例题SQL语句详解-数据库基本操作2

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1.4 表的操作

1.4.1 显示所有表

语法：

show tables

1.4.2 创建表

语法：

create table [if not exists] 表名(
    字段名 数据类型 [null|not null] [auto_increment] [primary key] [comment],
    字段名 数据类型 [default]…
)engine=存储引擎

单词

null | not null     空|非空
default             默认值
auto_increment      自动增长
primary key         主键
comment             备注
engine              引擎   innodb  myisam  memory  引擎是决定数据存储的方式

创建简单的表

mysql> create database itcast;
Query OK, 1 row affected (0.00 sec)

mysql> use itcast;
Database changed
mysql> show tables;
Empty set (0.05 sec)

# 创建表
mysql> create table stu(
    -> id int,
    -> name varchar(30)
    -> );
Query OK, 0 rows affected (0.13 sec)
# 查看创建的表
mysql> show tables;
+------------------+
| Tables_in_itcast |
+------------------+
| stu              |
+------------------+

创建复杂的表

mysql> set names gbk;   # 设置字符编码
Query OK, 0 rows affected (0.05 sec)

mysql> create table if not exists teacher(
    -> id int auto_increment primary key comment '主键',
    -> name varchar(20) not null comment '姓名',
    -> phone varchar(20) comment '电话号码',
    -> `add` varchar(100) default '地址不详' comment '地址'
    -> )engine=innodb;
Query OK, 0 rows affected (0.09 sec)

多学一招：create table 数据库名.表名，用于给指定的数据库创建表

mysql> create table data.stu(  #给data数据库中创建stu表
    -> id int,
    -> name varchar(10));
Query OK, 0 rows affected (0.00 sec)

1.4.3 显示创建表的语句

语法：

show create table 表名

显示创建teacher表的语句

mysql> show create table teacher;
+---------+--------------------------------------------------------------------------
-------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------
-------+
| Table   | Create Table


       |
+---------+--------------------------------------------------------------------------
-------------------------------------------------------------------------------------
-------------------------------------------------------------------------------------
-------+
| teacher | CREATE TABLE `teacher` (
  `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '主键',
  `name` varchar(20) NOT NULL COMMENT '姓名',
  `phone` varchar(20) DEFAULT NULL COMMENT '电话号码',
  `add` varchar(100) DEFAULT '地址不详' COMMENT '地址',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8                         |

将两个字段竖着排列 show create table 表名\G

mysql> show create table teacher\G;
*************************** 1. row ***************************
       Table: teacher
Create Table: CREATE TABLE `teacher` (
  `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '主键',
  `name` varchar(20) NOT NULL COMMENT '姓名',
  `phone` varchar(20) DEFAULT NULL COMMENT '电话号码',
  `add` varchar(100) DEFAULT '地址不详' COMMENT '地址',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

1.4.4 查看表结构

语法：

desc[ribe] 表名

查看teacher表的结构

mysql> describe teacher;
+-------+--------------+------+-----+----------+----------------+
| Field | Type         | Null | Key | Default  | Extra          |
+-------+--------------+------+-----+----------+----------------+
| id    | int(11)      | NO   | PRI | NULL     | auto_increment |
| name  | varchar(20)  | NO   |     | NULL     |                |
| phone | varchar(20)  | YES  |     | NULL     |                |
| add   | varchar(100) | YES  |     | 地址不详        |                |
+-------+--------------+------+-----+----------+----------------+
4 rows in set (0.08 sec)

mysql> desc teacher;
+-------+--------------+------+-----+----------+----------------+
| Field | Type         | Null | Key | Default  | Extra          |
+-------+--------------+------+-----+----------+----------------+
| id    | int(11)      | NO   | PRI | NULL     | auto_increment |
| name  | varchar(20)  | NO   |     | NULL     |                |
| phone | varchar(20)  | YES  |     | NULL     |                |
| add   | varchar(100) | YES  |     | 地址不详        |                |
+-------+--------------+------+-----+----------+----------------+
4 rows in set (0.01 sec)

1.4.5 删除表

语法：

drop table [if exists] 表1，表2,… 

删除表

mysql> drop table stu;
Query OK, 0 rows affected (0.08 sec)

如果删除一个不存在的表就会报错，删除的时候可以判断一下，存在就删除。

mysql> drop table stu;
ERROR 1051 (42S02): Unknown table 'stu'

mysql> drop table if exists stu;
Query OK, 0 rows affected, 1 warning (0.00 sec)

可以一次删除多个表

mysql> drop table a1,a2;
Query OK, 0 rows affected (0.00 sec)

1.4.6 修改表

语法：alter table 表名

1、添加字段：alter table 表名add [column] 字段名 数据类型 [位置]

例题一：添加字段

mysql> alter table teacher add age int;
Query OK, 0 rows affected (0.09 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc teacher;
+-------+--------------+------+-----+----------+----------------+
| Field | Type         | Null | Key | Default  | Extra          |
+-------+--------------+------+-----+----------+----------------+
| id    | int(11)      | NO   | PRI | NULL     | auto_increment |
| name  | varchar(20)  | NO   |     | NULL     |                |
| phone | varchar(20)  | YES  |     | NULL     |                |
| add   | varchar(100) | YES  |     | 地址不详        |                |
| age   | int(11)      | YES  |     | NULL     |                |
+-------+--------------+------+-----+----------+----------------+
5 rows in set (0.00 sec)

例题二：在第一个位置上添加字段

mysql> alter table teacher add email varchar(30) first;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc teacher;
+-------+--------------+------+-----+----------+----------------+
| Field | Type         | Null | Key | Default  | Extra          |
+-------+--------------+------+-----+----------+----------------+
| email | varchar(30)  | YES  |     | NULL     |                |
| id    | int(11)      | NO   | PRI | NULL     | auto_increment |
| name  | varchar(20)  | NO   |     | NULL     |                |
| phone | varchar(20)  | YES  |     | NULL     |                |
| add   | varchar(100) | YES  |     | 地址不详        |                |
| age   | int(11)      | YES  |     | NULL     |                |
+-------+--------------+------+-----+----------+----------------+

例题三：在指定的字段后添加字段

mysql> alter table teacher add sex varchar(2) after name;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc teacher;
+-------+--------------+------+-----+----------+----------------+
| Field | Type         | Null | Key | Default  | Extra          |
+-------+--------------+------+-----+----------+----------------+
| email | varchar(30)  | YES  |     | NULL     |                |
| id    | int(11)      | NO   | PRI | NULL     | auto_increment |
| name  | varchar(20)  | NO   |     | NULL     |                |
| sex   | varchar(2)   | YES  |     | NULL     |                |
| phone | varchar(20)  | YES  |     | NULL     |                |
| add   | varchar(100) | YES  |     | 地址不详        |                |
| age   | int(11)      | YES  |     | NULL     |                |
+-------+--------------+------+-----+----------+----------------+
7 rows in set (0.00 sec)

2、删除字段：alter table 表 drop [column] 字段名

mysql> alter table teacher drop email;
Query OK, 0 rows affected (0.06 sec)
Records: 0  Duplicates: 0  Warnings: 0

3、修改字段(改名改类型)：alter table 表 change [column] 原字段名 新字段名 数据类型 …

将字段sex改为xingbie，数据类型为int

mysql> alter table teacher change sex xingbie int;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

4、修改字段（不改名）:alter table 表 modify 字段名 字段属性…

将性别的数据类型改为varchar(2)

mysql> alter table teacher modify xingbie varchar(2);
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

5、修改引擎：alter table 表名 engine=引擎名

mysql> alter table teacher engine=myisam;
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

6、修改表名：alter table 表名 rename to 新表名

mysql> alter table teacher rename to stu;
Query OK, 0 rows affected (0.00 sec)

mysql> show tables;
+------------------+
| Tables_in_itcast |
+------------------+
| stu              |
+------------------+
1 row in set (0.00 sec)

1.4.7 复制表

语法一：create table 新表 select 字段 from 旧表

特点：不能复制父表的主键，能够复制父表的数据

mysql> create table stu1 select * from stu;
Query OK, 1 row affected (0.06 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> select * from stu1;    # 查看数据复制到新表中
+----+------+------+-------+
| id | name | addr | score |
+----+------+------+-------+
|  1 | rose | 上海    |    88 |
+----+------+------+-------+
1 row in set (0.00 sec)

mysql> desc stu1;    #  主键没有复制
+-------+-------------+------+-----+----------+-------+
| Field | Type        | Null | Key | Default  | Extra |
+-------+-------------+------+-----+----------+-------+
| id    | int(11)     | NO   |     | 0        |       |
| name  | varchar(20) | NO   |     | NULL     |       |
| addr  | varchar(50) | YES  |     | 地址不详        |       |
| score | int(11)     | YES  |     | NULL     |       |
+-------+-------------+------+-----+----------+-------+
4 rows in set (0.00 sec)

语法二：create table 新表 like 旧表

特点：只能复制表结构，不能复制表数据

Query OK, 0 rows affected (0.00 sec)

mysql> select * from stu2;   # 数据没有复制
Empty set (0.01 sec)

mysql> desc stu2;   # 主键复制了
+-------+-------------+------+-----+----------+----------------+
| Field | Type        | Null | Key | Default  | Extra          |
+-------+-------------+------+-----+----------+----------------+
| id    | int(11)     | NO   | PRI | NULL     | auto_increment |
| name  | varchar(20) | NO   |     | NULL     |                |
| addr  | varchar(50) | YES  |     | 地址不详        |                |
| score | int(11)     | YES  |     | NULL     |                |
+-------+-------------+------+-----+----------+----------------+
4 rows in set (0.00 sec)