数据库基础知识 (数据库基础)

发布时间：2023-05-29 11:19:29 所属栏目：MySql教程来源：

导读： sql是Structure Query Language(结构化查询语言）的缩写，它是关系型数据库的应用语言，由IBM在20世纪70年×××发，以实现关系型数据库中的信息检索。

在20世纪80年代初，美国国家标准局（A

sql是Structure Query Language(结构化查询语言）的缩写，它是关系型数据库的应用语言，由IBM在20世纪70年×××发，以实现关系型数据库中的信息检索。

在20世纪80年代初，美国国家标准局（ANSI)开始着手制定sql标准，最早的ANSI标准于1986年完成，就被叫做sql-86。正是由于sql语言的标准化，所以大多数关系型数据库都支持sql语言，它已经发展成为多种平台进行交互操作的底层会话语言。

sql的分类：

DDL：数据定义语言，即是对数据库内部对象进行创建、删除、修改等操作的语言，和DML最大区别在于DML仅对表内数据进行操作，而不涉及到表的定义、结构的修改，更不会涉及其它对象，DBA使用较多。常用关键字包括create、drop、alter等。

DML：数据操作语言，用于添加、删除、更新和查询表中的记录，并检查数据的完整性，开发人员使用较多。常用的语句包括insert、delete、update、和select等。

DCL：数据控制语言，用于管理系统中的对象权限时使用，常用语句有grant、revoke等。

1 DDL语句示例：

1）创建数据库

mysql> show engines; #查看支持的引擎，包括默认的引擎

+------------+---------+------------------------------------------------------------+--------------+------+------------+

| Engine | Support | Comment | Transactions | XA | Savepoints |

+------------+---------+------------------------------------------------------------+--------------+------+------------+

| MRG_MYISAM | YES | Collection of identical MyISAM tables | NO | NO | NO |

| CSV | YES | CSV storage engine | NO | NO | NO |

| MyISAM | DEFAULT | Default engine as of MysqL 3.23 with great performance | NO | NO | NO |

| InnoDB | YES | Supports transactions, row-level locking, and foreign keys | YES | YES | YES |

| MEMORY | YES | Hash based, stored in memory, useful for temporary tables | NO | NO | NO |

+------------+---------+------------------------------------------------------------+--------------+------+------------+

MysqL> show character set; #查看系统支持的字符集

MysqL> show variables like "character%"; #查看当前字符集设置

MysqL> show variables like "collation%"; #查看字符集校验设置

MysqL> create database test1; #创建test1数据库

Query OK, 1 row affected (0.05 sec)

MysqL> show databases; #查看数据库

+--------------------+

| Database |

+--------------------+

| information_schema |

| MysqL |

| performance_schema |

| test |

| test1 |

| testdb |

MysqL> create database t121 default character set utf8 collate utf8_general_ci;

Query OK, 1 row affected (0.00 sec) #创建时，也可以指定字符集

2）删除数据库

MysqL> drop database test1;

Query OK, 0 rows affected (0.01 sec)

MysqL> show databases;

+--------------------+

| Database |

+--------------------+

| information_schema |

| MysqL |

| performance_schema |

| test |

| testdb |

备注：数据库一旦被删除，库中所有的表也将被删除，因此，备份非常重要

3）创建表

MysqL> use zwj; #选择数据库

MysqL> create table emp #varchar(n)其中n代表字符数

-> (ename varchar(10),

-> hiredate date,

-> sal decimal(10,2),

-> deptno int(2));

Query OK, 0 rows affected (0.11 sec)

MysqL> desc zwj.emp; #查看zwj库中的emp表的结构

+----------+---------------+------+-----+---------+-------+

| Field | Type | Null | Key | Default | Extra |

+----------+---------------+------+-----+---------+-------+

| ename | varchar(10) | YES | | NULL | |

| hiredate | date | YES | | NULL | |

| sal | decimal(10,2) | YES | | NULL | |

| deptno | int(2) | YES | | NULL | |

+----------+---------------+------+-----+---------+-------+

MysqL> show create table zwj.emp\g #查看创建表的sql语句，包括使用的字符集

+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

| Table | Create Table |

+-------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

| emp | CREATE TABLE `emp` (

`ename` varchar(10) DEFAULT NULL,

`hiredate` date DEFAULT NULL,

`sal` decimal(10,2) DEFAULT NULL,

`deptno` int(2) DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=utf8 |

复制一张表

MysqL> create table t119 like zwj.t118;

Query OK, 0 rows affected (0.04 sec)

MysqL> insert into t119 select * from zwj.t118;

Query OK, 12 rows affected (0.00 sec)

Records: 12 Duplicates: 0 Warnings: 0

查看表的状态，了解两张表是否一致

MysqL> use zwj;

Database changed

MysqL> show table status\G

*************************** 1. row ***************************

Name: t118

Engine: MyISAM

Version: 10

Row_format: Fixed

Rows: 12

Avg_row_length: 30

Data_length: 360

Max_data_length: 8444249301319679

Index_length: 1024

Data_free: 0

Auto_increment: NULL

Create_time: 2017-04-29 08:20:18

Update_time: 2017-04-29 08:29:50

Check_time: NULL

Collation: latin1_swedish_ci

Checksum: NULL

Create_options:

Comment:

*************************** 2. row ***************************

Name: t119

Engine: MyISAM

Version: 10

Row_format: Fixed

Rows: 12

Avg_row_length: 30

Data_length: 360

Max_data_length: 8444249301319679

Index_length: 1024

Data_free: 0

Auto_increment: NULL

Create_time: 2017-04-29 09:09:12

Update_time: 2017-04-29 09:09:38

Check_time: NULL

Collation: latin1_swedish_ci

Checksum: NULL

Create_options:

Comment:

2 rows in set (0.00 sec)

查看指定表的状态信息

MysqL> show table status like 't118'\G

*************************** 1. row ***************************

Name: t118

Engine: MyISAM

Version: 10

Row_format: Fixed

Rows: 12

Avg_row_length: 30

Data_length: 360

Max_data_length: 8444249301319679

Index_length: 1024

Data_free: 0

Auto_increment: NULL

Create_time: 2017-04-29 08:20:18

Update_time: 2017-04-29 08:29:50

Check_time: NULL

Collation: latin1_swedish_ci

Checksum: NULL

Create_options:

Comment:

1 row in set (0.00 sec)

4）删除表

MysqL> drop table zwj.emp;

Query OK, 0 rows affected (0.05 sec)

5）修改表，需要用到alter table语句

修改表ename字段的定义，把varchar(10)改为varchar(20)

MysqL> alter table emp modify ename varchar(20); #关键字modify用于修改表中字段的定义

Query OK, 0 rows affected (0.17 sec)

Records: 0 Duplicates: 0 Warnings: 0

MysqL> desc emp;

+----------+---------------+------+-----+---------+-------+

| Field | Type | Null | Key | Default | Extra |

+----------+---------------+------+-----+---------+-------+

| ename | varchar(20) | YES | | NULL | |

| hiredate | date | YES | | NULL | |

| sal | decimal(10,2) | YES | | NULL | |

| deptno | int(2) | YES | | NULL | |

+----------+---------------+------+-----+---------+-------+

在表emp中新增字段age，类型为int(3):

MysqL> alter table emp add age int(3); #默认排在最后

Query OK, 0 rows affected (0.08 sec)

Records: 0 Duplicates: 0 Warnings: 0

MysqL> desc emp;

+----------+---------------+------+-----+---------+-------+

| Field | Type | Null | Key | Default | Extra |

+----------+---------------+------+-----+---------+-------+

| ename | varchar(20) | YES | | NULL | |

| hiredate | date | YES | | NULL | |

| sal | decimal(10,2) | YES | | NULL | |

| deptno | int(2) | YES | | NULL | |

| age | int(3) | YES | | NULL | |

+----------+---------------+------+-----+---------+-------+

5 rows in set (0.00 sec)

删除一个字段

MysqL> alter table emp drop age;

Query OK, 0 rows affected (0.08 sec)

Records: 0 Duplicates: 0 Warnings: 0

修改字段名称

MysqL> alter table emp change age age1 int(4); #关键字change可以修改表的定义，如字段名

Query OK, 0 rows affected (0.03 sec)

Records: 0 Duplicates: 0 Warnings: 0

MysqL> desc emp;

+----------+---------------+------+-----+---------+-------+

| Field | Type | Null | Key | Default | Extra |

+----------+---------------+------+-----+---------+-------+

| ename | varchar(20) | YES | | NULL | |

| hiredate | date | YES | | NULL | |

| sal | decimal(10,2) | YES | | NULL | |

| deptno | int(2) | YES | | NULL | |

| age1 | int(4) | YES | | NULL | |

+----------+---------------+------+-----+---------+-------+

修改字段排列顺序

MysqL> alter table emp add birth date after ename; #新增字段birth，排在ename之后

Query OK, 0 rows affected (0.08 sec)

Records: 0 Duplicates: 0 Warnings: 0

MysqL> desc emp;

+----------+---------------+------+-----+---------+-------+

| Field | Type | Null | Key | Default | Extra |

+----------+---------------+------+-----+---------+-------+

| ename | varchar(20) | YES | | NULL | |

| birth | date | YES | | NULL | |

| hiredate | date | YES | | NULL | |

| sal | decimal(10,2) | YES | | NULL | |

| deptno | int(2) | YES | | NULL | |

| age1 | int(4) | YES | | NULL | |

+----------+---------------+------+-----+---------+-------+

6 rows in set (0.00 sec)

把字段deptno放在age1后面

MysqL> alter table emp1 modify deptno int(2) after age1;

Query OK, 0 rows affected (0.14 sec)

Records: 0 Duplicates: 0 Warnings: 0

MysqL> desc emp1;

+--------+-------------+------+-----+---------+-------+

| Field | Type | Null | Key | Default | Extra |

+--------+-------------+------+-----+---------+-------+

| age1 | int(4) | YES | | NULL | |

| deptno | int(2) | YES | | NULL | |

| ename | varchar(20) | YES | | NULL | |

| birth | date | YES | | NULL | |

+--------+-------------+------+-----+---------+-------+

4 rows in set (0.00 sec)

把字段age1放在最前面

MysqL> alter table emp modify age1 int(4) first;

Query OK, 0 rows affected (0.15 sec)

Records: 0 Duplicates: 0 Warnings: 0

MysqL> desc emp;

+----------+---------------+------+-----+---------+-------+

| Field | Type | Null | Key | Default | Extra |

+----------+---------------+------+-----+---------+-------+

| age1 | int(4) | YES | | NULL | |

| ename | varchar(20) | YES | | NULL | |

| birth | date | YES | | NULL | |

| hiredate | date | YES | | NULL | |

| sal | decimal(10,2) | YES | | NULL | |

| deptno | int(2) | YES | | NULL | |

+----------+---------------+------+-----+---------+-------+

修改表名：

MysqL> alter table emp rename emp1;

Query OK, 0 rows affected (0.02 sec)

MysqL> show tables;

+---------------+

| Tables_in_zwj |

+---------------+

| emp1 |

+---------------+

1 row in set (0.00 sec)

2 DML语句示例

1）插入记录

MysqL> insert into emp1(age1,ename,birth,deptno) values('555','aaa','2016-10-30','5');

Query OK, 1 row affected (0.03 sec)

也可以不指定字段名称，但values后面的顺序应该和字段的排列顺序一致

MysqL> insert into emp1 values('666','bbb','2016-12-30','8');

MysqL> select * from emp1;

+------+-------+------------+--------+

| age1 | ename | birth | deptno |

+------+-------+------------+--------+

| 555 | aaa | 2016-10-30 | 5 |

| 666 | bbb | 2016-12-30 | 8 |

+------+-------+------------+--------+

一次插入多条记录

MysqL> insert into emp1(age1,ename,birth,deptno)

-> values ('111','ccc','2011-11-30','4'),

-> ('666','ddd','2014-12-22','11'),

-> ('888','eee','2015-11-30','22'),

-> ('333','fff','2011-04-30','8');

Query OK, 4 rows affected (0.02 sec)

Records: 4 Duplicates: 0 Warnings: 0

MysqL> select * from emp1;

+------+-------+------------+--------+

| age1 | ename | birth | deptno |

+------+-------+------------+--------+

| 555 | aaa | 2016-10-30 | 5 |

| 666 | bbb | 2016-12-30 | 8 |

| 111 | ccc | 2011-11-30 | 4 |

| 666 | ddd | 2014-12-22 | 11 |

| 888 | eee | 2015-11-30 | 22 |

| 333 | fff | 2011-04-30 | 8 |

+------+-------+------------+--------+

6 rows in set (0.00 sec)

2）更新记录，通过update命令进行更改

MysqL> update emp1 set age1=1000 where ename='aaa';

Query OK, 1 row affected (0.03 sec)

Rows matched: 1 Changed: 1 Warnings: 0

MysqL> select * from emp1;

+------+-------+------------+--------+

| age1 | ename | birth | deptno |

+------+-------+------------+--------+

| 1000 | aaa | 2016-10-30 | 5 |

| 666 | bbb | 2016-12-30 | 8 |

| 111 | ccc | 2011-11-30 | 4 |

| 666 | ddd | 2014-12-22 | 11 |

| 888 | eee | 2015-11-30 | 22 |

| 333 | fff | 2011-04-30 | 8 |

+------+-------+------------+--------+

6 rows in set (0.00 sec)

3）删除记录：

MysqL> delete from emp1 where ename='bbb';

Query OK, 1 row affected (0.02 sec)

MysqL> select * from emp1;

+------+--------+-------+------------+

| age1 | deptno | ename | birth |

+------+--------+-------+------------+

| 111 | 4 | ccc | 2011-11-30 |

| 666 | 11 | ddd | 2014-12-22 |

| 888 | 22 | eee | 2015-11-30 |

| 333 | 8 | fff | 2011-04-30 |

+------+--------+-------+------------+

4 rows in set (0.00 sec)

4）查询记录

MysqL> select age1,ename from zwj.emp1;

+------+-------+

| age1 | ename |

+------+-------+

| 666 | bbb |

| 111 | ccc |

| 666 | ddd |

| 888 | eee |

| 333 | fff |

+------+-------+

把表中的记录去掉重复后显示出来，

MysqL> select distinct age1 from emp1; #distinct是关键字，age1是字段名

条件查询

MysqL> select * from emp1 where age1='666';

+------+-------+------------+--------+

| age1 | ename | birth | deptno |

+------+-------+------------+--------+

| 666 | bbb | 2016-12-30 | 8 |

| 666 | ddd | 2014-12-22 | 11 |

+------+-------+------------+--------+

组合条件查询：

MysqL> select * from emp1 where ename='bbb' and birth<'2017-01-01';

+------+-------+------------+--------+

| age1 | ename | birth | deptno |

+------+-------+------------+--------+

| 666 | bbb | 2016-12-30 | 8 |

+------+-------+------------+--------+

MysqL> select * from emp1 where ename='bbb' or birth<'2017-01-30';

+------+-------+------------+--------+

| age1 | ename | birth | deptno |

+------+-------+------------+--------+

| 666 | bbb | 2016-12-30 | 8 |

| 111 | ccc | 2011-11-30 | 4 |

| 666 | ddd | 2014-12-22 | 11 |

| 888 | eee | 2015-11-30 | 22 |

| 333 | fff | 2011-04-30 | 8 |

+------+-------+------------+--------+

5 rows in set (0.00 sec)

模式匹配：

MysqL> select * from zwj.emp1 where ename regexp '^c'; #关键字regexp支持正则表达式

+------+--------+-------+------------+

| age1 | deptno | ename | birth |

+------+--------+-------+------------+

| 111 | 4 | ccc | 2011-11-30 |

+------+--------+-------+------------+

1 row in set (0.00 sec)

MysqL> select * from zwj.emp1 where ename like 'c_c'; #短横表示匹配任意单个字符

+------+--------+-------+------------+

| age1 | deptno | ename | birth |

+------+--------+-------+------------+

| 111 | 4 | ccc | 2011-11-30 |

+------+--------+-------+------------+

1 row in set (0.01 sec)

MysqL> select * from zwj.emp1 where ename like 'c%'; #%表示任意字符

+------+--------+-------+------------+

| age1 | deptno | ename | birth |

+------+--------+-------+------------+

| 111 | 4 | ccc | 2011-11-30 |

+------+--------+-------+------------+

1 row in set (0.00 sec)

排序和限制：关键字order by（默认升序排序）

MysqL> select * from emp1 order by age1;

+------+-------+------------+--------+

| age1 | ename | birth | deptno |

+------+-------+------------+--------+

| 111 | ccc | 2011-11-30 | 4 |

| 333 | fff | 2011-04-30 | 8 |

| 666 | bbb | 2016-12-30 | 8 |

| 666 | ddd | 2014-12-22 | 11 |

| 888 | eee | 2015-11-30 | 22 |

+------+-------+------------+--------+

5 rows in set (0.00 sec)

对age1相同的记录，如果把字段deptno从高到低排列，可使用如下命令，desc表示降序。

MysqL> select * from emp1 order by age1,deptno desc;

+------+-------+------------+--------+

| age1 | ename | birth | deptno |

+------+-------+------------+--------+

| 111 | ccc | 2011-11-30 | 4 |

| 333 | fff | 2011-04-30 | 8 |

| 666 | ddd | 2014-12-22 | 11 |

| 666 | bbb | 2016-12-30 | 8 |

| 888 | eee | 2015-11-30 | 22 |

+------+-------+------------+--------+

5 rows in set (0.01 sec)

对age1相同的记录，如果把字段deptno从低到高排列，可使用如下命令，asc表示升序。

MysqL> select * from emp1 order by age1,deptno asc;

+------+-------+------------+--------+

| age1 | ename | birth | deptno |

+------+-------+------------+--------+

| 111 | ccc | 2011-11-30 | 4 |

| 333 | fff | 2011-04-30 | 8 |

| 666 | bbb | 2016-12-30 | 8 |

| 666 | ddd | 2014-12-22 | 11 |

| 888 | eee | 2015-11-30 | 22 |

+------+-------+------------+--------+

5 rows in set (0.01 sec)

选择排序后的前3条记录

MysqL> select * from emp1 order by age1 limit 3;

+------+-------+------------+--------+

| age1 | ename | birth | deptno |

+------+-------+------------+--------+

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