姓名',chinese float DEFAULT 0.0 COMMENT '语文成绩',math float DEFAULT 0.0 COMMENT '数学成绩',english float DEFAULT 0.0 COMMENT '英语成绩'
);
-- 插入测试数据
INSERT INTO exam_result (name, chinese, math, english) VALUES('唐三藏', 67, 98, 56),('孙悟空', 87, 78, 77),('猪悟能', 88, 98, 90),('曹孟德', 82, 84, 67),('刘玄德', 55, 85, 45),('孙权', 70, 73, 78),('宋公明', 75, 65, 30);
Query OK, 7 rows affected (0.00 sec)
Records: 7 Duplicates: 0 Warnings: 0

1.SELECT列

1.1全列查询

– 通常情况下不建议使用 * 进行全列查询

– 1. 查询的列越多，意味着需要传输的数据量越大；

– 2. 可能会影响到索引的使用。

SELECT * FROM exam_result;

SELECT * FROM exam_result;
+----+-----------+-------+--------+--------+
| id | name | chinese | math | english |
+----+-----------+-------+--------+--------+
| 1 | 唐三藏 | 67 | 98 | 56 |
| 2 | 孙悟空 | 87 | 78 | 77 |
| 3 | 猪悟能 | 88 | 98 | 90 |
| 4 | 曹孟德 | 82 | 84 | 67 |
| 5 | 刘玄德 | 55 | 85 | 45 |
| 6 | 孙权 | 70 | 73 | 78 |
| 7 | 宋公明 | 75 | 65 | 30 |
+----+-----------+-------+--------+--------+
7 rows in set (0.00 sec)

1.2指定列查询

– 指定列的顺序不需要按定义表的顺序来

SELECT id, name, english FROM exam_result;

SELECT id, name, english FROM exam_result;
+----+-----------+--------+
| id | name | english |
+----+-----------+--------+
| 1 | 唐三藏 | 56 |
| 2 | 孙悟空 | 77 |
| 3 | 猪悟能 | 90 |
| 4 | 曹孟德 | 67 |
| 5 | 刘玄德 | 45 |
| 6 | 孙权 | 78 |
| 7 | 宋公明 | 30 |
+----+-----------+--------+
7 rows in set (0.00 sec)

1.3查询字段为表达式

– 表达式不包含字段

SELECT id, name, 10 FROM exam_result;

SELECT id, name, 10 FROM exam_result;
+----+-----------+----+
| id | name | 10 |
+----+-----------+----+
| 1 | 唐三藏 | 10 |
| 2 | 孙悟空 | 10 |
| 3 | 猪悟能 | 10 |
| 4 | 曹孟德 | 10 |
| 5 | 刘玄德 | 10 |
| 6 | 孙权 | 10 |
| 7 | 宋公明 | 10 |
+----+-----------+----+
7 rows in set (0.00 sec)

– 表达式包含一个字段

SELECT id, name, english + 10 FROM exam_result;

SELECT id, name, english + 10 FROM exam_result;
+----+-----------+-------------+
| id | name | english + 10 |
+----+-----------+-------------+
| 1 | 唐三藏 | 66 |
| 2 | 孙悟空 | 87 |
| 3 | 猪悟能 | 100 |
| 4 | 曹孟德 | 77 |
| 5 | 刘玄德 | 55 |
| 6 | 孙权 | 88 |
| 7 | 宋公明 | 40 |
+----+-----------+-------------+
7 rows in set (0.00 sec)

– 表达式包含多个字段

SELECT id, name, chinese + math + english FROM exam_result;

SELECT id, name, chinese + math + english FROM exam_result;
+----+-----------+-------------------------+
| id | name | chinese + math + english |
+----+-----------+-------------------------+
| 1 | 唐三藏 | 221 |
| 2 | 孙悟空 | 242 |
| 3 | 猪悟能 | 276 |
| 4 | 曹孟德 | 233 |
| 5 | 刘玄德 | 185 |
| 6 | 孙权 | 221 |
| 7 | 宋公明 | 170 |
+----+-----------+-------------------------+
7 rows in set (0.00 sec)

1.4为查询结果指定别名

语法：

SELECT column [AS] alias_name [...] FROM table_name;

SELECT id, name, chinese + math + english 总分 FROM exam_result;

+----+-----------+--------+
| id | name | 总分 |
+----+-----------+--------+
| 1 | 唐三藏 | 221 |
| 2 | 孙悟空 | 242 |
| 3 | 猪悟能 | 276 |
| 4 | 曹孟德 | 233 |
| 5 | 刘玄德 | 185 |
| 6 | 孙权 | 221 |
| 7 | 宋公明 | 170 |
+----+-----------+--------+
7 rows in set (0.00 sec)

1.5 结果去重 – distinct

-- 98 分重复了
SELECT math FROM exam_result;
+--------+
| math |
+--------+
| 98 |
| 78 |
| 98 |
| 84 |
| 85 |
| 73 |
| 65 |
+--------+
7 rows in set (0.00 sec)

-- 去重结果
SELECT DISTINCT math FROM exam_result;
+--------+
| math |
+--------+
| 98 |
| 78 |
| 84 |
| 85 |
| 73 |
| 65 |
+--------+
6 rows in set (0.00 sec)

2.WHERE 条件

比较运算符：

运算符
>, >=, <, <=	大于，大于等于，小于，小于等于
=	等于，NULL 不安全，例如 NULL = NULL 的结果是 NULL
<=>	等于，NULL 安全，例如 NULL <=> NULL 的结果是 TRUE(1)
!=, <>	不等于
BETWEEN a0 AND a1	范围匹配，[a0, a1]，如果 a0 <= value <= a1，返回 TRUE(1)
IN (option, …)	如果是 option 中的任意一个，返回 TRUE(1)
IS NULL	是 NULL
IS NOT NULL	不是 NULL
LIKE	模糊匹配。% 表示任意多个（包括 0 个）任意字符；_ 表示任意一个字符

逻辑运算符：

运算符	说明
AND	多个条件必须都为 TRUE(1)，结果才是TRUE(1)
OR	任意一个条件为 TRUE(1), 结果为 TRUE(1)
NOT	条件为 TRUE(1)，结果为 FALSE(0)

表的内容如下：

SELECT * FROM exam_result;
+----+-----------+-------+--------+--------+
| id | name | chinese | math | english |
+----+-----------+-------+--------+--------+
| 1 | 唐三藏 | 67 | 98 | 56 |
| 2 | 孙悟空 | 87 | 78 | 77 |
| 3 | 猪悟能 | 88 | 98 | 90 |
| 4 | 曹孟德 | 82 | 84 | 67 |
| 5 | 刘玄德 | 55 | 85 | 45 |
| 6 | 孙权 | 70 | 73 | 78 |
| 7 | 宋公明 | 75 | 65 | 30 |
+----+-----------+-------+--------+--------+
7 rows in set (0.00 sec)

1.英语不及格的同学及英语成绩 ( < 60 )

SELECT name, english FROM exam_result WHERE english < 60;
+-----------+--------+
| name | english |
+-----------+--------+
| 唐三藏 | 56 |
| 刘玄德 | 45 |
| 宋公明 | 30 |
+-----------+--------+
3 rows in set (0.01 sec)

2.语文成绩在 [80, 90] 分的同学及语文成绩

– 使用 AND 进行条件连接

SELECT name, chinese FROM exam_result WHERE chinese >= 80 AND chinese <= 90;
+-----------+-------+
| name | chinese |
+-----------+-------+
| 孙悟空 | 87 |
| 猪悟能 | 88 |
| 曹孟德 | 82 |
+-----------+-------+
3 rows in set (0.00 sec)

– 使用 BETWEEN … AND … 条件

SELECT name, chinese FROM exam_result WHERE chinese BETWEEN 80 AND 90;
+-----------+-------+
| name | chinese |
+-----------+-------+
| 孙悟空 | 87 |
| 猪悟能 | 88 |
| 曹孟德 | 82 |
+-----------+-------+
3 rows in set (0.00 sec)

3.数学成绩是58或者59或者98或者99分的同学及数学成绩

– 使用 OR 进行条件连接

SELECT name, math FROM exam_result
WHERE math = 58
OR math = 59
OR math = 98
OR math = 99;
+-----------+--------+
| name | math |
+-----------+--------+
| 唐三藏 | 98 |
| 猪悟能 | 98 |
+-----------+--------+
2 rows in set (0.01 sec)

– 使用 IN 条件

SELECT name, math FROM exam_result WHERE math IN (58, 59, 98, 99);
+-----------+--------+
| name | math |
+-----------+--------+
| 唐三藏 | 98 |
| 猪悟能 | 98 |
+-----------+--------+
2 rows in set (0.00 sec)

4.姓孙的同学及孙某同学

– % 匹配任意多个（包括 0 个）任意字符

SELECT name FROM exam_result WHERE name LIKE '孙%';
+-----------+
| name |
+-----------+
| 孙悟空 |
| 孙权 |
+-----------+
2 rows in set (0.00 sec)

– _ 匹配严格的一个任意字符

SELECT name FROM exam_result WHERE name LIKE '孙_';
+--------+
| name |
+--------+
| 孙权 |
+--------+
1 row in set (0.00 sec)

5.语文成绩好于英语成绩的同学

– WHERE 条件中比较运算符两侧都是字段

SELECT name, chinese, english FROM exam_result WHERE chinese > english;
+-----------+-------+--------+
| name | chinese | english |
+-----------+-------+--------+
| 唐三藏 | 67 | 56 |
| 孙悟空 | 87 | 77 |
| 曹孟德 | 82 | 67 |
| 刘玄德 | 55 | 45 |
| 宋公明 | 75 | 30 |
+-----------+-------+--------+
5 rows in set (0.00 sec)

6.总分在 200 分以下的同学

– WHERE 条件中使用表达式

– 别名不能用在 WHERE 条件中

SELECT name, chinese + math + english 总分 FROM exam_result
WHERE chinese + math + english < 200;
+-----------+--------+
| name | 总分 |
+-----------+--------+
| 刘玄德 | 185 |
| 宋公明 | 170 |
+-----------+--------+
2 rows in set (0.00 sec)

7.语文成绩 > 80 并且不姓孙的同学

– AND 与 NOT 的使用

SELECT name, chinese FROM exam_result
WHERE chinese > 80 AND name NOT LIKE '孙%';
+----+-----------+-------+--------+--------+
| id | name | chinese | math | english |
+----+-----------+-------+--------+--------+
| 3 | 猪悟能 | 88 | 98 | 90 |
| 4 | 曹孟德 | 82 | 84 | 67 |
+----+-----------+-------+--------+--------+
2 rows in set (0.00 sec)

8.孙某同学，否则要求总成绩 > 200 并且语文成绩 < 数学成绩并且英语成绩 > 80

SELECT name, chinese, math, english, chinese + math + english 总分 FROM exam_result WHERE name LIKE '孙_' OR (chinese + math + english > 200 AND chinese < math AND english > 80);+-----------+-------+--------+--------+--------+
| name | chinese | math | english | 总分 |
+-----------+-------+--------+--------+--------+
| 猪悟能 | 88 | 98 | 90 | 276 |
| 孙权 | 70 | 73 | 78 | 221 |
+-----------+-------+--------+--------+--------+
2 rows in set (0.00 sec)

9.NULL 的查询

表的内容如下：

-- 查询 students 表
+-----+-------+-----------+-------+
| id | sn | name | qq |
+-----+-------+-----------+-------+
| 100 | 10010 | 唐大师 | NULL |
| 101 | 10001 | 孙悟空 | 11111 |
| 103 | 20002 | 孙仲谋 | NULL |
| 104 | 20001 | 曹阿瞒 | NULL |
+-----+-------+-----------+-------+
4 rows in set (0.00 sec)

查询 qq 号已知的同学姓名

select name, qq from student where qq is not null;SELECT name, qq FROM students WHERE qq IS NOT NULL;
+-----------+-------+
| name | qq |
+-----------+-------+
| 孙悟空 | 11111 |
+-----------+-------+
1 row in set (0.00 sec)

NULL 和 NULL 的比较，= 和 <=> 的区别

SELECT NULL = NULL, NULL = 1, NULL = 0;
+-------------+----------+----------+
| NULL = NULL | NULL = 1 | NULL = 0 |
+-------------+----------+----------+
| NULL | NULL | NULL |
+-------------+----------+----------+
1 row in set (0.00 sec)SELECT NULL <=> NULL, NULL <=> 1, NULL <=> 0;
+---------------+------------+------------+
| NULL <=> NULL | NULL <=> 1 | NULL <=> 0 |
+---------------+------------+------------+
| 1 | 0 | 0 |
+---------------+------------+------------+
1 row in set (0.00 sec)

3.结果排序

语法：

-- ASC 为升序（从小到大）
-- DESC 为降序（从大到小）
-- 默认为 ASC
SELECT ... FROM table_name [WHERE ...]
ORDER BY column [ASC|DESC], [...];

注意：没有 ORDER BY 子句的查询，返回的顺序是未定义的，永远不要依赖这个顺序

表的内容如下：

SELECT * FROM exam_result;
+----+-----------+-------+--------+--------+
| id | name | chinese | math | english |
+----+-----------+-------+--------+--------+
| 1 | 唐三藏 | 67 | 98 | 56 |
| 2 | 孙悟空 | 87 | 78 | 77 |
| 3 | 猪悟能 | 88 | 98 | 90 |
| 4 | 曹孟德 | 82 | 84 | 67 |
| 5 | 刘玄德 | 55 | 85 | 45 |
| 6 | 孙权 | 70 | 73 | 78 |
| 7 | 宋公明 | 75 | 65 | 30 |
+----+-----------+-------+--------+--------+
7 rows in set (0.00 sec)-- 查询 students 表
+-----+-------+-----------+-------+
| id | sn | name | qq |
+-----+-------+-----------+-------+
| 100 | 10010 | 唐大师 | NULL |
| 101 | 10001 | 孙悟空 | 11111 |
| 103 | 20002 | 孙仲谋 | NULL |
| 104 | 20001 | 曹阿瞒 | NULL |
+-----+-------+-----------+-------+
4 rows in set (0.00 sec)

1.同学及数学成绩，按数学成绩升序显示

select name, math from exam_result order by math;SELECT name, math FROM exam_result ORDER BY math;
+-----------+--------+
| name | math |
+-----------+--------+
| 宋公明 | 65 |
| 孙权 | 73 |
| 孙悟空 | 78 |
| 曹孟德 | 84 |
| 刘玄德 | 85 |
| 唐三藏 | 98 |
| 猪悟能 | 98 |
+-----------+--------+
7 rows in set (0.00 sec)

2.同学及 qq 号，按 qq 号排序显示

select name, qq from students order by qq;-- NULL 视为比任何值都小，升序出现在最上面
SELECT name, qq FROM students ORDER BY qq;
+-----------+-------+
| name | qq |
+-----------+-------+
| 唐大师 | NULL |
| 孙仲谋 | NULL |
| 曹阿瞒 | NULL |
| 孙悟空 | 11111 |
+-----------+-------+
4 rows in set (0.00 sec)-- NULL 视为比任何值都小，降序出现在最下面
SELECT name, qq FROM students ORDER BY qq DESC;
+-----------+-------+
| name | qq |
+-----------+-------+
| 孙悟空 | 11111 |
| 唐大师 | NULL |
| 孙仲谋 | NULL |
| 曹阿瞒 | NULL |
+-----------+-------+
4 rows in set (0.00 sec)

3.查询同学各门成绩，依次按数学降序，英语升序，语文升序的方式显示

select chinese, math, english from exam_result 
order by myth desc,english asc,chinese asc;SELECT name, math, english, chinese FROM exam_result
ORDER BY math DESC, english, chinese;
+-----------+--------+--------+-------+
| name | math | english | chinese |
+-----------+--------+--------+-------+
| 唐三藏 | 98 | 56 | 67 |
| 猪悟能 | 98 | 90 | 88 |
| 刘玄德 | 85 | 45 | 55 |
| 曹孟德 | 84 | 67 | 82 |
| 孙悟空 | 78 | 77 | 87 |
| 孙权 | 73 | 78 | 70 |
| 宋公明 | 65 | 30 | 75 |
+-----------+--------+--------+-------+
7 rows in set (0.00 sec)

4.查询同学及总分，由高到低

select chinses+math+english 总分 from exam_result order by chinese+math+english desc;-- ORDER BY 中可以使用表达式
SELECT name, chinese + english + math FROM exam_result
ORDER BY chinese + english + math DESC;
+-----------+-------------------------+
| name | chinese + english + math |
+-----------+-------------------------+
| 猪悟能 | 276 |
| 孙悟空 | 242 |
| 曹孟德 | 233 |
| 唐三藏 | 221 |
| 孙权 | 221 |
| 刘玄德 | 185 |
| 宋公明 | 170 |
+-----------+-------------------------+
7 rows in set (0.00 sec)-- ORDER BY 子句中可以使用列别名
SELECT name, chinese + english + math 总分 FROM exam_result
ORDER BY 总分 DESC;
+-----------+--------+
| name | 总分 |
+-----------+--------+
| 猪悟能 | 276 |
| 孙悟空 | 242 |
| 曹孟德 | 233 |
| 唐三藏 | 221 |
| 孙权 | 221 |
| 刘玄德 | 185 |
| 宋公明 | 170 |
+-----------+--------+
7 rows in set (0.00 sec)

5.查询姓孙的同学或者姓曹的同学数学成绩，结果按数学成绩由高到低显示

select name, math from exam_result 
where name like '孙%' or name like '曹%' 
order by math desc;-- 结合 WHERE 子句 和 ORDER BY 子句
SELECT name, math FROM exam_result
WHERE name LIKE '孙%' OR name LIKE '曹%'
ORDER BY math DESC;
+-----------+--------+
| name | math |
+-----------+--------+
| 曹孟德 | 84 |
| 孙悟空 | 78 |
| 孙权 | 73 |
+-----------+--------+
3 rows in set (0.00 sec)

4.筛选分页结果

语法：

-- 起始下标为 0
-- 从 s 开始，筛选 n 条结果
SELECT ... FROM table_name [WHERE ...] [ORDER BY ...] LIMIT s, n
-- 从 0 开始，筛选 n 条结果
SELECT ... FROM table_name [WHERE ...] [ORDER BY ...] LIMIT n;
-- 从 s 开始，筛选 n 条结果，比第二种用法更明确，建议使用
SELECT ... FROM table_name [WHERE ...] [ORDER BY ...] LIMIT n OFFSET s;

建议：对未知表进行查询时，最好加一条LIMIT 1，避免因为表中数据过大，查询全表数据导致数据库卡死

按 id 进行分页，每页3 条记录，分别显示第1、2、3 页

-- 第 1 页
SELECT id, name, math, english, chinese FROM exam_result
ORDER BY id LIMIT 3 OFFSET 0;
+----+-----------+--------+--------+-------+
| id | name | math | english | chinese |
+----+-----------+--------+--------+-------+
| 1 | 唐三藏 | 98 | 56 | 67 |
| 2 | 孙悟空 | 78 | 77 | 87 |
| 3 | 猪悟能 | 98 | 90 | 88 |
+----+-----------+--------+--------+-------+
3 rows in set (0.02 sec)

-- 第 2 页
SELECT id, name, math, english, chinese FROM exam_result
ORDER BY id LIMIT 3 OFFSET 3;
+----+-----------+--------+--------+-------+
| id | name | math | english | chinese |
+----+-----------+--------+--------+-------+
| 4 | 曹孟德 | 84 | 67 | 82 |
| 5 | 刘玄德 | 85 | 45 | 55 |
| 6 | 孙权 | 73 | 78 | 70 |
+----+-----------+--------+--------+-------+
3 rows in set (0.00 sec)

-- 第 3 页，如果结果不足 3 个，不会有影响
SELECT id, name, math, english, chinese FROM exam_result
ORDER BY id LIMIT 3 OFFSET 6;
+----+-----------+--------+--------+-------+
| id | name | math | english | chinese |
+----+-----------+--------+--------+-------+
| 7 | 宋公明 | 65 | 30 | 75 |
+----+-----------+--------+--------+-------+
1 row in set (0.00 sec)

三、更新–Update

语法：

UPDATE table_name SET column = expr [, column = expr ...]
[WHERE ...] [ORDER BY ...] [LIMIT ...]

对查询到的结果进行列值更新

案例：

1.将孙悟空同学的数学成绩变更为80分

-- 更新值为具体值
-- 查看原数据
SELECT name, math FROM exam_result WHERE name = '孙悟空';
+-----------+--------+
| name | math |
+-----------+--------+
| 孙悟空 | 78 |
+-----------+--------+
1 row in set (0.00 sec)
-- 数据更新
UPDATE exam_result SET math = 80 WHERE name = '孙悟空';
Query OK, 1 row affected (0.04 sec)
Rows matched: 1 Changed: 1 Warnings: 0
-- 查看更新后数据
SELECT name, math FROM exam_result WHERE name = '孙悟空';
+-----------+--------+
| name | math |
+-----------+--------+
| 孙悟空 | 80 |
+-----------+--------+
1 row in set (0.00 sec)

2.将曹孟德同学的数学成绩变更为 60 分，语文成绩变更为 70 分

update exam_result set math = 60, chinese = 70 where name = '曹孟德';-- 一次更新多个列
-- 查看原数据
SELECT name, math, chinese FROM exam_result WHERE name = '曹孟德';
+-----------+--------+-------+
name | math | chinese |
+-----------+--------+-------+
| 曹孟德 | 84 | 82 |
+-----------+--------+-------+
1 row in set (0.00 sec)
-- 数据更新
UPDATE exam_result SET math = 60, chinese = 70 WHERE name = '曹孟德';
Query OK, 1 row affected (0.14 sec)
Rows matched: 1 Changed: 1 Warnings: 0
-- 查看更新后数据
SELECT name, math, chinese FROM exam_result WHERE name = '曹孟德';
+-----------+--------+-------+
| name | math | chinese |
+-----------+--------+-------+
| 曹孟德 | 60 | 70 |
+-----------+--------+-------+
1 row in set (0.00 sec)

3.将总成绩倒数前三的 3 位同学的数学成绩加上 30 分

update exam_result set math = math + 30
order by chinese + math + english asc limit 3;-- 更新值为原值基础上变更
-- 查看原数据
-- 别名可以在ORDER BY中使用
SELECT name, math, chinese + math + english 总分 FROM exam_result
ORDER BY 总分 LIMIT 3;
+-----------+--------+--------+
| name | math | 总分 |
+-----------+--------+--------+
| 宋公明 | 65 | 170 |
| 刘玄德 | 85 | 185 |
| 曹孟德 | 60 | 197 |
+-----------+--------+--------+
3 rows in set (0.00 sec)
-- 数据更新，不支持 math += 30 这种语法
UPDATE exam_result SET math = math + 30
ORDER BY chinese + math + english LIMIT 3;
-- 查看更新后数据
-- 思考：这里还可以按总分升序排序取前 3 个么？
SELECT name, math, chinese + math + english 总分 FROM exam_result
WHERE name IN ('宋公明', '刘玄德', '曹孟德');
+-----------+--------+--------+
| name | math | 总分 |
+-----------+--------+--------+
| 曹孟德 | 90 | 227 |
| 刘玄德 | 115 | 215 |
| 宋公明 | 95 | 200 |
+-----------+--------+--------+
3 rows in set (0.00 sec)
-- 按总成绩排序后查询结果
SELECT name, math, chinese + math + english 总分 FROM exam_result
ORDER BY 总分 LIMIT 3;
+-----------+--------+--------+
| name | math | 总分 |
+-----------+--------+--------+
| 宋公明 | 95 | 200 |
| 刘玄德 | 115 | 215 |
| 唐三藏 | 98 | 221 |
+-----------+--------+--------+
3 rows in set (0.00 sec)

4.将所有同学的语文成绩更新为原来的 2 倍

update exam_result chinese = chinese * 2;-- 没有 WHERE 子句，则更新全表
-- 查看原数据
SELECT * FROM exam_result;
+----+-----------+-------+--------+--------+
| id | name | chinese | math | english |
+----+-----------+-------+--------+--------+
| 1 | 唐三藏 | 67 | 98 | 56 |
| 2 | 孙悟空 | 87 | 80 | 77 |
| 3 | 猪悟能 | 88 | 98 | 90 |
| 4 | 曹孟德 | 70 | 90 | 67 |
| 5 | 刘玄德 | 55 | 115 | 45 |
| 6 | 孙权 | 70 | 73 | 78 |
| 7 | 宋公明 | 75 | 95 | 30 |
+----+-----------+-------+--------+--------+
7 rows in set (0.00 sec)
-- 数据更新
UPDATE exam_result SET chinese = chinese * 2;
Query OK, 7 rows affected (0.00 sec)
Rows matched: 7 Changed: 7 Warnings: 0
-- 查看更新后数据
SELECT * FROM exam_result;
+----+-----------+-------+--------+--------+
| id | name | chinese | math | english |
+----+-----------+-------+--------+--------+
| 1 | 唐三藏 | 134 | 98 | 56 |
| 2 | 孙悟空 | 174 | 80 | 77 |
| 3 | 猪悟能 | 176 | 98 | 90 |
| 4 | 曹孟德 | 140 | 90 | 67 |
| 5 | 刘玄德 | 110 | 115 | 45 |
| 6 | 孙权 | 140 | 73 | 78 |
| 7 | 宋公明 | 150 | 95 | 30 |
+----+-----------+-------+--------+--------+
7 rows in set (0.00 sec)

四、删除–Delete

1.删除数据

语法：

DELETE FROM table_name [WHERE ...] [ORDER BY ...] [LIMIT ...]

案例：

1.删除孙悟空同学的考试成绩

delete from exam_result where name = '孙悟空';-- 查看原数据
SELECT * FROM exam_result WHERE name = '孙悟空';
+----+-----------+-------+--------+--------+
| id | name | chinese | math | english |
+----+-----------+-------+--------+--------+
| 2 | 孙悟空 | 174 | 80 | 77 |
+----+-----------+-------+--------+--------+
1 row in set (0.00 sec)
-- 删除数据
DELETE FROM exam_result WHERE name = '孙悟空';
Query OK, 1 row affected (0.17 sec)
-- 查看删除结果
SELECT * FROM exam_result WHERE name = '孙悟空';
Empty set (0.00 sec)

2.删除整张表数据

-- 准备测试表
CREATE TABLE for_delete (id INT PRIMARY KEY AUTO_INCREMENT,name VARCHAR(20)
);
Query OK, 0 rows affected (0.16 sec)-- 插入测试数据
INSERT INTO for_delete (name) VALUES ('A'), ('B'), ('C');
Query OK, 3 rows affected (1.05 sec)
Records: 3 Duplicates: 0 Warnings: 0-- 查看测试数据
SELECT * FROM for_delete;
+----+------+
| id | name |
+----+------+
| 1 | A |
| 2 | B |
| 3 | C |
+----+------+
3 rows in set (0.00 sec)-- 删除整表数据
DELETE FROM for_delete;
Query OK, 3 rows affected (0.00 sec)-- 查看删除结果
SELECT * FROM for_delete;
Empty set (0.00 sec)-- 再插入一条数据，自增 id 在原值上增长
INSERT INTO for_delete (name) VALUES ('D');
Query OK, 1 row affected (0.00 sec)-- 查看数据
SELECT * FROM for_delete;
+----+------+
| id | name |
+----+------+
| 4 | D |
+----+------+
1 row in set (0.00 sec)-- 查看表结构，会有 AUTO_INCREMENT=n 项
SHOW CREATE TABLE for_delete\G
*************************** 1. row ***************************
Table: for_delete
Create Table: CREATE TABLE `for_delete` (`id` int(11) NOT NULL AUTO_INCREMENT,`name` varchar(20) DEFAULT NULL,PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

我们发现删除表之后，自增长的数字依然在增加，并没有重新从1开始增长

2.截断表

语法：

TRUNCATE [TABLE] table_name

注意：这个操作慎用

1.只能对整表操作，不能像 DELETE 一样针对部分数据操作；

2.实际上 MySQL 不对数据操作，所以比 DELETE 更快，但是TRUNCATE在删除数据的时候，并不经过真正的事物，所以无法回滚

3.会重置 AUTO_INCREMENT 项

创建表并插入数据

-- 准备测试表
CREATE TABLE for_truncate (id INT PRIMARY KEY AUTO_INCREMENT,name VARCHAR(20)
);Query OK, 0 rows affected (0.16 sec)
-- 插入测试数据
INSERT INTO for_truncate (name) VALUES ('A'), ('B'), ('C');
Query OK, 3 rows affected (1.05 sec)
Records: 3 Duplicates: 0 Warnings: 0-- 查看测试数据
SELECT * FROM for_truncate;
+----+------+
| id | name |
+----+------+
| 1 | A |
| 2 | B |
| 3 | C |
+----+------+
3 rows in set (0.00 sec)

截断整表数据

-- 截断整表数据，注意影响行数是 0，所以实际上没有对数据真正操作
TRUNCATE for_truncate;
Query OK, 0 rows affected (0.10 sec)-- 查看删除结果
SELECT * FROM for_truncate;
Empty set (0.00 sec)

我们发现影响行数是 0，所以实际上没有对数据真正操作，但是表已经删除

-- 再插入一条数据，自增 id 在重新增长
INSERT INTO for_truncate (name) VALUES ('D');
Query OK, 1 row affected (0.00 sec)

我们再插入一条数据，发现自增 id 在重新增长

-- 查看数据
SELECT * FROM for_truncate;
+----+------+
| id | name |
+----+------+
| 1 | D |
+----+------+
1 row in set (0.00 sec)-- 查看表结构，会有 AUTO_INCREMENT=2 项
SHOW CREATE TABLE for_truncate\G
*************************** 1. row ***************************
Table: for_truncate
Create Table: CREATE TABLE `for_truncate` (`id` int(11) NOT NULL AUTO_INCREMENT,`name` varchar(20) DEFAULT NULL,PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

总结：

delete整张表之后，自增长的数据会继续增长，并不会继续从1开始增长

truncate整张表之后，没有经过事物，无法回滚，自增长会从1开始增长

五、插入查询结果

语法：

INSERT INTO table_name [(column [, column ...])] SELECT ...

案例：删除表中的的重复复记录，重复的数据只能有一份

-- 创建原数据表
CREATE TABLE duplicate_table (id int, name varchar(20));
Query OK, 0 rows affected (0.01 sec)-- 插入测试数据
INSERT INTO duplicate_table VALUES
(100, 'aaa'),
(100, 'aaa'),
(200, 'bbb'),
(200, 'bbb'),
(200, 'bbb'),
(300, 'ccc');
Query OK, 6 rows affected (0.00 sec)
Records: 6 Duplicates: 0 Warnings: 0

思路：

创建一张空表 no_duplicate_table，结构和 duplicate_table 一样

CREATE TABLE no_duplicate_table LIKE duplicate_table;
Query OK, 0 rows affected (0.00 sec)

将 duplicate_table 的去重数据插入到 no_duplicate_table

INSERT INTO no_duplicate_table SELECT DISTINCT * FROM duplicate_table;
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0

通过重命名表，实现原子的去重操作

RENAME TABLE duplicate_table TO old_duplicate_table,
no_duplicate_table TO duplicate_table;
Query OK, 0 rows affected (0.00 sec)

查看最终结果

SELECT * FROM duplicate_table;
+------+------+
| id | name |
+------+------+
| 100 | aaa |
| 200 | bbb |
| 300 | ccc |
+------+------+
3 rows in set (0.00 sec)

六、聚合函数

函数	说明
COUNT([DISTINCT] expr)	返回查询到的数据的数量
SUM([DISTINCT] expr)	返回查询到的数据的总和，不是数字没有意义
AVG([DISTINCT] expr)	返回查询到的数据的平均值，不是数字没有意义
MAX([DISTINCT] expr)	返回查询到的数据的最大值，不是数字没有意义
MIN([DISTINCT] expr)	返回查询到的数据的最小值，不是数字没有意义

案例：

表的内容如下：

-- 查询 students 表
+-----+-------+-----------+-------+
| id | sn | name | qq |
+-----+-------+-----------+-------+
| 100 | 10010 | 唐大师 | NULL |
| 101 | 10001 | 孙悟空 | 11111 |
| 103 | 20002 | 孙仲谋 | NULL |
| 104 | 20001 | 曹阿瞒 | NULL |
+-----+-------+-----------+-------+
4 rows in set (0.00 sec)SELECT * FROM exam_result;
+----+-----------+-------+--------+--------+
| id | name | chinese | math | english |
+----+-----------+-------+--------+--------+
| 1 | 唐三藏 | 67 | 98 | 56 |
| 2 | 孙悟空 | 87 | 78 | 77 |
| 3 | 猪悟能 | 88 | 98 | 90 |
| 4 | 曹孟德 | 82 | 84 | 67 |
| 5 | 刘玄德 | 55 | 85 | 45 |
| 6 | 孙权 | 70 | 73 | 78 |
| 7 | 宋公明 | 75 | 65 | 30 |
+----+-----------+-------+--------+--------+
7 rows in set (0.00 sec)

1.统计班级共有多少同学

select count(*) from students;-- 使用 * 做统计，不受 NULL 影响
SELECT COUNT(*) FROM students;
+----------+
| COUNT(*) |
+----------+
| 4 |
+----------+
1 row in set (0.00 sec)-- 使用表达式做统计
SELECT COUNT(1) FROM students;
+----------+
| COUNT(1) |
+----------+
| 4 |
+----------+
1 row in set (0.00 sec)

2.统计班级收集的 qq 号有多少

select count(qq) from students;-- NULL 不会计入结果
SELECT COUNT(qq) FROM students;
+-----------+
| COUNT(qq) |
+-----------+
| 1 |
+-----------+
1 row in set (0.00 sec)

3.统计本次考试的数学成绩分数个数

select count(distinct math) from exam_result;-- COUNT(math) 统计的是全部成绩
SELECT COUNT(math) FROM exam_result;
+---------------+
| COUNT(math) |
+---------------+
| 6 |
+---------------+
1 row in set (0.00 sec)-- COUNT(DISTINCT math) 统计的是去重成绩数量
SELECT COUNT(DISTINCT math) FROM exam_result;
+------------------------+
| COUNT(DISTINCT math) |
+------------------------+
| 5 |
+------------------------+
1 row in set (0.00 sec)

4.统计数学成绩总分

select sum(math) from exam_result;SELECT SUM(math) FROM exam_result;
+-------------+
| SUM(math) |
+-------------+
| 569 |
+-------------+
1 row in set (0.00 sec)-- 不及格 < 60 的总分，没有结果，返回 NULL
SELECT SUM(math) FROM exam_result WHERE math < 60;
+-------------+
| SUM(math) |
+-------------+
| NULL |
+-------------+
1 row in set (0.00 sec)

5.统计平均总分

select avg(chinese + math + english) from exam_result;SELECT AVG(chinese + math + english) 平均总分 FROM exam_result;
+--------------+
| 平均总分 |
+--------------+
| 297.5 |
+--------------+

6.返回英语最高分

select max(english) from exam_result;SELECT MAX(english) FROM exam_result;
+-------------+
| MAX(english) |
+-------------+
| 90 |
+-------------+
1 row in set (0.00 sec)

7.返回 > 70 分以上的数学最低分

select min(math) from exam_result where math > 70;SELECT MIN(math) FROM exam_result WHERE math > 70;
+-------------+
| MIN(math) |
+-------------+
| 73 |
+-------------+
1 row in set (0.00 sec)

七、group by子句的使用

在select中使用group by 子句可以对指定列进行分组查询

select column1, column2, .. from table group by column;

案例：

准备工作，创建一个雇员信息表（来自oracle 9i的经典测试表）

EMP员工表

DEPT部门表

SALGRADE工资等级表

DROP database IF EXISTS `scott`;
CREATE database IF NOT EXISTS `scott` DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci;USE `scott`;DROP TABLE IF EXISTS `dept`;
CREATE TABLE `dept` (`deptno` int(2) unsigned zerofill NOT NULL COMMENT '部门编号',`dname` varchar(14) DEFAULT NULL COMMENT '部门名称',`loc` varchar(13) DEFAULT NULL COMMENT '部门所在地点'
);DROP TABLE IF EXISTS `emp`;
CREATE TABLE `emp` (`empno` int(6) unsigned zerofill NOT NULL COMMENT '雇员编号',`ename` varchar(10) DEFAULT NULL COMMENT '雇员姓名',`job` varchar(9) DEFAULT NULL COMMENT '雇员职位',`mgr` int(4) unsigned zerofill DEFAULT NULL COMMENT '雇员领导编号',`hiredate` datetime DEFAULT NULL COMMENT '雇佣时间',`sal` decimal(7,2) DEFAULT NULL COMMENT '工资月薪',`comm` decimal(7,2) DEFAULT NULL COMMENT '奖金',`deptno` int(2) unsigned zerofill DEFAULT NULL COMMENT '部门编号'
);DROP TABLE IF EXISTS `salgrade`;
CREATE TABLE `salgrade` (`grade` int(11) DEFAULT NULL COMMENT '等级',`losal` int(11) DEFAULT NULL COMMENT '此等级最低工资',`hisal` int(11) DEFAULT NULL COMMENT '此等级最高工资'
);insert into dept (deptno, dname, loc)
values (10, 'ACCOUNTING', 'NEW YORK');
insert into dept (deptno, dname, loc)
values (20, 'RESEARCH', 'DALLAS');
insert into dept (deptno, dname, loc)
values (30, 'SALES', 'CHICAGO');
insert into dept (deptno, dname, loc)
values (40, 'OPERATIONS', 'BOSTON');insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7369, 'SMITH', 'CLERK', 7902, '1980-12-17', 800, null, 20);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7499, 'ALLEN', 'SALESMAN', 7698, '1981-02-20', 1600, 300, 30);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7521, 'WARD', 'SALESMAN', 7698, '1981-02-22', 1250, 500, 30);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7566, 'JONES', 'MANAGER', 7839, '1981-04-02', 2975, null, 20);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7654, 'MARTIN', 'SALESMAN', 7698, '1981-09-28', 1250, 1400, 30);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7698, 'BLAKE', 'MANAGER', 7839, '1981-05-01', 2850, null, 30);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7782, 'CLARK', 'MANAGER', 7839, '1981-06-09', 2450, null, 10);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7788, 'SCOTT', 'ANALYST', 7566, '1987-04-19', 3000, null, 20);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7839, 'KING', 'PRESIDENT', null, '1981-11-17', 5000, null, 10);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7844, 'TURNER', 'SALESMAN', 7698,'1981-09-08', 1500, 0, 30);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7876, 'ADAMS', 'CLERK', 7788, '1987-05-23', 1100, null, 20);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7900, 'JAMES', 'CLERK', 7698, '1981-12-03', 950, null, 30);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7902, 'FORD', 'ANALYST', 7566, '1981-12-03', 3000, null, 20);insert into emp (empno, ename, job, mgr, hiredate, sal, comm, deptno)
values (7934, 'MILLER', 'CLERK', 7782, '1982-01-23', 1300, null, 10);insert into salgrade (grade, losal, hisal) values (1, 700, 1200);
insert into salgrade (grade, losal, hisal) values (2, 1201, 1400);
insert into salgrade (grade, losal, hisal) values (3, 1401, 2000);
insert into salgrade (grade, losal, hisal) values (4, 2001, 3000);
insert into salgrade (grade, losal, hisal) values (5, 3001, 9999);

将上面的内容放到一个文件中，然后使用source 命令即可创建三张表

表的内容如下：

mysql> select * from emp;
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
| 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |
| 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |
| 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |
| 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |
| 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |
| 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |
| 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |
| 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |
| 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |
| 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |
| 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |
| 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |
| 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |
| 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |
+--------+--------+-----------+------+---------------------+---------+---------+--------+
14 rows in set (0.00 sec)mysql> select * from dept;
+--------+------------+----------+
| deptno | dname      | loc      |
+--------+------------+----------+
|     10 | ACCOUNTING | NEW YORK |
|     20 | RESEARCH   | DALLAS   |
|     30 | SALES      | CHICAGO  |
|     40 | OPERATIONS | BOSTON   |
+--------+------------+----------+
4 rows in set (0.00 sec)

1.如何显示每个部门的平均工资和最高工资

select deptno, avg(sal) 平均工资, max(sal) 最高工资 from emp group by deptno;mysql> select deptno, avg(sal) 平均工资, max(sal) 最高工资 from emp group by deptno;
+--------+--------------+--------------+
| deptno | 平均工资     | 最高工资     |
+--------+--------------+--------------+
|     10 |  2916.666667 |      5000.00 |
|     20 |  2175.000000 |      3000.00 |
|     30 |  1566.666667 |      2850.00 |
+--------+--------------+--------------+
3 rows in set (0.00 sec)

2.显示每个部门的每种岗位的平均工资和最低工资

mysql> select deptno,job,avg(sal),min(sal) from emp group by deptno,job;
+--------+-----------+-------------+----------+
| deptno | job       | avg(sal)    | min(sal) |
+--------+-----------+-------------+----------+
|     10 | CLERK     | 1300.000000 |  1300.00 |
|     10 | MANAGER   | 2450.000000 |  2450.00 |
|     10 | PRESIDENT | 5000.000000 |  5000.00 |
|     20 | ANALYST   | 3000.000000 |  3000.00 |
|     20 | CLERK     |  950.000000 |   800.00 |
|     20 | MANAGER   | 2975.000000 |  2975.00 |
|     30 | CLERK     |  950.000000 |   950.00 |
|     30 | MANAGER   | 2850.000000 |  2850.00 |
|     30 | SALESMAN  | 1400.000000 |  1250.00 |
+--------+-----------+-------------+----------+
9 rows in set (0.00 sec)

3.显示平均工资低于2000的部门和它的平均工资

1.统计各个部门的平均工资

select deptno, avg(sal) from emp group by deptno;mysql> select deptno,avg(sal) from emp group by deptno;
+--------+-------------+
| deptno | avg(sal)    |
+--------+-------------+
|     10 | 2916.666667 |
|     20 | 2175.000000 |
|     30 | 1566.666667 |
+--------+-------------+
3 rows in set (0.00 sec)

2.having和group by配合使用，对group by结果进行过滤

select deptno, avg(sal) from emp group by deptno having avg(sal) < 2000;
mysql> select deptno, avg(sal) from emp group by deptno having avg(sal) < 2000;
+--------+-------------+
| deptno | avg(sal)    |
+--------+-------------+
|     30 | 1566.666667 |
+--------+-------------+
1 row in set (0.00 sec)

–having经常和group by搭配使用，作用是对分组进行筛选，作用有些像where