文章目录
- 什么是CRUD？
- 新增（Create）
- 单行数据 + 全列插入
- 多行数据 + 指定列插入
- 查询（Retrieve）
- 全列查询
- 指定列查询
- 查询字段为表达式
- 起别名查询
- 去重查询
- 排序查询
- 条件查询
- 分页查询
- 修改（Update）
- 删除（Delete）
什么是CRUD？
CRUD，即增加（Create）、查找（Retrieve）、修改（Update）、删除（Delete）四个单词的首字母缩写。
在进行下面所有操作的前提都是得选中一个数据库，并且已经创建了可以用来操作的表。
默认我们现在已经创建了learning数据库，在数据库中有一张student表。后续操作都基于此进行
新增（Create）
insert into 表名 values（值，值，值...）;
注：这里值的类型和个数要和表的 列的类型和个数匹配。
单行数据 + 全列插入
mysql> insert into student values (1,"zhangsan");
Query OK, 1 row affected (0.00 sec)
注：
- 在SQL中没有字符串类型，所以既可以用‘ ’来引用字符串，又可以使用“ ”来引用字符串；
- 还可以直接插入中文字符（需要把数据库字符集改为UTF-8）
多行数据 + 指定列插入
mysql> insert into student values (1,"zhangsan")，（2，"lisi"）,(3,"wangwu");
Query OK, 3 row affected (0.00 sec)
注：比一条一条插入更快
查询（Retrieve）
MySQL是一个客户端—服务器结构的程序，显示在客户端的查询结果是一个“临时表”，服务器端的数据并不是这样的组织形式。
全列查询
select* from 表名;
mysql> select * from student;
+------+----------+
| id | name |
+------+----------+
| 1 | zhangsan |
| 2 | lisi |
| 3 | wangwu |
+------+----------+
3 rows in set (0.00 sec)
注：
- 通配符* 表示匹配所有的列，即查询所有列，把所有的数据都查询出来；
- 这是一个危险操作（当数据量大的时候）：进行此操作的时候，服务器要先读取磁盘，把这些数据都查出来，再通过网卡，把这些数据传输给客户端，由于数据量非常大，极有可能把磁盘IO（输入输出）吃满，或者网络带宽吃满。这时其他数据就无法正常返回了，最直观的感受就是客户端感受到卡顿。
指定列查询
select 列名，列名，列名.... from 表名;
mysql> select id from student;
+------+
| id |
+------+
| 1 |
| 2 |
| 3 |
+------+
3 rows in set (0.00 sec)
注：当我们查询时省去一些不必要的列时，就可以节约大量的磁盘IO和网络带宽了。
查询字段为表达式
select 表达式 from 表名;
- 演示此操作需要创建一个新的表：
mysql> create table exam_result (id int, name varchar(20), chinese decimal(3,1),math decimal(3,1), english decimal(3,1));
Query OK, 0 rows affected (0.01 sec)
//decimal（3，1） 表示共有三位有效数字，保留一位小数。 比如：32.1、10.5
- 查看一下表结构：
mysql> desc exam_result;
+---------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
| chinese | decimal(3,1) | YES | | NULL | |
| math | decimal(3,1) | YES | | NULL | |
| english | decimal(3,1) | YES | | NULL | |
+---------+--------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
- 插入数据
mysql> INSERT INTO exam_result (id,name, chinese, math, english) VALUES
-> (1,'zhangsan', 67, 98, 56),
-> (2,'lisi', 87.5, 78, 77),
-> (3,'wangwu', 88, 98.5, 90),
-> (4,'zhaoliu', 82, 84, 67),
-> (5,'sunqi', 55.5, 85, 45),
-> (6,'zhouba', 70, 73, 78.5),
-> (7,'wujiu', 75, 65, 30);
Query OK, 7 rows affected (0.00 sec)
Records: 7 Duplicates: 0 Warnings: 0
- 查询表内全部数据
mysql> select * from exam_result;
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| 1 | zhangsan | 67.0 | 98.0 | 56.0 |
| 2 | lisi | 87.5 | 78.0 | 77.0 |
| 3 | wangwu | 88.0 | 98.5 | 90.0 |
| 4 | zhaoliu | 82.0 | 84.0 | 67.0 |
| 5 | sunqi | 55.5 | 85.0 | 45.0 |
| 6 | zhouba | 70.0 | 73.0 | 78.5 |
| 7 | wujiu | 75.0 | 65.0 | 30.0 |
+------+-----------+---------+------+---------+
7 rows in set (0.00 sec)
- 带表达式的查询：让所有人的语文成绩+10分
mysql> select name,chinese + 10 from exam_result;
+-----------+--------------+
| name | chinese + 10 |
+-----------+--------------+
| zhangsan | 77.0 |
| lisi | 97.5 |
| wangwu | 98.0 |
| zhaoliu | 92.0 |
| sunqi | 65.5 |
| zhouba | 80.0 |
| wujiu | 85.0 |
+-----------+--------------+
7 rows in set (0.00 sec)
起别名查询
select 表达式 as 别名 from 表名;
- 普通情况查询语、数、英三科总分
mysql> select name, chinese + math + english from exam_result;
+-----------+--------------------------+
| name | chinese + math + english |
+-----------+--------------------------+
| zhangsan | 221.0 |
| lisi | 242.5 |
| wangwu | 276.5 |
| zhaoliu | 233.0 |
| sunqi | 185.5 |
| zhouba | 221.5 |
| wujiu | 170.0 |
+-----------+--------------------------+
7 rows in set (0.00 sec)
- 起别名查询语、数、英三科总分
mysql> select name, chinese + english + math as total from exam_result;
+-----------+-------+
| name | total |
+-----------+-------+
| zhangsan | 221.0 |
| lisi | 242.5 |
| wangwu | 276.5 |
| zhaoliu | 233.0 |
| sunqi | 185.5 |
| zhouba | 221.5 |
| wujiu | 170.0 |
+-----------+-------+
7 rows in set (0.00 sec)
注：as可以写着，也可以省略。 建议写着！
去重查询
select distinct 列名 from 表名;
- 演示此操作需增加相同信息
mysql> insert into exam_result (name, math) values ('zhangsan', 98.0);
Query OK, 1 row affected (0.00 sec)
- 查看当前表的全部信息
mysql> select * from exam_result;
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| 1 | zhangsan | 67.0 | 98.0 | 56.0 |
| 2 | lisi | 87.5 | 78.0 | 77.0 |
| 3 | wangwu | 88.0 | 98.5 | 90.0 |
| 4 | zhaoliu | 82.0 | 84.0 | 67.0 |
| 5 | sunqi | 55.5 | 85.0 | 45.0 |
| 6 | zhouba | 70.0 | 73.0 | 78.5 |
| 7 | wujiu | 75.0 | 65.0 | 30.0 |
| NULL | zhangsan | NULL | 98.0 | NULL |
+------+-----------+---------+------+---------+
8 rows in set (0.00 sec)
- 进行去重查询
mysql> select distinct name, math from exam_result;
+-----------+------+
| name | math |
+-----------+------+
| zhangsan | 98.0 |
| lisi | 78.0 |
| wangwu | 98.5 |
| zhaoliu | 84.0 |
| sunqi | 85.0 |
| zhouba | 73.0 |
| wujiu | 65.0 |
+-----------+------+
7 rows in set (0.00 sec)
注：当用distinct指定多个列时，必须是这几个列的值同时相同时才会去重。
排序查询
select 列名 from 表名 order by 列名;
- 按语文成绩升序排序
mysql> select * from exam_result order by chinese;
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| NULL | zhangsan | NULL | 98.0 | NULL |
| 5 | sunqi | 55.5 | 85.0 | 45.0 |
| 1 | zhangsan | 67.0 | 98.0 | 56.0 |
| 6 | zhouba | 70.0 | 73.0 | 78.5 |
| 7 | wujiu | 75.0 | 65.0 | 30.0 |
| 4 | zhaoliu | 82.0 | 84.0 | 67.0 |
| 2 | lisi | 87.5 | 78.0 | 77.0 |
| 3 | wangwu | 88.0 | 98.5 | 90.0 |
+------+-----------+---------+------+---------+
8 rows in set (0.00 sec)
- 按语文成绩降序排序
mysql> select * from exam_result order by chinese desc;
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| 3 | wangwu | 88.0 | 98.5 | 90.0 |
| 2 | lisi | 87.5 | 78.0 | 77.0 |
| 4 | zhaoliu | 82.0 | 84.0 | 67.0 |
| 7 | wujiu | 75.0 | 65.0 | 30.0 |
| 6 | zhouba | 70.0 | 73.0 | 78.5 |
| 1 | zhangsan | 67.0 | 98.0 | 56.0 |
| 5 | sunqi | 55.5 | 85.0 | 45.0 |
| NULL | zhangsan | NULL | 98.0 | NULL |
+------+-----------+---------+------+---------+
8 rows in set (0.00 sec)
- order by 也可以针对别名进行排序
mysql> select name, chinese + math + english as total from exam_result order by total desc;
+-----------+-------+
| name | total |
+-----------+-------+
| wangwu | 276.5 |
| lisi | 242.5 |
| zhaoliu | 233.0 |
| zhouba | 221.5 |
| zhangsan | 221.0 |
| sunqi | 185.5 |
| wujiu | 170.0 |
| zhangsan | NULL |
+-----------+-------+
8 rows in set (0.00 sec)
- order by 进行排序的时候还可以指定多个列进行排序 效果是：先以第一列为标准进行比较，如果第一列不分胜负，那么继续按照第二列进行比较，一次类推
mysql> select * from exam_result order by math desc,chinese;
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| 3 | wangwu | 88.0 | 98.5 | 90.0 |
| NULL | zhangsan | NULL | 98.0 | NULL |
| 1 | zhangsan | 67.0 | 98.0 | 56.0 |
| 5 | sunqi | 55.5 | 85.0 | 45.0 |
| 4 | zhaoliu | 82.0 | 84.0 | 67.0 |
| 2 | lisi | 87.5 | 78.0 | 77.0 |
| 6 | zhouba | 70.0 | 73.0 | 78.5 |
| 7 | wujiu | 75.0 | 65.0 | 30.0 |
+------+-----------+---------+------+---------+
8 rows in set (0.00 sec)
注：
- 升序排序末尾加asc 但默认是升序排序可以省略，降序排序末尾加desc
- 在SQL中，拿NULL和其他类型进行混合计算，结果仍然是NULL
- 在select操作中，如果没有使用order by 那么查询结果的顺序是不确定的。
条件查询
select* from 表名 where 条件;
引入where子句，对条件进行筛选，即：用where子句对最初的每一行查询结果进行筛选，如果满足条件，就把这一行放入到最终的查询结果；如果不满足条件，则舍弃这一行；最后返回最终查询结果。
- 比较运算符
注：
- 在SQL中没有== 使用=进行比较
- 在SQL中，NULL = NULL 结果还是NUULL ，相当于false； NULL <=> NULL 结果是true
- like进行模糊匹配，匹配过程中可以带上通配符
- 逻辑运算符
注：
- 在where条件中，可以使用表达式，但不能使用别名；
- and的优先级高于or，在使用时注意次序或者加（）
- 基本查询：查询语文成绩比英语成绩好的人
mysql> select * from exam_result where chinese > english;
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| 1 | zhangsan | 67.0 | 98.0 | 56.0 |
| 2 | lisi | 87.5 | 78.0 | 77.0 |
| 4 | zhaoliu | 82.0 | 84.0 | 67.0 |
| 5 | sunqi | 55.5 | 85.0 | 45.0 |
| 7 | wujiu | 75.0 | 65.0 | 30.0 |
+------+-----------+---------+------+---------+
5 rows in set (0.00 sec)
- and / or查询
mysql> select * from exam_result where chinese > 80 or english > 70 and math > 70;
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| 2 | lisi | 87.5 | 78.0 | 77.0 |
| 3 | wangwu | 88.0 | 98.5 | 90.0 |
| 4 | zhaoliu | 82.0 | 84.0 | 67.0 |
| 6 | zhouba | 70.0 | 73.0 | 78.5 |
+------+-----------+---------+------+---------+
4 rows in set (0.00 sec)
- 范围查询：查询语文成绩在80-90之间的人
mysql> select * from exam_result where chinese >= 80 and chinese <= 90;
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| 2 | lisi | 87.5 | 78.0 | 77.0 |
| 3 | wangwu | 88.0 | 98.5 | 90.0 |
| 4 | zhaoliu | 82.0 | 84.0 | 67.0 |
+------+-----------+---------+------+---------+
3 rows in set (0.00 sec)
mysql> select * from exam_result where chinese between 80 and 90;
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| 2 | lisi | 87.5 | 78.0 | 77.0 |
| 3 | wangwu | 88.0 | 98.5 | 90.0 |
| 4 | zhaoliu | 82.0 | 84.0 | 67.0 |
+------+-----------+---------+------+---------+
3 rows in set (0.00 sec)
- in 查询 查询数学成绩是58 或者59 或者98 或者99的人
mysql> select * from exam_result where math in (58,59,98,99);
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| 1 | zhangsan | 67.0 | 98.0 | 56.0 |
+------+-----------+---------+------+---------+
1 rows in set (0.00 sec)
mysql> select * from exam_result where math = 58 or math = 59 or math = 98 or math = 99;
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| 1 | zhangsan | 67.0 | 98.0 | 56.0 |
+------+-----------+---------+------+---------+
1 rows in set (0.00 sec)
- 模糊查询like 不一定完全相同，只要有一部分匹配即可。
mysql> select * from exam_result where name like 'w%';
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| 3 | wangwu | 88 | 98.5 | 90 |
| 7 | wujiu | 75 | 65 | 30 |
+------+-----------+---------+------+---------+
2 rows in set (0.00 sec)
注： % 可以替代任意个字符，_ 可以替代任意一个字符
- NULL的查询
mysql> select * from exam_result;
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| 1 | zhangsan | 67.0 | 98.0 | 56.0 |
| 2 | lisi | 87.5 | 78.0 | 77.0 |
| 3 | wangwu | 88.0 | 98.5 | 90.0 |
| 4 | zhaoliu | 82.0 | 84.0 | 67.0 |
| 5 | sunqi | 55.5 | 85.0 | 45.0 |
| 6 | zhouba | 70.0 | 73.0 | 78.5 |
| 7 | wujiu | 75.0 | 65.0 | 30.0 |
| NULL | zhangsan | NULL | 98.0 | NULL |
+------+-----------+---------+------+---------+
8 rows in set (0.00 sec)
mysql> select * from exam_result where chinese = NULL;
Empty set (0.00 sec)
mysql> select * from exam_result where chinese <=> NULL;
+------+--------+---------+------+---------+
| id | name | chinese | math | english |
+------+--------+---------+------+---------+
| NULL |zhangsan| NULL | 98.0 | NULL |
+------+--------+---------+------+---------+
1 row in set (0.00 sec)
mysql> select * from exam_result where chinese is NULL;
+------+--------+---------+------+---------+
| id | name | chinese | math | english |
+------+--------+---------+------+---------+
| NULL |zhangsan| NULL | 98.0 | NULL |
+------+--------+---------+------+---------+
1 row in set (0.00 sec)
注：
- 直接使用 = 来进行匹配是不能正确进行筛选的
- 使用 <=> 可以正确和NULL匹配
- 使用 is NULL也可以正确和NULL匹配
分页查询
select 列名 from 表名 limit N offset M;
select 列名 from 表名 limit M,N;
N：返回结果的条数 M：跳过M条结果再开始返回
- 从M条开始查询 最多返回N条结果
mysql> select * from exam_result;
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| 1 | zhangsan | 67.0 | 98.0 | 56.0 |
| 2 | lisi | 87.5 | 78.0 | 77.0 |
| 3 | wangwu | 88.0 | 98.5 | 90.0 |
| 4 | zhaoliu | 82.0 | 84.0 | 67.0 |
| 5 | sunqi | 55.5 | 85.0 | 45.0 |
| 6 | zhouba | 70.0 | 73.0 | 78.5 |
| 7 | wujiu | 75.0 | 65.0 | 30.0 |
| NULL | zhangsan | NULL | 98.0 | NULL |
+------+-----------+---------+------+---------+
8 rows in set (0.00 sec)
mysql> select * from exam_result limit 3;
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| 1 | zhangsan | 67.0 | 98.0 | 56.0 |
| 2 | lisi | 87.5 | 78.0 | 77.0 |
| 3 | wangwu | 88.0 | 98.5 | 90.0 |
+------+-----------+---------+------+---------+
3 rows in set (0.00 sec)
mysql> select * from exam_result limit 3 offset 3;
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| 4 | zhaoliu | 82.0 | 84.0 | 67.0 |
| 5 | sunqi | 55.5 | 85.0 | 45.0 |
| 6 | zhouba | 70.0 | 73.0 | 78.5 |
+------+-----------+---------+------+---------+
3 rows in set (0.00 sec)
注：select* 这样的操作容易把数据库搞挂了，除了select*外，其他的查询操作只要你返回的结果足够多，都有可能把数据库搞挂；即使你加上了where子句进行筛选，但是返回的结果仍然可能很多。最保险的办法就是加上limit
修改（Update）
update 表名 set 列名 = 值..... where 条件;
- 把lisi 的数学成绩修改为80分
mysql> update exam_result set math = 80 where name = 'lisi';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from exam_result;
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| 1 | zhangsan | 67.0 | 98.0 | 56.0 |
| 2 | lisi | 87.5 | 80.0 | 77.0 |
| 3 | wangwu | 88.0 | 98.5 | 90.0 |
| 4 | zhaoliu | 82.0 | 84.0 | 67.0 |
| 5 | sunqi | 55.5 | 85.0 | 45.0 |
| 6 | zhouba | 70.0 | 73.0 | 78.5 |
| 7 | wujiu | 75.0 | 65.0 | 30.0 |
| NULL | zhangsan | NULL | 98.0 | NULL |
+------+-----------+---------+------+---------+
8 rows in set (0.00 sec)
注： update后面的where条件很重要。加上条件，表示修改符合条件某些行；不加条件，表示修改所有行。
删除（Delete）
delete from 表名 where 条件;
- 删除zhangsan的信息
mysql> delete from exam_result where name = 'zhangsan';
Query OK, 1 row affected (0.00 sec)
mysql> select * from exam_result;
+------+-----------+---------+------+---------+
| id | name | chinese | math | english |
+------+-----------+---------+------+---------+
| 2 | lisi | 87.5 | 80.0 | 77.0 |
| 3 | wangwu | 88.0 | 98.5 | 90.0 |
| 4 | zhaoliu | 70.0 | 60.0 | 67.0 |
| 5 | sunqi | 55.5 | 85.0 | 45.0 |
| 6 | zhouba | 70.0 | 73.0 | 78.5 |
| 7 | wujiu | 75.0 | 65.0 | 30.0 |
+------+-----------+---------+------+---------+
6 rows in set (0.00 sec)
注：
- delete后面的 where 条件很重要。加上条件，表示删除符合条件某些行；不加条件，表示删除表中的全部信息。
- delete from 表名；表示删除表内的所有信息，但是表还在。
drop table 表名；表示删除整个表，表也不存在了。
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