MySQL —— 基本查询

文章目录

- 1. 向表中插入数据
- 2. 查询操作
- - 2.1 全列查询
  - 2.2 指定列查询
  - 2.3 查询字段带表达式
  - 2.4 为查询结果指定别名
  - 2.5 去重操作
- 3. where 条件
- - 3.1 比较运算符和逻辑预算符的运用
  - 3.2 like的细节
  - 3. 3 null查询
- 4. 对查询的结果进行排序
- - 4.1 对单一字段进行排序
  - 4.2 对多个字段排序
  - 4.3 对字段排序结果进行分页
- 5. 更改表中的数据
- - 5.1 更新单列数据
  - 5.2 更新多列的数据
- 6. 删除表中数据
- - 6.1 删除表中一个数据或或者多个数据
  - 6.2 删除全表的数据
  - 6.3 截断表，不可以恢复的删除
- 7. 去重操作
- 8. 聚合函数

前言： 本文主要讲解MySQL中对表进行一些基本的查询操作，同时在讲一些对表的其他操作，所谓其他操作就是之前博客里面没有涉及到的。

1. 向表中插入数据

向表中插入数据再补充一下，如果插入的数据和主键或者唯一键冲突了的情况。

先创建

mysql> 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))character set utf8;
Query OK, 0 rows affected (0.04 sec)

mysql> desc students;
+-------+------------------+------+-----+---------+----------------+
| Field | Type             | Null | Key | Default | Extra          |
+-------+------------------+------+-----+---------+----------------+
| id    | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| sn    | int(11)          | NO   | UNI | NULL    |                |
| name  | varchar(20)      | NO   |     | NULL    |                |
| qq    | varchar(20)      | YES  |     | NULL    |                |
+-------+------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

向表中插入数据，单行全列插入：

mysql> insert into students values(1,100,"张三",22222);
Query OK, 1 row affected (0.01 sec)

mysql> insert into students values(2,101,"李四",33333);
Query OK, 1 row affected (0.00 sec)

mysql> select * from students;
+----+-----+--------+-------+
| id | sn  | name   | qq    |
+----+-----+--------+-------+
|  1 | 100 | 张三   | 22222 |
|  2 | 101 | 李四   | 33333 |
+----+-----+--------+-------+
2 rows in set (0.00 sec)

再向表中插入数据，多行全列插入：

mysql> insert into students values(11,111,"王五",4444),(12,222,"刘六",55555);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from students;
+----+-----+--------+-------+
| id | sn  | name   | qq    |
+----+-----+--------+-------+
|  1 | 100 | 张三   | 22222 |
|  2 | 101 | 李四   | 33333 |
| 11 | 111 | 王五   | 4444  |
| 12 | 222 | 刘六   | 55555 |
+----+-----+--------+-------+
4 rows in set (0.00 sec)

这里需要注意的就是，多行插入时，() () 之间用，隔开。

当然还可以指定列插入，现在来演示插入和主键，唯一键冲突了的情况：

mysql> insert into students values(2,101,"开心",33333);
ERROR 1062 (23000): Duplicate entry '2' for key 'PRIMARY'

很明显这是主键冲突，但是我如果就是想要更新一下name信息，但是发生冲突，导致无法完成插入操作，这就需要这样插入：

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

mysql> insert into students values(2,101,"开心",33333) on duplicate key update name="开心";
Query OK, 2 rows affected (0.01 sec)

mysql> select * from students;
+----+-----+--------+-------+
| id | sn  | name   | qq    |
+----+-----+--------+-------+
|  1 | 100 | 张三   | 22222 |
|  2 | 101 | 开心   | 33333 |
| 11 | 111 | 王五   | 4444  |
| 12 | 222 | 刘六   | 55555 |
+----+-----+--------+-------+
4 rows in set (0.00 sec)

主键冲突，想要更新其余字段就是这样操作。唯一键冲突，和上面一样的操作。这里需要注意一点，那就是主键冲突，更新的字段不能包括主键，唯一键冲突，更新的字段不能包括唯一键。这是因为你更新了，就相当于不冲突了，那就可以直接插入了，而不是利用上面的更新操作。

上面发生冲突后，采取的是更新的方式，其实还有一种方式，那就是替换。

replace into students (sn, name) values (111, '王五');

如果发生了键冲突，那么就删除原来的，直接插入新的。这就是替换。
如果没有发生键冲突，就是直接插入。

2. 查询操作

我们先创建一个表，然后进行一些查询操作：

mysql> create table exam(id int unsigned primary key auto_increment, name varchar(20) not null comment '同学姓名',  chinese float deefault 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');
Query OK, 0 rows affected (0.05 sec)

mysql> desc exam;
+---------+------------------+------+-----+---------+----------------+
| Field   | Type             | Null | Key | Default | Extra          |
+---------+------------------+------+-----+---------+----------------+
| id      | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| name    | varchar(20)      | NO   |     | NULL    |                |
| chinese | float            | YES  |     | 0       |                |
| math    | float            | YES  |     | 0       |                |
| english | float            | YES  |     | 0       |                |
+---------+------------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

插入一批数据：

mysql> insert into exam values(1,'bob',88.5,75.4,98.5),(2,'xm',88,76,93),(3,'gg',76,77,98);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> insert into exam values(4,'ss',82.5,72.4,98.5),(5,'ll',88,74,97),(6,'ff',75,72,93);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

2.1 全列查询

select * from 表名

全列查询，这个就是咱们实验用的比较多，一般情况下，很少用，因为全列显示，意味着需要传输的数据量越大，可能会影响到索引的使用。

比如：

mysql> select * from exam;
+----+------+---------+------+---------+
| id | name | chinese | math | english |
+----+------+---------+------+---------+
|  1 | bob  |    88.5 | 75.4 |    98.5 |
|  2 | xm   |      88 |   76 |      93 |
|  3 | gg   |      76 |   77 |      98 |
|  4 | ss   |    82.5 | 72.4 |    98.5 |
|  5 | ll   |      88 |   74 |      97 |
|  6 | ff   |      75 |   72 |      93 |
+----+------+---------+------+---------+
6 rows in set (0.00 sec)

2.2 指定列查询

就是指定部分列进行查询，不是全列查询，需要指定：

select 指定列 from exam:

mysql> select id,name,math from exam;
+----+------+------+
| id | name | math |
+----+------+------+
|  1 | bob  | 75.4 |
|  2 | xm   |   76 |
|  3 | gg   |   77 |
|  4 | ss   | 72.4 |
|  5 | ll   |   74 |
|  6 | ff   |   72 |
+----+------+------+
6 rows in set (0.00 sec)

2.3 查询字段带表达式

比如：现在要求显示出来的math都加10:

select id,name,math+10 from exam:

mysql> select id,name,math+10 from exam;
+----+------+------------------+
| id | name | math+10          |
+----+------+------------------+
|  1 | bob  | 85.4000015258789 |
|  2 | xm   |               86 |
|  3 | gg   |               87 |
|  4 | ss   | 82.4000015258789 |
|  5 | ll   |               84 |
|  6 | ff   |               82 |
+----+------+------------------+

注意这个+10，只影响显示结果，不会影响表中的原始数据。

现在要求显示出总分：

mysql> select id,name,math+chinese+english from exam;
+----+------+----------------------+
| id | name | math+chinese+english |
+----+------+----------------------+
|  1 | bob  |    262.4000015258789 |
|  2 | xm   |                  257 |
|  3 | gg   |                  251 |
|  4 | ss   |    253.4000015258789 |
|  5 | ll   |                  259 |
|  6 | ff   |                  240 |
+----+------+----------------------+

2.4 为查询结果指定别名

把上面的math+chinese+english，起别名为总分：

mysql> select id,name,math+chinese+english 总分 from exam ;
+----+------+-------------------+
| id | name | 总分              |
+----+------+-------------------+
|  1 | bob  | 262.4000015258789 |
|  2 | xm   |               257 |
|  3 | gg   |               251 |
|  4 | ss   | 253.4000015258789 |
|  5 | ll   |               259 |
|  6 | ff   |               240 |
+----+------+-------------------+
6 rows in set (0.00 sec)

2.5 去重操作

select distinct 字段名 from 表名；

mysql> select chinese from exam;
+---------+
| chinese |
+---------+
|    88.5 |
|      88 |
|      76 |
|    82.5 |
|      88 |
|      75 |
+---------+

可以看到里面有重复的情况，那就是88和88重复了，那么去重就是：

mysql> select distinct chinese from exam;
+---------+
| chinese |
+---------+
|    88.5 |
|      88 |
|      76 |
|    82.5 |
|      75 |
+---------+

3. where 条件

比较运算符：

在这里插入图片描述
逻辑运算符：

3.1 比较运算符和逻辑预算符的运用

假如：我要语文成绩在[80,90]之间的同学名单：

mysql> select name,chinese from exam where chinese>=80 and chinese<=90;
+------+---------+
| name | chinese |
+------+---------+
| bob  |    88.5 |
| xm   |      88 |
| ss   |    82.5 |
| ll   |      88 |
+------+---------+
4 rows in set (0.00 sec)

先在要求：数学成绩大于75或者语文成绩大于85：

mysql> select name,chinese,math from exam where math>75 or chinese>=85;
+------+---------+------+
| name | chinese | math |
+------+---------+------+
| bob  |    88.5 | 75.4 |
| xm   |      88 |   76 |
| gg   |      76 |   77 |
| ll   |      88 |   74 |
+------+---------+------+
4 rows in set (0.00 sec)

3.2 like的细节

找出名字为bob的同学：

mysql> select * from exam where name like 'bob';
+----+------+---------+------+---------+
| id | name | chinese | math | english |
+----+------+---------+------+---------+
|  1 | bob  |    88.5 | 75.4 |    98.5 |
+----+------+---------+------+---------+
1 row in set (0.00 sec)

其实这里也可以用 name = ‘bob’ ，不过这里用like 。

关于like的两个细节：%，_。

比如我要搜索 b后面只有一个字母的人，bb之类的。如果不限制后面字母数那就是 %:

mysql> select * from exam where name like 'b_';
+----+------+---------+------+---------+
| id | name | chinese | math | english |
+----+------+---------+------+---------+
|  7 | bb   |      87 |   86 |      76 |
+----+------+---------+------+---------+
1 row in set (0.00 sec)

mysql> select * from exam where name like 'b%';
+----+------+---------+------+---------+
| id | name | chinese | math | english |
+----+------+---------+------+---------+
|  1 | bob  |    88.5 | 75.4 |    98.5 |
|  7 | bb   |      87 |   86 |      76 |
+----+------+---------+------+---------+
2 rows in set (0.00 sec)

3. 3 null查询

is null
is not null

null不参与一般的运算，如果要让它参与比较运算，也是可以的，那就是用 is null和is not null，这样好用。如果非要用比较运算符的话，那就是<=>等于。

mysql> select * from students;
+----+-----+--------+-------+
| id | sn  | name   | qq    |
+----+-----+--------+-------+
|  3 | 102 | 开学   | 33333 |
| 11 | 111 | 王五   | 4444  |
| 12 | 222 | 刘六   | 55555 |
| 13 | 100 | 发财   | NULL  |
+----+-----+--------+-------+

比如:上面那个表，其中发财的qq是空。

mysql> select * from students where qq is not null;
+----+-----+--------+-------+
| id | sn  | name   | qq    |
+----+-----+--------+-------+
|  3 | 102 | 开学   | 33333 |
| 11 | 111 | 王五   | 4444  |
| 12 | 222 | 刘六   | 55555 |
+----+-----+--------+-------+
3 rows in set (0.00 sec)

mysql> select * from students where qq is null;
+----+-----+--------+------+
| id | sn  | name   | qq   |
+----+-----+--------+------+
| 13 | 100 | 发财   | NULL |
+----+-----+--------+------+
1 row in set (0.00 sec)

mysql> select * from students where qq <=> null;
+----+-----+--------+------+
| id | sn  | name   | qq   |
+----+-----+--------+------+
| 13 | 100 | 发财   | NULL |
+----+-----+--------+------+
1 row in set (0.00 sec)

4. 对查询的结果进行排序

-- asc 为升序（从小到大）
-- desc 为降序（从大到小）
-- 默认为 asc

4.1 对单一字段进行排序

假如：对数学成绩进行排序：

mysql> select id,name,math  from exam order by math;
+----+------+------+
| id | name | math |
+----+------+------+
|  6 | ff   |   72 |
|  4 | ss   | 72.4 |
|  5 | ll   |   74 |
|  1 | bob  | 75.4 |
|  2 | xm   |   76 |
|  3 | gg   |   77 |
|  7 | bb   |   86 |
+----+------+------+
7 rows in set (0.00 sec)

对吧，很明显是升序，也就是默认asc。

那么如果要排降序呢，那就是选desc：

mysql> select id,name,math  from exam order by math desc;
+----+------+------+
| id | name | math |
+----+------+------+
|  7 | bb   |   86 |
|  3 | gg   |   77 |
|  2 | xm   |   76 |
|  1 | bob  | 75.4 |
|  5 | ll   |   74 |
|  4 | ss   | 72.4 |
|  6 | ff   |   72 |
+----+------+------+
7 rows in set (0.00 sec)

4.2 对多个字段排序

对多个字段进行排序：
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：

注意其实多字段排序是不能完成的，你想想吧，有的数学高，有的英语高，这不能在一张表中排序，所以多字段排序其实就是你写的要排序的第一个字段：

mysql> select * from exam order by math desc,english,chinese;
+----+------+---------+------+---------+
| id | name | chinese | math | english |
+----+------+---------+------+---------+
|  7 | bb   |      87 |   86 |      76 |
|  3 | gg   |      76 |   77 |      98 |
|  2 | xm   |      88 |   76 |      93 |
|  1 | bob  |    88.5 | 75.4 |    98.5 |
|  5 | ll   |      88 |   74 |      97 |
|  4 | ss   |    82.5 | 72.4 |    98.5 |
|  6 | ff   |      75 |   72 |      93 |
+----+------+---------+------+---------+
7 rows in set (0.00 sec)

这只对math进行了排序。

4.3 对字段排序结果进行分页

起始下标为 0
-- 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
SELECT ... FROM table_name [WHERE ...] [ORDER BY ...] LIMIT s, 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;

比如：我只想看一些，数学的第一名：

mysql> select id,name,math from exam order by math desc limit 1 offset 0;
+----+------+------+
| id | name | math |
+----+------+------+
|  7 | bb   |   86 |
+----+------+------+
1 row in set (0.00 sec)

我想看数学的前三名：

mysql> select id,name,math from exam order by math desc limit 3 offset 0;
+----+------+------+
| id | name | math |
+----+------+------+
|  7 | bb   |   86 |
|  3 | gg   |   77 |
|  2 | xm   |   76 |
+----+------+------+
3 rows in set (0.00 sec)

我想看从[4,6]名：

mysql> select id,name,math from exam order by math desc limit 3 offset 3;
+----+------+------+
| id | name | math |
+----+------+------+
|  1 | bob  | 75.4 |
|  5 | ll   |   74 |
|  4 | ss   | 72.4 |
+----+------+------+
3 rows in set (0.00 sec)

5. 更改表中的数据

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

5.1 更新单列数据

比如我想用更改bob的语文成绩：

mysql> select name,chinese from exam where name like'bob';
+------+---------+
| name | chinese |
+------+---------+
| bob  |    88.5 |
+------+---------+
1 row in set (0.00 sec)

mysql> update exam  set chinese=90 where name like'bob';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select name,chinese from exam where name like'bob';
+------+---------+
| name | chinese |
+------+---------+
| bob  |      90 |
+------+---------+
1 row in set (0.00 sec)

mysql>

5.2 更新多列的数据

假如我要更新bob的语文和数学成绩：

mysql> update exam  set chinese=84,math=90 where name like'bob';
Query OK, 1 row affected (0.02 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select name,chinese,math from exam where name like'bob';
+------+---------+------+
| name | chinese | math |
+------+---------+------+
| bob  |      84 |   90 |
+------+---------+------+
1 row in set (0.00 sec)

更新表中数据，如果不加上 where条件句的限制，那就是全列更新。但是全列更新这个操作用的不多，要慎用。update操作当然不仅是简单的修改值，还可以：比如说要把某个人的语文成绩+30，update exam set chinese=chinese+30 where name like'bob';或者说语文成绩变成原来的2倍，update exam set chinese=chinese*2 where name like'bob。像上面的操作都是可以的，不过不支持什么 += ，*= 等复合操作。

6. 删除表中数据

6.1 删除表中一个数据或或者多个数据

要求删除bob的成绩：

mysql> select * from exam;
+----+------+---------+------+---------+
| id | name | chinese | math | english |
+----+------+---------+------+---------+
|  1 | bob  |      84 |   90 |    98.5 |
|  2 | xm   |      88 |   76 |      93 |
|  3 | gg   |      76 |   77 |      98 |
|  4 | ss   |    82.5 | 72.4 |    98.5 |
|  5 | ll   |      88 |   74 |      97 |
|  6 | ff   |      75 |   72 |      93 |
|  7 | bb   |      87 |   86 |      76 |
+----+------+---------+------+---------+
7 rows in set (0.00 sec)

mysql> delete from exam where name='bob';
Query OK, 1 row affected (0.02 sec)

mysql> select * from exam;
+----+------+---------+------+---------+
| id | name | chinese | math | english |
+----+------+---------+------+---------+
|  2 | xm   |      88 |   76 |      93 |
|  3 | gg   |      76 |   77 |      98 |
|  4 | ss   |    82.5 | 72.4 |    98.5 |
|  5 | ll   |      88 |   74 |      97 |
|  6 | ff   |      75 |   72 |      93 |
|  7 | bb   |      87 |   86 |      76 |
+----+------+---------+------+---------+
6 rows in set (0.01 sec)

我现在要求删除xm和gg的成绩：

mysql> delete from exam where name='xm' or  name='gg';
Query OK, 2 rows affected (0.00 sec)

mysql> select * from exam;
+----+------+---------+------+---------+
| id | name | chinese | math | english |
+----+------+---------+------+---------+
|  4 | ss   |    82.5 | 72.4 |    98.5 |
|  5 | ll   |      88 |   74 |      97 |
|  6 | ff   |      75 |   72 |      93 |
|  7 | bb   |      87 |   86 |      76 |
+----+------+---------+------+---------+
4 rows in set (0.00 sec)

6.2 删除全表的数据

这个操作要慎用：

delete from 表名。

这个操作就不演示了，不过会在下面进行讲解。

6.3 截断表，不可以恢复的删除

像是上面的delete操作，之后都是可以恢复数据的，这个可以通过MySQL的日志功能进行恢复。但是truncate操作之后，是不能恢复数据的。

我创建一个表test，专门用于实验这点:

mysql> CREATE TABLE for_delete (
    -> id INT PRIMARY KEY AUTO_INCREMENT,
    -> name VARCHAR(20));
Query OK, 0 rows affected (0.03 sec)

mysql> INSERT INTO for_delete (name) VALUES ('A'), ('B'), ('C');
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from for_delete;
+----+------+
| id | name |
+----+------+
|  1 | A    |
|  2 | B    |
|  3 | C    |
+----+------+
3 rows in set (0.00 sec)

mysql> delete from for_delete;
Query OK, 3 rows affected (0.01 sec)

mysql> select * from for_delete;
Empty set (0.00 sec)

mysql> insert into for_delete(name) values('W');
Query OK, 1 row affected (0.00 sec)

mysql> select * from for_delete;
+----+------+
| id | name |
+----+------+
|  4 | W    |
+----+------+
1 row in set (0.00 sec)

mysql>

可以看到，delete全表后，继续插入数据，自增的id 是有记录的，它自动从id=3后进行增加。为什么？我们来查看表结构：

在这里插入图片描述
这里是有记录的，说明下次自增的话，id就会是5。

那么我们再来实验一下，truncate操作：

mysql> CREATE TABLE for_truncate( id INT PRIMARY KEY AUTO_INCREMENT, name VARCHAR(20));
Query OK, 0 rows affected (0.02 sec)

mysql> INSERT INTO for_truncate (name) VALUES ('A'), ('B'), ('C');
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from for_truncate;
+----+------+
| id | name |
+----+------+
|  1 | A    |
|  2 | B    |
|  3 | C    |
+----+------+
3 rows in set (0.00 sec)

mysql> truncate for_truncate;
Query OK, 0 rows affected (0.02 sec)

mysql> select * from for_truncate;
Empty set (0.00 sec)

mysql> 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 DEFAULT CHARSET=latin1
1 row in set (0.00 sec)

ERROR: 
No query specified

mysql> insert into for_truncate (name) values('W');
Query OK, 1 row affected (0.02 sec)

mysql> select * from for_truncate;
+----+------+
| id | name |
+----+------+
|  1 | W    |
+----+------+
1 row in set (0.00 sec)

mysql>

从这里就能看到，truncate是没有保留以前的自增长的。

7. 去重操作

我创建一个新的表，演示去重操作：

mysql> create table test (id int,name varchar(20));
Query OK, 0 rows affected (0.02 sec)

mysql> insert into test values(1,'bb'),(2,'yy'),(3,'bb'),(1,'bb');
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> select * from test;
+------+------+
| id   | name |
+------+------+
|    1 | bb   |
|    2 | yy   |
|    3 | bb   |
|    1 | bb   |
+------+------+
4 rows in set (0.00 sec)

mysql> select distinct * from test ;
+------+------+
| id   | name |
+------+------+
|    1 | bb   |
|    2 | yy   |
|    3 | bb   |
+------+------+
3 rows in set (0.00 sec)

用distinct修饰字段，就是去重操作。

mysql> select distinct name from test ;
+------+
| name |
+------+
| bb   |
| yy   |
+------+

8. 聚合函数

count ：返回查询到的数据的数量总和
sum: 返回查询到的数据的总和
avg：返回查询到的数据的平均值
max：返回查询到的数据的最大值
min: 返回查询到的数据的最小值

注意：null不会加入聚合函数的计算。

比如：

我想看一下表中有多少数据：

mysql> select * from students;
+----+-----+--------+-------+
| id | sn  | name   | qq    |
+----+-----+--------+-------+
|  3 | 102 | 开学   | 33333 |
| 11 | 111 | 王五   | 4444  |
| 12 | 222 | 刘六   | 55555 |
| 13 | 100 | 发财   | NULL  |
+----+-----+--------+-------+
4 rows in set (0.00 sec)

mysql> select count(*) from students;
+----------+
| count(*) |
+----------+
|        4 |
+----------+
1 row in set (0.00 sec)

mysql> select count(id) from students;
+-----------+
| count(id) |
+-----------+
|         4 |
+-----------+
1 row in set (0.00 sec

聚合函数中的sum使用，sum就是给一列进行求和：

mysql> select * from exam;
+----+------+---------+------+---------+
| id | name | chinese | math | english |
+----+------+---------+------+---------+
|  4 | ss   |    82.5 | 72.4 |    98.5 |
|  5 | ll   |      88 |   74 |      97 |
|  6 | ff   |      75 |   72 |      93 |
|  7 | bb   |      87 |   86 |      76 |
+----+------+---------+------+---------+
4 rows in set (0.00 sec)

mysql> select sum(math) from exam;
+-------------------+
| sum(math)         |
+-------------------+
| 304.4000015258789 |
+-------------------+
1 row in set (0.00 sec)

mysql> select sum(math+chinese) from exam;
+-------------------+
| sum(math+chinese) |
+-------------------+
| 636.9000015258789 |
+-------------------+
1 row in set (0.00 sec)

mysql> select sum(math+chinese+english) from exam;
+---------------------------+
| sum(math+chinese+english) |
+---------------------------+
|        1001.4000015258789 |
+---------------------------+
1 row in set (0.00 sec)

mysql>

其实像上面math+chinese，math+chinese+english都可以看作一列。

统计平均分用avg：

mysql> select avg(math+chinese+english) from exam;
+---------------------------+
| avg(math+chinese+english) |
+---------------------------+
|        250.35000038146973 |
+---------------------------+
1 row in set (0.00 sec)

看一下数学最高分，上面也演示过看最高分，不过用的方法是order by + limit的方式：

mysql> select max(math) from exam;
+-----------+
| max(math) |
+-----------+
|        86 |
+-----------+
1 row in set (0.00 sec)

看一下最低数学成绩：

mysql> select min(math) from exam;
+-----------+
| min(math) |
+-----------+
|        72 |
+-----------+
1 row in set (0.00 sec)