MySQL基本查询

表的增删查改

CRUD : Create(创建), Retrieve(读取)， Update(更新)， Delete（删除）

Create（创建）

基本语法：

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

案例：

mysql> create table students (-> id int unsigned primary key auto_increment,-> sn int not null unique comment '学号',-> name varchar(20) not null,-> email varchar(20)-> )engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.03 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    |                |
| email | varchar(20)      | YES  |     | NULL    |                |
+-------+------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

---

单行数据 + 全列插入

插入两条记录，当value_list 数量和定义表的列的数量及顺序一致时，就可以省略value_list。注意，这里在插入的时候，也可以不用指定id，mysql会使用默认的值进行自增。

mysql> insert into students values (100, 1000, 'Curry', NULL);
Query OK, 1 row affected (0.01 sec)mysql> insert into students values (101, 1001, 'Durant', '[e-mail]');
Query OK, 1 row affected (0.00 sec)mysql> select * from students;
+-----+------+--------+--------------+
| id  | sn   | name   | email        |
+-----+------+--------+--------------+
| 100 | 1000 | Curry  | NULL         |
| 101 | 1001 | Durant | [e-mail] |
+-----+------+--------+--------------+
2 rows in set (0.00 sec)

---

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

mysql> insert into students (id, sn, name) values (102, 1002, 'Kobe'), (103, 1003, 'Klay');
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0mysql> select * from students;
+-----+------+--------+--------------+
| id  | sn   | name   | email        |
+-----+------+--------+--------------+
| 100 | 1000 | Curry  | NULL         |
| 101 | 1001 | Durant | [e-mail] |
| 102 | 1002 | Kobe   | NULL         |
| 103 | 1003 | Klay   | NULL         |
+-----+------+--------+--------------+
4 rows in set (0.00 sec)

---

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：

mysql> insert into students (id, sn, name) values (100, 1004, 'Brown');
ERROR 1062 (23000): Duplicate entry '100' for key 'PRIMARY'

唯一键冲突：

mysql> insert into students (id, sn, name) values (104, 1003, 'Bryant');
ERROR 1062 (23000): Duplicate entry '1003' 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] ...

mysql> insert into students (id, sn, name) values (104, 1003, 'Bryant')-> on duplicate key update id=104, name='Bryant';
Query OK, 2 rows affected (0.01 sec)mysql> select * from students;
+-----+------+--------+--------------+
| id  | sn   | name   | email        |
+-----+------+--------+--------------+
| 100 | 1000 | Curry  | NULL         |
| 101 | 1001 | Durant | [e-mail] |
| 102 | 1002 | Kobe   | NULL         |
| 104 | 1003 | Bryant | NULL         |
+-----+------+--------+--------------+
4 rows in set (0.00 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

---

替换

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

mysql> replace into students (sn, name) values (1002, 'Mitchell');
Query OK, 2 rows affected (0.00 sec)mysql> select * from students;
+-----+------+----------+--------------+
| id  | sn   | name     | email        |
+-----+------+----------+--------------+
| 100 | 1000 | Curry    | NULL         |
| 101 | 1001 | Durant   | [e-mail] |
| 104 | 1003 | Bryant   | NULL         |
| 105 | 1002 | Mitchell | NULL         |
+-----+------+----------+--------------+
4 rows in set (0.00 sec)

• 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

---

Retrieve（读取）

基础语法：

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

案例：

创建表结构：

mysql> create table exam_result (-> id int unsigned primary key auto_increment,-> name varchar(20) not null comment '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'-> )engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.02 sec)

插入测试数据：

mysql> 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

---

SELECT列

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；

mysql> 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)

---

指定列查询

指定列的顺序不需要按定义表的顺序来

mysql> select id, name, math from exam_result;
+----+-----------+------+
| id | name      | math |
+----+-----------+------+
|  1 | 唐三藏    |   98 |
|  2 | 孙悟空    |   78 |
|  3 | 猪悟能    |   98 |
|  4 | 曹孟德    |   84 |
|  5 | 刘玄德    |   85 |
|  6 | 孙权      |   73 |
|  7 | 宋公明    |   65 |
+----+-----------+------+
7 rows in set (0.00 sec)

---

查询字段为表达式

表达式不包含字段：

mysql> 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：

mysql> select id, name, math+10 from exam_result;
+----+-----------+---------+
| id | name      | math+10 |
+----+-----------+---------+
|  1 | 唐三藏    |     108 |
|  2 | 孙悟空    |      88 |
|  3 | 猪悟能    |     108 |
|  4 | 曹孟德    |      94 |
|  5 | 刘玄德    |      95 |
|  6 | 孙权      |      83 |
|  7 | 宋公明    |      75 |
+----+-----------+---------+
7 rows in set (0.00 sec)

表达式包含多个字段：

mysql> select id, name, math+chinese+english from exam_result;
+----+-----------+----------------------+
| id | name      | math+chinese+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)

---

查询结果指定别名

基础语法：

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

mysql> select id, name, math+chinese+english total from exam_result;
+----+-----------+-------+
| id | name      | total |
+----+-----------+-------+
|  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)

---

结果去重

查询结果重复：

mysql> select math from exam_result;
+------+
| math |
+------+
|   98 |
|   78 |
|   98 |
|   84 |
|   85 |
|   73 |
|   65 |
+------+
7 rows in set (0.00 sec)

查询结果去重：

mysql> select distinct math from exam_result;
+------+
| math |
+------+
|   98 |
|   78 |
|   84 |
|   85 |
|   73 |
|   65 |
+------+
6 rows in set (0.00 sec)

---

WHERE 条件

基本比较

英语不及格的同学及英语成绩 ( < 60 )：

mysql> 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 (0.00 sec)

---

BETWEEN AND 条件连接

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

mysql> 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 条件连接

mysql> 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)

---

OR 条件连接

数学成绩是 58 或者 59 或者 98 或者 99 分的同学及数学成绩：

mysql> 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 (0.00 sec)

---

IN 条件连接

数学成绩是 58 或者 59 或者 98 或者 99 分的同学及数学成绩：

mysql> 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)

---

LIKE 条件匹配

查找姓孙的同学：% 匹配任意多个（包括 0 个）任意字符

mysql> select name from exam_result where name like '孙%';
+-----------+
| name      |
+-----------+
| 孙悟空    |
| 孙权      |
+-----------+
2 rows in set (0.00 sec)

查找孙某同学： _ 匹配严格的一个任意字符

mysql> select name from exam_result where name like '孙_';
+--------+
| name   |
+--------+
| 孙权   |
+--------+
1 row in set (0.00 sec)

---

WHERE 条件中使用表达式

总分在 200 分以下的同学：

mysql> select name, chinese+math+english total from exam_result where total<200;
ERROR 1054 (42S22): Unknown column 'total' in 'where clause'

这里我们发现一个问题，where条件查询中不能使用指定别名，这是因为chinese+math+english这个字句比where total<200字句先执行，所以MySQL并不认识total这个别名，就会报错。

正确写法：

mysql> select name, chinese+math+english total from exam_result where chinese+math+english<200;
+-----------+-------+
| name      | total |
+-----------+-------+
| 刘玄德    |   185 |
| 宋公明    |   170 |
+-----------+-------+
2 rows in set (0.00 sec)

---

AND 与 NOT 的使用

语文成绩 > 80 并且不姓孙的同学：

mysql> select name,chinese from exam_result where chinese>80 and name not like '孙%';
+-----------+---------+
| name      | chinese |
+-----------+---------+
| 猪悟能    |      88 |
| 曹孟德    |      82 |
+-----------+---------+
2 rows in set (0.00 sec)

---

综合性查询

查询孙某同学，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：

mysql> select name,chinese,math,english,chinese+math+english total from exam_result-> where (name like '孙_') or (chinese+math+english>200 and chinese<math and english>80);
+-----------+---------+------+---------+-------+
| name      | chinese | math | english | total |
+-----------+---------+------+---------+-------+
| 猪悟能    |      88 |   98 |      90 |   276 |
| 孙权      |      70 |   73 |      78 |   221 |
+-----------+---------+------+---------+-------+
2 rows in set (0.00 sec)

---

NULL的查询

查询 email 号已知的同学姓名：

mysql> select name from students where email is not null;
+--------+
| name   |
+--------+
| Durant |
+--------+
1 row in set (0.00 sec)

NULL 和 NULL 的比较，= 和 <=> 的区别：

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

---

结果排序

基本语法：

-- 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], [...];

升序显示

查询姓名及数学成绩，按数学成绩升序显示：

mysql> 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)

---

降序排序

查询姓名 及 eamil，按 eamil排序显示：

mysql> select name,email from students order by email;
+----------+--------------+
| name     | email        |
+----------+--------------+
| Curry    | NULL         |
| Bryant   | NULL         |
| Mitchell | NULL         |
| Durant   | [e-mail] |
+----------+--------------+
4 rows in set (0.00 sec)

NULL 视为比任何值都小，升序出现在最上面

---

多字段排序

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：

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

多字段排序，排序优先级随书写顺序

---

ORDER BY 使用表达式

查询同学及总分，由高到低：

mysql> select name, chinese+math+english from exam_result order by chinese+math+english desc;
+-----------+----------------------+
| name      | chinese+math+english |
+-----------+----------------------+
| 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：

mysql> select name, chinese+math+english total from exam_result order by total desc;
+-----------+-------+
| name      | total |
+-----------+-------+
| 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)

---

结合 WHERE 子句 和 ORDER BY 子句

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：

mysql> 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)

---

筛选分页结果

基础语法：

-- 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;

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。

案例：第 1 页：

mysql> select id, name, chinese, math, english from exam_result order by id limit 3 offset 0;
+----+-----------+---------+------+---------+
| 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 |
+----+-----------+---------+------+---------+
3 rows in set (0.00 sec)

第 2 页：

mysql> select id, name, chinese, math, english from exam_result order by id limit 3 offset 3;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  4 | 曹孟德    |      82 |   84 |      67 |
|  5 | 刘玄德    |      55 |   85 |      45 |
|  6 | 孙权      |      70 |   73 |      78 |
+----+-----------+---------+------+---------+
3 rows in set (0.00 sec)

第 3 页，如果结果不足 3 个，不会有影响：

mysql> select id, name, chinese, math, english from exam_result order by id limit 3 offset 6;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  7 | 宋公明    |      75 |   65 |      30 |
+----+-----------+---------+------+---------+
1 row in set (0.00 sec)

---

Update（更新）

基本语法：

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

更新单列

将孙悟空同学的数学成绩变更为 80 分：

mysql> select name, math from exam_result where name='孙悟空';
+-----------+------+
| name      | math |
+-----------+------+
| 孙悟空    |   78 |
+-----------+------+
1 row in set (0.00 sec)mysql> update exam_result set math=80 where name='孙悟空';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0mysql> select name, math from exam_result where name='孙悟空';
+-----------+------+
| name      | math |
+-----------+------+
| 孙悟空    |   80 |
+-----------+------+
1 row in set (0.00 sec)

---

更新多列

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：

mysql> 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)mysql> update exam_result set math=60, chinese=70 where name='曹孟德';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0mysql> 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)

---

更新值为原值基础上变更

将总成绩倒数前三的 3 位同学的数学成绩加上 30 分：

mysql> update exam_result set math=math+30 order by chinese+math+english limit 3;
Query OK, 3 rows affected (0.00 sec)
Rows matched: 3  Changed: 3  Warnings: 0mysql> select name, math from exam_result;
+-----------+------+
| name      | math |
+-----------+------+
| 唐三藏    |   98 |
| 孙悟空    |   80 |
| 猪悟能    |   98 |
| 曹孟德    |   90 |
| 刘玄德    |  115 |
| 孙权      |   73 |
| 宋公明    |   95 |
+-----------+------+
7 rows in set (0.00 sec)

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更新全表

将所有同学的语文成绩更新为原来的 2 倍：

mysql> select name, chinese from exam_result;
+-----------+---------+
| name      | chinese |
+-----------+---------+
| 唐三藏    |      67 |
| 孙悟空    |      87 |
| 猪悟能    |      88 |
| 曹孟德    |      70 |
| 刘玄德    |      55 |
| 孙权      |      70 |
| 宋公明    |      75 |
+-----------+---------+
7 rows in set (0.00 sec)mysql> update exam_result set chinese=chinese*2;
Query OK, 7 rows affected (0.00 sec)
Rows matched: 7  Changed: 7  Warnings: 0mysql> select name, chinese from exam_result;
+-----------+---------+
| name      | chinese |
+-----------+---------+
| 唐三藏    |     134 |
| 孙悟空    |     174 |
| 猪悟能    |     176 |
| 曹孟德    |     140 |
| 刘玄德    |     110 |
| 孙权      |     140 |
| 宋公明    |     150 |
+-----------+---------+
7 rows in set (0.00 sec)

---

Delete（删除）

基础语法：

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

删除单条记录

删除孙悟空同学的考试成绩：

mysql> 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)mysql> delete from exam_result where name='孙悟空';
Query OK, 1 row affected (0.00 sec)mysql> select * from exam_result where name='孙悟空';
Empty set (0.00 sec)

---

删除整表

注意：删除整表操作要慎用！

准备测试表：

mysql> create table for_delete (-> id int unsigned primary key auto_increment,-> name varchar(20)-> )engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.02 sec)

插入测试数据：

mysql> insert into for_delete (name) values ('a'), ('b'), ('c');
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0mysql> 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：

mysql> delete from for_delete;
Query OK, 3 rows affected (0.00 sec)mysql> 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：

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

mysql> show create table for_delete \G
*************************** 1. row ***************************Table: for_delete
Create Table: CREATE TABLE `for_delete` (`id` int(10) unsigned 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)

说明：虽然delete语句删除了整表，但是再向删除后的表插入时，表中的自增值会在之前的原数据的基础之上增加。

---

截断表

基础语法：

TRUNCATE [TABLE] table_name

1. TRUNCATE 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

准备测试表：

mysql> create table for_truncate (-> id int unsigned primary key auto_increment,-> name varchar(20)-> )engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.02 sec)

插入测试数据：

mysql> insert into for_truncate (name) values ('a'), ('b'), ('c');
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0mysql> 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，注意影响行数是 0，所以实际上没有对数据真正操作：

mysql> truncate for_truncate;
Query OK, 0 rows affected (0.02 sec)mysql> select * from for_truncate;
Empty set (0.00 sec)

再插入一条数据，自增 id 在重新增长：

mysql> insert into for_truncate (name) values ('d');
Query OK, 1 row affected (0.00 sec)mysql> 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：

mysql> show create table for_truncate \G
*************************** 1. row ***************************Table: for_truncate
Create Table: CREATE TABLE `for_truncate` (`id` int(10) unsigned 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)

---

插入查询结果

基础语法：

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

案例：删除表中的的重复记录，重复的数据只能有一份

创建原数据表，插入测试数据：

mysql> create table duplicate_table (-> id int,-> name varchar(20)-> );
Query OK, 0 rows affected (0.04 sec)mysql> 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

创建一张空表 no_duplicate_table结构和 duplicate_table结构一样：

mysql> create table no_duplicate_table like duplicate_table;
Query OK, 0 rows affected (0.01 sec)mysql> desc no_duplicate_table;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

将 duplicate_table 的去重数据插入到 no_duplicate_table：

mysql> insert into no_duplicate_table select distinct * from duplicate_table;
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0mysql> select * from no_duplicate_table;
+------+------+
| id   | name |
+------+------+
|  100 | aaa  |
|  200 | bbb  |
|  300 | ccc  |
+------+------+
3 rows in set (0.00 sec)

通过重命名表，实现原子的去重操作：

mysql> alter table duplicate_table rename to duplicate_table_bak;
Query OK, 0 rows affected (0.00 sec)mysql> alter table no_duplicate_table rename to duplicate_table;
Query OK, 0 rows affected (0.01 sec)

---

聚合函数

案例：

统计班级共有多少同学：

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

统计班级收集的 email 有多少：

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

统计本次考试的数学成绩分数个数：

统计全部成绩：

mysql> select count(math) from exam_result;
+-------------+
| count(math) |
+-------------+
|           6 |
+-------------+
1 row in set (0.00 sec)

统计去重成绩数量：

mysql> select count(distinct math) from exam_result;
+----------------------+
| count(distinct math) |
+----------------------+
|                    5 |
+----------------------+
1 row in set (0.00 sec)

统计数学成绩总分：

mysql> select sum(math) from exam_result;
+-----------+
| sum(math) |
+-----------+
|       569 |
+-----------+
1 row in set (0.00 sec)

统计平均总分：

mysql> select avg(chinese+math+english) from exam_result;
+---------------------------+
| avg(chinese+math+english) |
+---------------------------+
|                     297.5 |
+---------------------------+
1 row in set (0.00 sec)

返回英语最高分：

mysql> select max(english) from exam_result;
+--------------+
| max(english) |
+--------------+
|           90 |
+--------------+
1 row in set (0.00 sec)

返回 > 70 分以上的数学最低分：

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

---

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

基本语法：

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

案例：

准备工作，创建一个雇员信息表

• EMP员工表
• DEPT部门表
• SALGRADE工资等级表

mysql> desc dept;
+--------+--------------------------+------+-----+---------+-------+
| Field  | Type                     | Null | Key | Default | Extra |
+--------+--------------------------+------+-----+---------+-------+
| deptno | int(2) unsigned zerofill | NO   |     | NULL    |       |
| dname  | varchar(14)              | YES  |     | NULL    |       |
| loc    | varchar(13)              | YES  |     | NULL    |       |
+--------+--------------------------+------+-----+---------+-------+
3 rows in set (0.00 sec)mysql> desc emp;
+----------+--------------------------+------+-----+---------+-------+
| Field    | Type                     | Null | Key | Default | Extra |
+----------+--------------------------+------+-----+---------+-------+
| empno    | int(6) unsigned zerofill | NO   |     | NULL    |       |
| ename    | varchar(10)              | YES  |     | NULL    |       |
| job      | varchar(9)               | YES  |     | NULL    |       |
| mgr      | int(4) unsigned zerofill | YES  |     | NULL    |       |
| hiredate | datetime                 | YES  |     | NULL    |       |
| sal      | decimal(7,2)             | YES  |     | NULL    |       |
| comm     | decimal(7,2)             | YES  |     | NULL    |       |
| deptno   | int(2) unsigned zerofill | YES  |     | NULL    |       |
+----------+--------------------------+------+-----+---------+-------+
8 rows in set (0.00 sec)mysql> desc salgrade;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| grade | int(11) | YES  |     | NULL    |       |
| losal | int(11) | YES  |     | NULL    |       |
| hisal | int(11) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
3 rows in set (0.00 sec)

显示每个部门的平均工资和最高工资：

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

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) avg, min(sal) min from emp group by deptno, job;
+--------+-----------+-------------+---------+
| deptno | job       | avg         | min     |
+--------+-----------+-------------+---------+
|     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)

平均工资低于2000的部门和它的平均工资：

mysql> select deptno, avg(sal) avg from emp group by deptno having avg < 2000;
+--------+-------------+
| deptno | avg         |
+--------+-------------+
|     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，但是having通常在数据where选择完，group by进行分组，再执行having筛选。

---