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前言

CURD : create(创建),retrieve(读取),update(更新),delete(删除).

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正文开始!!!

一、create

语法

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

案例:

# 创建一张学生表
mysql> create table if not exists students(-> id int unsigned primary key auto_increment,-> sn int unsigned unique key not null comment '学生的学号',-> name varchar(64) not null comment '学生的姓名',-> qq varchar(20) unique key-> );
Query OK, 0 rows affected (0.02 sec)

1. 单行数据 + 全列插入

-- 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。
mysql> insert into students values (1,61,'路飞','111@qq.com');
Query OK, 1 row affected (0.00 sec)mysql> insert into students values (2,6,'索隆',null);
Query OK, 1 row affected (0.01 sec)-- 查看插入结果
mysql> select * from students;
+----+----+--------+------------+
| id | sn | name   | qq         |
+----+----+--------+------------+
|  1 | 61 | 路飞   | 111@qq.com |
|  2 |  6 | 索隆   | NULL       |
+----+----+--------+------------+
2 rows in set (0.00 sec

2. 多行数据 + 指定列插入

-- 插入三条记录，value_list 数量必须和指定列数量及顺序一致
mysql> insert into students (sn,name,qq) values (62,'乌索普','222@qq.com'),-> (63,'娜美','333@qq.com'),-> (64,'乔巴',null);
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0-- 查看插入结果
mysql> select * from students;
+----+----+-----------+------------+
| id | sn | name      | qq         |
+----+----+-----------+------------+
|  1 | 61 | 路飞      | 111@qq.com |
|  2 |  6 | 索隆      | NULL       |
|  3 | 62 | 乌索普    | 222@qq.com |
|  4 | 63 | 娜美      | 333@qq.com |
|  5 | 64 | 乔巴      | NULL       |
+----+----+-----------+------------+
5 rows in set (0.00 sec)

3. 插入冲突否则更新

由于 主键 和 唯一键 对于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 (1,66,'弗兰奇');
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'-- 唯一键冲突
mysql> insert into students (id,sn,name) values (10,6,'弗兰奇');
ERROR 1062 (23000): Duplicate entry '6' for key 'sn'

可以选择性的进行同步更新操作语法:

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

mysql> select * from students;
+----+----+-----------+------------+
| id | sn | name      | qq         |
+----+----+-----------+------------+
|  1 | 61 | 路飞      | 111@qq.com |
|  2 |  6 | 索隆      | NULL       |
|  3 | 62 | 乌索普    | 222@qq.com |
|  4 | 63 | 娜美      | 333@qq.com |
|  5 | 64 | 乔巴      | NULL       |
+----+----+-----------+------------+
5 rows in set (0.00 sec)mysql> insert into students (id,sn,name) values (2,66,'索隆') on duplicate key update sn=65,name='索隆';
Query OK, 2 rows affected (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mysql> select ROW_COUNT();
+-------------+
| ROW_COUNT() |
+-------------+
|           2 |
+-------------+
1 row in set (0.00 sec)mysql> select * from students;
+----+----+-----------+------------+
| id | sn | name      | qq         |
+----+----+-----------+------------+
|  1 | 61 | 路飞      | 111@qq.com |
|  2 | 65 | 索隆      | NULL       |
|  3 | 62 | 乌索普    | 222@qq.com |
|  4 | 63 | 娜美      | 333@qq.com |
|  5 | 64 | 乔巴      | NULL       |
+----+----+-----------+------------+
5 rows in set (0.00 sec) 
-- ON DUPLICATE KEY 当发生重复key的时候

4. 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-- 主键和唯一键都冲突了
mysql> replace into students (id,sn,name) values (2,62,'索隆');
Query OK, 3 rows affected (0.01 sec)mysql> select * from students;
+----+----+--------+------------+
| id | sn | name   | qq         |
+----+----+--------+------------+
|  1 | 61 | 路飞   | 111@qq.com |
|  2 | 62 | 索隆   | NULL       |
|  4 | 63 | 娜美   | 333@qq.com |
|  5 | 64 | 乔巴   | NULL       |
+----+----+--------+------------+
4 rows in set (0.00 sec)-- 1 row affected: 表中没有冲突数据，数据被插入
-- n row affected: 表中有冲突数据，删除后重新插入

二、retrieve

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 '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'-> );
Query OK, 0 rows affected (0.01 sec)-- 插入测试数据
mysql> INSERT INTO exam_result (name, chinese, math, english) VALUES('赖益烨', 67, 98, 56),('郭雨妍', 87, 78, 77),('马师傅', 88, 98, 90),('洋妈', 82, 84, 67),('李观洋', 55, 85, 45),('郭昊', 70, 73, 78),('张照洋', 75, 65, 30);
Query OK, 7 rows affected (0.01 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。（索引待后面课程讲解）
mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  1 | 赖益烨    |      67 |   98 |      56 |
|  2 | 郭雨妍    |      87 |   78 |      77 |
|  3 | 马师傅    |      88 |   98 |      90 |
|  4 | 洋妈      |      82 |   84 |      67 |
|  5 | 李观洋    |      55 |   85 |      45 |
|  6 | 郭昊      |      70 |   73 |      78 |
|  7 | 张照洋    |      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mysql> select name,id,math from exam_result;
+-----------+----+------+
| name      | id | math |
+-----------+----+------+
| 赖益烨    |  1 |   98 |
| 郭雨妍    |  2 |   78 |
| 马师傅    |  3 |   98 |
| 洋妈      |  4 |   84 |
| 李观洋    |  5 |   85 |
| 郭昊      |  6 |   73 |
| 张照洋    |  7 |   65 |
+-----------+----+------+
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
mysql> SELECT id, name, 10 FROM exam_result;
+----+-----------+----+
| id | name      | 10 |
+----+-----------+----+
|  1 | 赖益烨    | 10 |
|  2 | 郭雨妍    | 10 |
|  3 | 马师傅    | 10 |
|  4 | 洋妈      | 10 |
|  5 | 李观洋    | 10 |
|  6 | 郭昊      | 10 |
|  7 | 张照洋    | 10 |
+----+-----------+----+
7 rows in set (0.00 sec)mysql> SELECT id, name, 10+10 FROM exam_result;
+----+-----------+-------+
| id | name      | 10+10 |
+----+-----------+-------+
|  1 | 赖益烨    |    20 |
|  2 | 郭雨妍    |    20 |
|  3 | 马师傅    |    20 |
|  4 | 洋妈      |    20 |
|  5 | 李观洋    |    20 |
|  6 | 郭昊      |    20 |
|  7 | 张照洋    |    20 |
+----+-----------+-------+
7 rows in set (0.00 sec)mysql> select id,name,chinese+math+english from exam_result;
+----+-----------+----------------------+
| id | name      | chinese+math+english |
+----+-----------+----------------------+
|  1 | 赖益烨    |                  221 |
|  2 | 郭雨妍    |                  242 |
|  3 | 马师傅    |                  276 |
|  4 | 洋妈      |                  233 |
|  5 | 李观洋    |                  185 |
|  6 | 郭昊      |                  221 |
|  7 | 张照洋    |                  170 |
+----+-----------+----------------------+
7 rows in set (0.00 sec)mysql> select id,name,chinese+50 from exam_result;
+----+-----------+------------+
| id | name      | chinese+50 |
+----+-----------+------------+
|  1 | 赖益烨    |        117 |
|  2 | 郭雨妍    |        137 |
|  3 | 马师傅    |        138 |
|  4 | 洋妈      |        132 |
|  5 | 李观洋    |        105 |
|  6 | 郭昊      |        120 |
|  7 | 张照洋    |        125 |
+----+-----------+------------+
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;

mysql> select id,name,chinese+math+english 总分 from exam_result;
+----+-----------+--------+
| id | name      | 总分   |
+----+-----------+--------+
|  1 | 赖益烨    |    221 |
|  2 | 郭雨妍    |    242 |
|  3 | 马师傅    |    276 |
|  4 | 洋妈      |    233 |
|  5 | 李观洋    |    185 |
|  6 | 郭昊      |    221 |
|  7 | 张照洋    |    170 |
+----+-----------+--------+
7 rows in set (0.00 sec)

1.5 结果去重

mysql> select math from exam_result;
+------+
| math |
+------+
|   98 |
|   78 |
|   98 |
|   84 |
|   85 |
|   73 |
|   65 |
+------+
7 rows in set (0.00 sec)
# 98分重复啦mysql> 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	说明
>, >=, <, <=	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)

案例：

2.1 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 |
+-----------+---------+
| 赖益烨    |      56 |
| 李观洋    |      45 |
| 张照洋    |      30 |
+-----------+---------+
3 rows in set (0.00 sec)

2.2 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
mysql> select name,chinese from exam_result where chinese>=80 and chinese<=90;
+-----------+---------+
| name      | chinese |
+-----------+---------+
| 郭雨妍    |      87 |
| 马师傅    |      88 |
| 洋妈      |      82 |
+-----------+---------+
3 rows in set (0.00 sec)-- 使用 BETWEEN ... AND ... 条件
mysql> select name,chinese from exam_result where chinese between 80 and 90;
+-----------+---------+
| name      | chinese |
+-----------+---------+
| 郭雨妍    |      87 |
| 马师傅    |      88 |
| 洋妈      |      82 |
+-----------+---------+
3 rows in set (0.01 sec)

2.3 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
mysql> select name,math from exam_result where math=58 or math=59 or math=98 or math=99;
+-----------+------+
| name      | math |
+-----------+------+
| 赖益烨    |   98 |
| 马师傅    |   98 |
+-----------+------+
2 rows in set (0.00 sec)-- 使用 IN 条件
mysql> select name,math from exam_result where math in (58,59,98,99);
+-----------+------+
| name      | math |
+-----------+------+
| 赖益烨    |   98 |
| 马师傅    |   98 |
+-----------+------+
2 rows in set (0.00 sec)

2.4 姓郭的同学 及 郭某同学

-- % 匹配任意多个（包括 0 个）任意字符
mysql> select name from exam_result where name like '郭%';
+-----------+
| name      |
+-----------+
| 郭雨妍    |
| 郭昊      |
+-----------+
2 rows in set (0.00 sec)-- _ 匹配严格的一个任意字符
mysql> select name from exam_result where name like '郭_';
+--------+
| name   |
+--------+
| 郭昊   |
+--------+
1 row in set (0.00 sec)mysql> select name from exam_result where name like '郭__';
+-----------+
| name      |
+-----------+
| 郭雨妍    |
+-----------+
1 row in set (0.00 sec)

2.5 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
mysql> select name,chinese,english from exam_result where chinese>english;
+-----------+---------+---------+
| name      | chinese | english |
+-----------+---------+---------+
| 赖益烨    |      67 |      56 |
| 郭雨妍    |      87 |      77 |
| 洋妈      |      82 |      67 |
| 李观洋    |      55 |      45 |
| 张照洋    |      75 |      30 |
+-----------+---------+---------+
5 rows in set (0.00 sec)

2.6 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
mysql> select name,chinese+math+english total from exam_result where chinese+math+english<200;
+-----------+-------+
| name      | total |
+-----------+-------+
| 李观洋    |   185 |
| 张照洋    |   170 |
+-----------+-------+
2 rows in set (0.00 sec)

2.7 语文成绩 > 80 并且不姓孙的同学

-- AND 与 NOT 的使用
mysql> select name,chinese from exam_result where chinese>80 and name not like '孙%';
+-----------+---------+
| name      | chinese |
+-----------+---------+
| 郭雨妍    |      87 |
| 马师傅    |      88 |
| 洋妈      |      82 |
+-----------+---------+
3 rows in set (0.00 sec)

2.8 郭某同学，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

-- 综合性查询
mysql> select *,chinese+math+english total from exam_result where (name like '郭_') or (chinese+math+english>200 and chinese<math and english>80);
+----+-----------+---------+------+---------+-------+
| id | name      | chinese | math | english | total |
+----+-----------+---------+------+---------+-------+
|  3 | 马师傅    |      88 |   98 |      90 |   276 |
|  6 | 郭昊      |      70 |   73 |      78 |   221 |
+----+-----------+---------+------+---------+-------+
2 rows in set (0.00 sec)

2.9 NULL 的查询

-- 查询 students 表
mysql> select * from students where qq is null;
+----+----+--------+------+
| id | sn | name   | qq   |
+----+----+--------+------+
|  2 | 62 | 索隆   | NULL |
|  5 | 64 | 乔巴   | NULL |
+----+----+--------+------+
2 rows in set (0.00 sec)mysql> select * from students where qq<=> null;
+----+----+--------+------+
| id | sn | name   | qq   |
+----+----+--------+------+
|  2 | 62 | 索隆   | NULL |
|  5 | 64 | 乔巴   | NULL |
+----+----+--------+------+
2 rows in set (0.00 sec)-- 查询 qq 号已知的同学姓名
mysql> select * from students where qq is not null;
+----+----+--------+------------+
| id | sn | name   | qq         |
+----+----+--------+------------+
|  1 | 61 | 路飞   | 111@qq.com |
|  4 | 63 | 娜美   | 333@qq.com |
+----+----+--------+------------+
2 rows 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)

如何理解mysql在当前的select执行的时候,是怎么执行的???

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!!!

案例:

3.1 同学及数学成绩，按数学成绩升序显示

mysql> select name,math from exam_result order by math asc;
+-----------+------+
| name      | math |
+-----------+------+
| 张照洋    |   65 |
| 郭昊      |   73 |
| 郭雨妍    |   78 |
| 洋妈      |   84 |
| 李观洋    |   85 |
| 赖益烨    |   98 |
| 马师傅    |   98 |
+-----------+------+
7 rows in set (0.00 sec)

3.2 同学及 qq 号，按 qq 号排序显示

mysql> select * from students;
+----+----+-----------+------------+
| id | sn | name      | qq         |
+----+----+-----------+------------+
|  1 | 61 | 路飞      | 111@qq.com |
|  2 | 62 | 索隆      | NULL       |
|  4 | 63 | 娜美      | 333@qq.com |
|  5 | 64 | 乔巴      | NULL       |
|  6 | 65 | 弗兰奇    | 444@qq.com |
|  7 | 66 | 罗宾      | 555@qq.com |
|  8 | 67 | 布鲁克    | 123@qq.com |
+----+----+-----------+------------+
7 rows in set (0.00 sec)# 顺序
-- NULL 视为比任何值都小，升序出现在最上面
mysql> select name,qq from students order by qq asc;
+-----------+------------+
| name      | qq         |
+-----------+------------+
| 索隆      | NULL       |
| 乔巴      | NULL       |
| 路飞      | 111@qq.com |
| 布鲁克    | 123@qq.com |
| 娜美      | 333@qq.com |
| 弗兰奇    | 444@qq.com |
| 罗宾      | 555@qq.com |
+-----------+------------+
7 rows in set (0.00 sec)# 逆序
-- NULL 视为比任何值都小，降序出现在最下面
mysql> select name,qq from students order by qq desc;
+-----------+------------+
| name      | qq         |
+-----------+------------+
| 罗宾      | 555@qq.com |
| 弗兰奇    | 444@qq.com |
| 娜美      | 333@qq.com |
| 布鲁克    | 123@qq.com |
| 路飞      | 111@qq.com |
| 索隆      | NULL       |
| 乔巴      | NULL       |
+-----------+------------+
7 rows in set (0.00 sec)

3.3 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

-- 多字段排序，排序优先级随书写顺序
mysql> select name,math,english,chinese from exam_result order by math desc,english asc,chinese asc;
+-----------+------+---------+---------+
| name      | math | english | chinese |
+-----------+------+---------+---------+
| 赖益烨    |   98 |      56 |      67 |
| 马师傅    |   98 |      90 |      88 |
| 李观洋    |   85 |      45 |      55 |
| 洋妈      |   84 |      67 |      82 |
| 郭雨妍    |   78 |      77 |      87 |
| 郭昊      |   73 |      78 |      70 |
| 张照洋    |   65 |      30 |      75 |
+-----------+------+---------+---------+
7 rows in set (0.00 sec)

3.4 查询同学及总分，由高到低

-- ORDER BY 中可以使用表达式
mysql> select name,chinese+math+english total from exam_result order by chinese+math+english desc;
+-----------+-------+
| name      | total |
+-----------+-------+
| 马师傅    |   276 |
| 郭雨妍    |   242 |
| 洋妈      |   233 |
| 赖益烨    |   221 |
| 郭昊      |   221 |
| 李观洋    |   185 |
| 张照洋    |   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 |
+-----------+-------+
| 马师傅    |   276 |
| 郭雨妍    |   242 |
| 洋妈      |   233 |
| 赖益烨    |   221 |
| 郭昊      |   221 |
| 李观洋    |   185 |
| 张照洋    |   170 |
+-----------+-------+
7 rows in set (0.00 sec)

3.5 查询姓郭的同学或者姓洋的同学数学成绩，结果按数学成绩由高到低显示

-- 结合 WHERE 子句 和 ORDER BY 子句mysql> select name,math from exam_result where name like '郭%' or name like '洋%' order by math desc;
+-----------+------+
| name      | math |
+-----------+------+
| 洋妈      |   84 |
| 郭雨妍    |   78 |
| 郭昊      |   73 |
+-----------+------+
3 rows in set (0.00 sec)

4. 筛选分页结果

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!!!
案例:

mysql> select name,math from exam_result limit 3;
+-----------+------+
| name      | math |
+-----------+------+
| 赖益烨    |   98 |
| 郭雨妍    |   78 |
| 马师傅    |   98 |
+-----------+------+
3 rows in set (0.00 sec)mysql> select name,math from exam_result limit 3,3;
+-----------+------+
| name      | math |
+-----------+------+
| 洋妈      |   84 |
| 李观洋    |   85 |
| 郭昊      |   73 |
+-----------+------+
3 rows in set (0.00 sec)mysql> select name,math from exam_result limit 3 offset 6;
+-----------+------+
| name      | math |
+-----------+------+
| 张照洋    |   65 |
+-----------+------+
1 row in set (0.00 sec)mysql> select name,math from exam_result limit 3 offset 0;
+-----------+------+
| name      | math |
+-----------+------+
| 赖益烨    |   98 |
| 郭雨妍    |   78 |
| 马师傅    |   98 |
+-----------+------+
3 rows in set (0.00 sec)

按id进行分页,每页3条记录,分别显示 第1,2,3 页!!!

-- 第 1 页
mysql> SELECT id, name, math, english, chinese FROM exam_result-> ORDER BY id LIMIT 3 OFFSET 0;
+----+-----------+------+---------+---------+
| id | name      | math | english | chinese |
+----+-----------+------+---------+---------+
|  1 | 赖益烨    |   98 |      56 |      67 |
|  2 | 郭雨妍    |   78 |      77 |      87 |
|  3 | 马师傅    |   98 |      90 |      88 |
+----+-----------+------+---------+---------+
3 rows in set (0.00 sec)-- 第 2 页
mysql> SELECT id, name, math, english, chinese FROM exam_result-> ORDER BY id LIMIT 3 OFFSET 3;
+----+-----------+------+---------+---------+
| id | name      | math | english | chinese |
+----+-----------+------+---------+---------+
|  4 | 洋妈      |   84 |      67 |      82 |
|  5 | 李观洋    |   85 |      45 |      55 |
|  6 | 郭昊      |   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
mysql> SELECT id, name, math, english, chinese FROM exam_result-> ORDER BY id LIMIT 3 OFFSET 6;
+----+-----------+------+---------+---------+
| id | name      | math | english | chinese |
+----+-----------+------+---------+---------+
|  7 | 张照洋    |   65 |      30 |      75 |
+----+-----------+------+---------+---------+
1 row in set (0.00 sec)

四、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. 将郭雨妍同学的数学成绩变更为 80 分

-- 更新值为具体值-- 查看原数据
mysql> select name,math from exam_result where name='郭雨妍';
+-----------+------+
| name      | math |
+-----------+------+
| 郭雨妍    |   78 |
+-----------+------+
1 row in set (0.00 sec)-- 数据更新
mysql> update exam_result set math=80 where name='郭雨妍';
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0-- 查看更新后数据
mysql> select name,math from exam_result where name='郭雨妍';
+-----------+------+
| name      | math |
+-----------+------+
| 郭雨妍    |   80 |
+-----------+------+
1 row in set (0.01 sec)

2. 将洋同学的数学成绩变更为 60 分，语文成绩变更为 70 分

-- 一次更新多个列-- 查看原数据
mysql> select name,math,chinese from exam_result where name='洋妈';
+--------+------+---------+
| name   | math | chinese |
+--------+------+---------+
| 洋妈   |   84 |      82 |
+--------+------+---------+
1 row in set (0.00 sec)-- 数据更新
mysql> update exam_result set math=60,chinese=70 where name='洋妈';
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0-- 查看更新后数据
mysql> select name,math,chinese from exam_result where name='洋妈';
+--------+------+---------+
| name   | math | chinese |
+--------+------+---------+
| 洋妈   |   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

-- 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
mysql> select name,math,chinese+math+english total from exam_result order by total asc limit 3;
+-----------+------+-------+
| name      | math | total |
+-----------+------+-------+
| 张照洋    |   65 |   170 |
| 李观洋    |   85 |   185 |
| 洋妈      |   60 |   197 |
+-----------+------+-------+
3 rows in set (0.00 sec)-- 数据更新，不支持 math += 30 这种语法
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: 0-- 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？
mysql> select name,math,chinese+math+english total from exam_result where name in ('张照洋','李观洋','洋妈');
+-----------+------+-------+
| name      | math | total |
+-----------+------+-------+
| 洋妈      |   90 |   227 |
| 李观洋    |  115 |   215 |
| 张照洋    |   95 |   200 |
+-----------+------+-------+
3 rows in set (0.00 sec)
# 不可以,因为数学成绩+30分以后,成绩就不一定在前三名了!!!-- 按总成绩排序后查询结果
mysql> select name,math,chinese+math+english total from exam_result order by total asc limit 3;
+-----------+------+-------+
| name      | math | total |
+-----------+------+-------+
| 张照洋    |   95 |   200 |
| 李观洋    |  115 |   215 |
| 赖益烨    |   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

-- 没有 WHERE 子句，则更新全表-- 查看原数据
mysql> select name,chinese from exam_result;
+-----------+---------+
| name      | chinese |
+-----------+---------+
| 赖益烨    |      67 |
| 郭雨妍    |      87 |
| 马师傅    |      88 |
| 洋妈      |      70 |
| 李观洋    |      55 |
| 郭昊      |      70 |
| 张照洋    |      75 |
+-----------+---------+
7 rows in set (0.00 sec)-- 数据更新
mysql> 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
mysql> select name,chinese from exam_result;
+-----------+---------+
| name      | chinese |
+-----------+---------+
| 赖益烨    |     134 |
| 郭雨妍    |     174 |
| 马师傅    |     176 |
| 洋妈      |     140 |
| 李观洋    |     110 |
| 郭昊      |     140 |
| 张照洋    |     150 |
+-----------+---------+
7 rows in set (0.00 sec)

四、Delete

1. 删除数据

语法:

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

案例：

1.1 删除郭雨妍同学的考试成绩

-- 查看原数据
mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  1 | 赖益烨    |     134 |   98 |      56 |
|  2 | 郭雨妍    |     174 |   80 |      77 |
|  3 | 马师傅    |     176 |   98 |      90 |
|  4 | 洋妈      |     140 |   90 |      67 |
|  5 | 李观洋    |     110 |  115 |      45 |
|  6 | 郭昊      |     140 |   73 |      78 |
|  7 | 张照洋    |     150 |   95 |      30 |
+----+-----------+---------+------+---------+
7 rows in set (0.00 sec)-- 删除数据
mysql> delete from exam_result where name='郭雨妍';
Query OK, 1 row affected (0.00 sec)-- 查看删除结果
mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  1 | 赖益烨    |     134 |   98 |      56 |
|  3 | 马师傅    |     176 |   98 |      90 |
|  4 | 洋妈      |     140 |   90 |      67 |
|  5 | 李观洋    |     110 |  115 |      45 |
|  6 | 郭昊      |     140 |   73 |      78 |
|  7 | 张照洋    |     150 |   95 |      30 |
+----+-----------+---------+------+---------+
6 rows in set (0.00 sec)

2.2 删除整张表数据

注意：删除整表操作要慎用！

-- 查看测试数据
mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  1 | 赖益烨    |     134 |   98 |      56 |
|  3 | 马师傅    |     176 |   98 |      90 |
|  4 | 洋妈      |     140 |   90 |      67 |
|  5 | 李观洋    |     110 |  115 |      45 |
|  6 | 郭昊      |     140 |   73 |      78 |
|  7 | 张照洋    |     150 |   95 |      30 |
+----+-----------+---------+------+---------+
6 rows in set (0.00 sec)-- 删除整表数据
mysql> delete from exam_result;
Query OK, 6 rows affected (0.00 sec)-- 查看删除结果
mysql> select * from exam_result;
Empty set (0.00 sec)-- 再插入一条数据，自增 id 在原值上增长
mysql> insert into exam_result (name,chinese,math,english) values ('魏师傅',111,90,78);
Query OK, 1 row affected (0.00 sec)-- 查看数据
mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  8 | 魏师傅    |     111 |   90 |      78 |
+----+-----------+---------+------+---------+
1 row in set (0.00 sec)-- 查看表结构，会有 AUTO_INCREMENT=n 项
mysql> show create table exam_result\\G
*************************** 1. row ***************************Table: exam_result
Create Table: CREATE TABLE `exam_result` (`id` int(10) unsigned NOT NULL AUTO_INCREMENT,`name` varchar(20) NOT NULL COMMENT '同学姓名',`chinese` float DEFAULT '0' COMMENT '语文成绩',`math` float DEFAULT '0' COMMENT '数学成绩',`english` float DEFAULT '0' COMMENT '英语成绩',PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

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,并不经过真正的事务,所以无法回滚.
3. 会重置 auot_increment项.

案例:

-- 查看测试数据
mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  9 | 赖益烨    |      67 |   98 |      56 |
| 10 | 郭雨妍    |      87 |   78 |      77 |
| 11 | 马师傅    |      88 |   98 |      90 |
| 12 | 洋妈      |      82 |   84 |      67 |
| 13 | 李观洋    |      55 |   85 |      45 |
| 14 | 郭昊      |      70 |   73 |      78 |
| 15 | 张照洋    |      75 |   65 |      30 |
+----+-----------+---------+------+---------+
7 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  exam_result;
Query OK, 0 rows affected (0.01 sec)-- 查看删除结果
mysql> select * from exam_result;
Empty set (0.00 sec)-- 再插入一条数据，自增 id 在重新增长
mysql> insert into exam_result (name,chinese,math,english) values ('魏师傅',111,90,78);
Query OK, 1 row affected (0.00 sec)-- 查看数据
mysql> select * from exam_result;
+----+-----------+---------+------+---------+
| id | name      | chinese | math | english |
+----+-----------+---------+------+---------+
|  1 | 魏师傅    |     111 |   90 |      78 |
+----+-----------+---------+------+---------+
1 row in set (0.00 sec)-- 查看表结构，会有 AUTO_INCREMENT=2 项
mysql> show create table exam_result\\G
*************************** 1. row ***************************Table: exam_result
Create Table: CREATE TABLE `exam_result` (`id` int(10) unsigned NOT NULL AUTO_INCREMENT,`name` varchar(20) NOT NULL COMMENT '同学姓名',`chinese` float DEFAULT '0' COMMENT '语文成绩',`math` float DEFAULT '0' COMMENT '数学成绩',`english` float DEFAULT '0' COMMENT '英语成绩',PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

五、插入查询结果

语法:

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

案例:

-- 创建原数据表
mysql> create table duplicate_table(id int,name varchar(20));
Query OK, 0 rows affected (0.02 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 一样
mysql>  create table duplicate_table_backup like duplicate_table;
Query OK, 0 rows affected (0.02 sec)-- 将 duplicate_table 的去重数据插入到 no_duplicate_table
mysql> insert into duplicate_table_backup 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
mysql> alter table duplicate_table rename duplicate_table_old;
Query OK, 0 rows affected (0.01 sec)mysql> alter table duplicate_table_backup rename duplicate_table;
Query OK, 0 rows affected (0.00 sec)-- 查看最终结果
mysql> 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：

1. 统计班级共有多少同学

-- 使用 * 做统计，不受 NULL 影响
mysql> select count(*) 总人数 from exam_result;
+-----------+
| 总人数    |
+-----------+
|         7 |
+-----------+
1 row in set (0.00 sec)-- 使用表达式做统计
mysql> select count(1) 总人数 from exam_result;
+-----------+
| 总人数    |
+-----------+
|         7 |
+-----------+
1 row in set (0.00 sec)

2. 统计班级收集的 qq 号有多少

mysql> select * from students;
+----+----+-----------+------------+
| id | sn | name      | qq         |
+----+----+-----------+------------+
|  1 | 61 | 路飞      | 111@qq.com |
|  2 | 62 | 索隆      | NULL       |
|  4 | 63 | 娜美      | 333@qq.com |
|  5 | 64 | 乔巴      | NULL       |
|  6 | 65 | 弗兰奇    | 444@qq.com |
|  7 | 66 | 罗宾      | 555@qq.com |
|  8 | 67 | 布鲁克    | 123@qq.com |
|  9 | 68 | 山治      |            |
+----+----+-----------+------------+
8 rows in set (0.00 sec)-- NULL 不会计入结果
mysql> select count(qq) from students;
+-----------+
| count(qq) |
+-----------+
|         6 |
+-----------+
1 row in set (0.00 sec)

3.统计本次考试的数学成绩分数个数

-- COUNT(math) 统计的是全部成绩
mysql> select count(math) from exam_result;
+-------------+
| count(math) |
+-------------+
|           7 |
+-------------+
1 row in set (0.00 sec)-- count(distinct math)统计的是去重后的成绩
mysql> select count(distinct math) from exam_result;
+----------------------+
| count(distinct math) |
+----------------------+
|                    6 |
+----------------------+
1 row in set (0.00 sec)

4.统计数学成绩总分

mysql> select sum(math) from exam_result;
+-----------+
| sum(math) |
+-----------+
|       581 |
+-----------+
1 row in set (0.00 sec)-- 不及格 < 60 的总分，没有结果，返回 NULL
mysql> 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

mysql> SELECT AVG(chinese + math + english) 平均总分 FROM exam_result;
+--------------------+
| 平均总分           |
+--------------------+
| 221.14285714285714 |
+--------------------+
1 row in set (0.00 sec)

6.返回英语最高分

mysql> 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

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

聚合函数:聚合统计一定是直接或者间接统计列方向的某些数据.–>一定是相同属性的!

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;

我们以前select的时候,都是将数据作为一个整体.其实mysql可以支持按照指定的列进行对数据做分组,我们可以让特定的sql在特定的组内进行某种操作!

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
    oracle 9i的经典测试表

1.查看表结构

mysql> source scott_data.sql;mysql> use scott;
Database changed
mysql> show tables;
+-----------------+
| Tables_in_scott |
+-----------------+
| dept            |
| emp             |
| salgrade        |
+-----------------+
3 rows in set (0.00 sec)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)

2. 如何显示每个部门的平均工资和最高工资

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

3. 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)

4.显示平均工资低于2000的部门和它的平均工资

• 统计各个部门的平均工资

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)

• having和group by配合使用，对group by结果进行过滤

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。mysql> select deptno,avg(sal) from emp where 1=1 group by deptno having avg(sal)<2000;
+--------+-------------+
| deptno | avg(sal)    |
+--------+-------------+
|     30 | 1566.666667 |
+--------+-------------+
1 row in set (0.00 sec)
# 下面这个例子证明了where和having是可以共存的.mysql> select deptno,avg(sal) from emp where sal>1000 group by deptno having avg(sal)<2000;
+--------+-------------+
| deptno | avg(sal)    |
+--------+-------------+
|     30 | 1690.000000 |
+--------+-------------+
1 row in set (0.00 sec)

注意:

• group by是通过分组这样的手段,为未来进行聚合统计提供基本的功能支持(group一定是配合聚合统计使用的);
• group by 后面跟的都是分组的字段依据,只有在group by后面出现的字段,未来在聚合统计的时候,在select中才能出现.
• where VS having 不是冲突的,是互相补充的,having通常是在完成整个分组聚合统计,然后在进行筛选;where通常是在表中数据初步被筛选的时候起效果的.

---

总结

SQL查询中各个关键字的执行先后顺序 from > on> join > where > group by > with > having > select
> distinct > order by > limit;
(本章完!)