Mysql 数据库管理之 —— SQL 语句
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文章目录
一、SQL 语言简介
SQL 语句用于维护管理数据库，包括数据查询、数据更新、访问控制、对象管理等功能。
SQL语言分类
- DDL：数据定义语言，用于创建数据库对象，如库、表、索引等。
- DML：数据操纵语言，用于对表中的数据进行管理，用来插入、删除和修改数据库中的数据。
- DQL：数据查询语言，用于从数据表中查找符合条件的数据记录。
- DCL：数据控制语言，用于设置或者更改数据库用户或角色权限（数据控制语句，用于控制不同数据段直接的许可和访问级别的语句，这些语句定义了数据库、表、字段、用户的访问权限和安全级别，如 COMMIT、ROLLBACK、GRANT、REVOKE …）
二、DDL 语句
数据定义语言，用于创建数据库对象，如库、表、索引等
常用命令：
create #创建
drop #删除
alter #修改
1. 创建新的数据库
CREATE DATABASE <数据库名>;
2. 创建新的表
- create table 表名 (字段1 数据类型,字段2 数据类型,[…],parmary key (主键名));
- 主键一般选择能代表唯一性的字段，不允许取空值（NULL），一个表只能有一个主键
CREATE TABLE web_demo (id int NOT NULL,name char(10) NOT NULL,score decimal(5,2),passwd char(48) DEFAULT '',PRIMARY KEY (id));
# NOT NULL 不允许为空值
# name char(10) 名称最长 10 位
# decimal(5,2) 整数部分最长五位，小数两位
# DEFAULT '' 密码默认值为空
# PRIMARY KEY 主键一般选择没有重复并且不为空值的字段
3. 删除表
drop table <数据库名>.<表名>;
#不 use 进入库中需加上数据库名
4. 删除数据库
drop database <数据库名>;
三、DML 语句
数据操纵语言，用于对表中的数据进行管理，插入、删除和修改数据库中的数据
常用命令：
insert #插入
update #修改，更新
delete #删除
1. 在表中插入数据
格式：
INSERT INTO 表名(字段1,字段2,...) VALUES(字段1的值，字段2的值,...);
INSERT INTO 表名 VALUES(字段1的值，字段2的值,...);
示例：
insert into web_demo (id,name,score,passwd) values(1,'zhangsan',70,PASSWORD('123456'));
insert into web_demo values(2,'lisi',85.5,PASSWORD('123456'));
————————————————————————————————————
mysql> show databases;
+--------------------+
| Database |
+--------------------+
| information_schema |
| game |
| mysql |
| performance_schema |
| sys |
| test1 |
+--------------------+
6 rows in set (0.00 sec)
mysql> use game;
Database changed
mysql> show tables;
+----------------+
| Tables_in_game |
+----------------+
| web_demo |
+----------------+
1 row in set (0.00 sec)
mysql> select * from web_demo;
Empty set (0.00 sec)
mysql> insert into web_demo (id,name,score,passwd) values(1,'zhangsan',70,PASSWORD('123456'));
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> select * from web_demo;
+----+----------+-------+-------------------------------------------+
| id | name | score | passwd |
+----+----------+-------+-------------------------------------------+
| 1 | zhangsan | 70.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
+----+----------+-------+-------------------------------------------+
1 row in set (0.00 sec)
mysql> desc web_demo;
+--------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+--------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | char(10) | NO | | NULL | |
| score | decimal(5,2) | YES | | NULL | |
| passwd | char(48) | YES | | | |
+--------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
mysql> insert into web_demo values(2,'lisi',85.5,PASSWORD('123456'));
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> select * from web_demo;
+----+----------+-------+-------------------------------------------+
| id | name | score | passwd |
+----+----------+-------+-------------------------------------------+
| 1 | zhangsan | 70.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
| 2 | lisi | 85.50 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
+----+----------+-------+-------------------------------------------+
2 rows in set (0.00 sec)
PS：
select * from <tables>; #查看表中的所有信息
desc <tables>; #查看表结构属性，describe 的缩写
密码字串以加密形式显示，若不使用 PASSWORD()，查询时以明文显示
desc 显示字段含义：
————————————————
Field：字段名称
type：数据类型
Null：是否允许为空
Key：主键
Default：默认值
Extra：扩展属性（例如：标志符列（增量/步长）1 2）
2. 修改、更新数据表中的数据记录
格式：
UPDATE 表名 SET 字段名1=字段值1[,字段名2=字段值2] [WHERE 条件表达式];
示例：
update web_demo set passwd=PASSWORD('') where name='zhangsan';
update web_demo set name='lisi',passwd='' where id=2;
——————————————————————————————————————————————————————————————
mysql> update web_demo set passwd=PASSWORD('') where name='zhangsan';
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> update web_demo set name='lisi',passwd='' where id=2;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from web_demo;
+----+----------+-------+--------+
| id | name | score | passwd |
+----+----------+-------+--------+
| 1 | zhangsan | 70.00 | |
| 2 | lisi | 85.50 | |
+----+----------+-------+--------+
2 rows in set (0.00 sec)
3. 在数据表中删除指定的数据记录
格式：
DELETE FROM 表名 [WHERE 条件表达式];
示例：
delete from web_demo where id=2;
————————————————————————————————————
mysql> select * from web_demo;
+----+----------+-------+--------+
| id | name | score | passwd |
+----+----------+-------+--------+
| 1 | zhangsan | 70.00 | |
| 2 | lisi | 85.50 | |
+----+----------+-------+--------+
2 rows in set (0.00 sec)
mysql> delete from web_demo where id=2;
Query OK, 1 row affected (0.00 sec)
mysql> select * from web_demo;
+----+----------+-------+--------+
| id | name | score | passwd |
+----+----------+-------+--------+
| 1 | zhangsan | 70.00 | |
+----+----------+-------+--------+
1 row in set (0.00 sec)
四、DQL 语句
用于从数据表中查找符合条件的数据记录
常用命令：
select
格式：
SELECT 字段名1,字段名2,[...] FROM 表名[WHERE 条件表达式];
——————————————————————————————————————————————————————
select * from web_demo;
select id,name,score from web_demo where id=2;
select name from web_demo\\G #以列表方式竖向显示
select * from web_demo limit 2; #只显示头 2 行
select * from web_demo limit 2,3; #显示第 2 行后的前 3 行
示例：
mysql> select * from web_demo;
+----+----------+-------+-------------------------------------------+
| id | name | score | passwd |
+----+----------+-------+-------------------------------------------+
| 1 | zhangsan | 70.00 | |
| 2 | lisi | 85.50 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
| 3 | wangwu | 86.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
| 4 | xiaohong | 65.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
| 5 | xiaobai | 70.50 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
| 6 | xiaolan | 99.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
+----+----------+-------+-------------------------------------------+
6 rows in set (0.00 sec)
mysql> select id,name,score from web_demo where id=2;
+----+------+-------+
| id | name | score |
+----+------+-------+
| 2 | lisi | 85.50 |
+----+------+-------+
1 row in set (0.00 sec)
mysql> select name from web_demo\\G;
*************************** 1. row ***************************
name: zhangsan
*************************** 2. row ***************************
name: lisi
*************************** 3. row ***************************
name: wangwu
*************************** 4. row ***************************
name: xiaohong
*************************** 5. row ***************************
name: xiaobai
*************************** 6. row ***************************
name: xiaolan
6 rows in set (0.00 sec)
ERROR:
No query specified
mysql> select * from web_demo limit 2;
+----+----------+-------+-------------------------------------------+
| id | name | score | passwd |
+----+----------+-------+-------------------------------------------+
| 1 | zhangsan | 70.00 | |
| 2 | lisi | 85.50 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
+----+----------+-------+-------------------------------------------+
2 rows in set (0.00 sec)
mysql> select * from web_demo limit 2,3;
+----+----------+-------+-------------------------------------------+
| id | name | score | passwd |
+----+----------+-------+-------------------------------------------+
| 3 | wangwu | 86.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
| 4 | xiaohong | 65.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
| 5 | xiaobai | 70.50 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
+----+----------+-------+-------------------------------------------+
3 rows in set (0.00 sec)
五、DCL 语句
(1) 修改表名和表结构
ALTER TABLE 旧表名 RENAME 新表名;
示例：
mysql> show tables;
+----------------+
| Tables_in_game |
+----------------+
| web_demo |
+----------------+
1 row in set (0.00 sec)
mysql> alter table web_demo rename web;
Query OK, 0 rows affected (0.01 sec)
mysql> show tables;
+----------------+
| Tables_in_game |
+----------------+
| web |
+----------------+
1 row in set (0.00 sec)
mysql> alter table web rename web_demo;
Query OK, 0 rows affected (0.00 sec)
mysql> show tables;
+----------------+
| Tables_in_game |
+----------------+
| web_demo |
+----------------+
1 row in set (0.00 sec)
(2) 扩展表结构（增加字段）
已知在二维表中，每一行称作一个记录，每一列称作一个字段，扩展表结构即为增加字段。
ALTER TABLE 表名 ADD address varchar(50) default '地址不详';
# default '地址不详'：表示此字段设置默认值为地址不详，可与 NOT NULL 配合使用
ALTER TABLE yyy ADD address varchar(50) NOT NULL default '地址不详';
示例：
alter table web_demo add address varchar(50) default '地址不详';
alter table web_demo add wife varchar(50) NOT NULL default '未知';
————————————————————————————————————————————————————————————————
mysql> select * from web_demo;
+----+----------+-------+-------------------------------------------+
| id | name | score | passwd |
+----+----------+-------+-------------------------------------------+
| 1 | zhangsan | 70.00 | |
| 2 | lisi | 85.50 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
| 3 | wangwu | 86.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
| 4 | xiaohong | 65.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
| 5 | xiaobai | 70.50 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
| 6 | xiaolan | 99.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 |
+----+----------+-------+-------------------------------------------+
6 rows in set (0.00 sec)
mysql> alter table web_demo add address varchar(50) default '地址不详';
Query OK, 0 rows affected (0.04 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> select * from web_demo;
+----+----------+-------+-------------------------------------------+--------------+
| id | name | score | passwd | address |
+----+----------+-------+-------------------------------------------+--------------+
| 1 | zhangsan | 70.00 | | 地址不详 |
| 2 | lisi | 85.50 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 |
| 3 | wangwu | 86.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 |
| 4 | xiaohong | 65.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 |
| 5 | xiaobai | 70.50 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 |
| 6 | xiaolan | 99.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 |
+----+----------+-------+-------------------------------------------+--------------+
6 rows in set (0.00 sec)
mysql> alter table web_demo add wife varchar(50) NOT NULL default '未知';
Query OK, 0 rows affected (0.04 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> select * from web_demo;
+----+----------+-------+-------------------------------------------+--------------+--------+
| id | name | score | passwd | address | wife |
+----+----------+-------+-------------------------------------------+--------------+--------+
| 1 | zhangsan | 70.00 | | 地址不详 | 未知 |
| 2 | lisi | 85.50 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
| 3 | wangwu | 86.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
| 4 | xiaohong | 65.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
| 5 | xiaobai | 70.50 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
| 6 | xiaolan | 99.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
+----+----------+-------+-------------------------------------------+--------------+--------+
6 rows in set (0.00 sec)
(3) 修改字段（列）名，添加唯一键
ALTER TABLE 表名 CHANGE 旧列名 新列名 数据类型 [unique key];
# unique key：唯一键（特性：唯一，但可以为空，空值只允许出现一次）
# Primary key：唯一且非空（主键一般选择能代表唯一性的字段不允许取空值（NULL），一个表只能有一个主键)
# CHANGE 可修改字段名、数据类型、约束等所有项
示例：
alter table web_demo change name username varchar(10) unique key;
#注意列每行内容不能有重复的，不然无法修改
————————————————————————————————————————————————————————————————
mysql> select * from web_demo;
+----+----------+-------+-------------------------------------------+--------------+--------+
| id | name | score | passwd | address | wife |
+----+----------+-------+-------------------------------------------+--------------+--------+
| 1 | zhangsan | 70.00 | | 地址不详 | 未知 |
| 2 | lisi | 85.50 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
| 3 | wangwu | 86.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
| 4 | xiaohong | 65.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
| 5 | xiaobai | 70.50 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
| 6 | xiaolan | 99.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
+----+----------+-------+-------------------------------------------+--------------+--------+
6 rows in set (0.00 sec)
mysql> alter table web_demo change name username varchar(10) unique key;
Query OK, 0 rows affected, 1 warning (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 1
mysql> select * from web_demo;
+----+----------+-------+-------------------------------------------+--------------+--------+
| id | username | score | passwd | address | wife |
+----+----------+-------+-------------------------------------------+--------------+--------+
| 1 | zhangsan | 70.00 | | 地址不详 | 未知 |
| 2 | lisi | 85.50 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
| 3 | wangwu | 86.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
| 4 | xiaohong | 65.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
| 5 | xiaobai | 70.50 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
| 6 | xiaolan | 99.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
+----+----------+-------+-------------------------------------------+--------------+--------+
6 rows in set (0.00 sec)
(4) 删除字段
格式：
ALTER TABLE 表名 DROP 字段名;
示例：
alter table web_demo drop address;
_______________________________________
mysql> select * from web_demo;
+----+----------+-------+-------------------------------------------+--------------+--------+
| id | name | score | passwd | address | wife |
+----+----------+-------+-------------------------------------------+--------------+--------+
| 1 | zhangsan | 70.00 | | 地址不详 | 未知 |
| 2 | lisi | 85.50 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
| 3 | wangwu | 86.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
| 4 | xiaohong | 65.00 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
| 5 | xiaobai | 70.50 | *6BB4837EB74329105EE4568DDA7DC67ED2CA2AD9 | 地址不详 | 未知 |
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