第三章 数据表的基本操作
第三章 数据表的基本操作
4.1 创建数据表
在创建完数据库之后,接下来我们就需要创建数据表。创建数据表是指在已经创建好的数据库中建立新表。创建数据表的过程是规定数据列的属性的过程,同时也是实施数据完整性约束的过程。
4.1.1 创建表的语法形式
数据表属于数据库,在创建数据表之前,应该使用语句“USE <数据库名>” 指定操作是从哪个数据库中进行,如果没有选择数据库,会报错
语法:
CREATE TABLE <表名>
(
字段名1, 数据类型 [列级别约束条件] [默认值],
字段名2, 数据类型 [列级别约束条件] [默认值],
... ...
):
例如:
mysql> CREATE TABLE tb_emp1
-> (
-> id INT(11),
-> name VARCHAR(25),
-> deptId INT(11),
-> salary FLOAT
-> );
Query OK, 0 rows affected (0.08 sec)
mysql> SHOW TABLES;
+----------------+
| Tables_in_test |
+----------------+
| tb_emp1 |
+----------------+
1 row in set (0.05 sec)
4.1.2 使用主键约束
主键约束要求主键列的数据唯一,并且不允许为空。
1、单字段主键
语法:
字段名 数据类型 PRIMARY KEY [默认值]
例如:
mysql> CREATE TABLE tb_emp2
-> (
-> id INT(11) PRIMARY KEY,
-> name VARCHAR(25),
-> deptID INT(11),
-> salary FLOAT
-> );
Query OK, 0 rows affected (0.05 sec)
2、在定义完所有列之后指定主键
语法:
[CONSTRAINT <约束名>] PRIMARY KEY [字段名]
例如:
mysql> CREATE TABLE tb_emp3
-> (
-> id INT(11),
-> name VARCHAR(25),
-> deptId INT(11),
-> salary FLOAT,
-> PRIMARY KEY(id)
-> );
Query OK, 0 rows affected (0.01 sec)
3、多字段联合主键
语法:
PRIMARY KEY [字段1,字段2,....]
例如:
mysql> CREATE TABLE tb_emp4
-> (
-> name VARCHAR(25),
-> deptId INT(11),
-> salary FLOAT,
-> PRIMARY KEY(name,deptId)
-> );
Query OK, 0 rows affected (0.00 sec)
4.1.3 使用外键约束
外键用来在两个表数据之间建立连接,它可以是一列或者多列
语法:
[CONSTRAINT<外键名>] FOREIGN KEY [字段名1,字段名2...]
REFERENCES<主表名> 主键列1[主键列2...]
例如:
mysql> CREATE TABLE tb_dept1
-> (
-> id INT(11) PRIMARY KEY,
-> name VARCHAR(22) NOT NULL,
-> location VARCHAR(50)
-> );
Query OK, 0 rows affected (0.00 sec)
mysql> CREATE TABLE tb_emp5
(
id INT(11) PRIMARY KEY,
name VARCHAR(25),
deptId INT(11),
salary FLOAT,
CONSTRAINT fk_emp_dept1 FOREIGN KEY(deptId) REFERENCES tb_dept1(id)
);
Query OK, 0 rows affected (0.06 sec)
需要有相同的 主键,俩张表可以放在一起进行查看
4.1.4 使用非空约束
非空约束指字段的值不能为空。
语法:
字段名 数据类型 not null
例如:
mysql> CREATE TABLE tb_emp6
-> (
-> id INT(11) PRIMARY KEY ,
-> name VARCHAR(25) NOT NULL,
-> deptId INT(11),
-> salary FLOAT
-> );
Query OK, 0 rows affected (0.06 sec)
或者
语法:
[CONSTRATIN <约束名>] UNIQUE (<字段名>)
例如:
mysql> CREATE TABLE tb_dept3
-> (
-> id INT(11) PRIMARY KEY,
-> name VARCHAR(22),
-> location VARCHAR(50),
-> CONSTRAINT STH UNIQUE(name)
-> );
Query OK, 0 rows affected (0.00 sec)
使用唯一性条件约束
unique
4.1.6 使用默认约束
默认约束指定某列的默认值。
语法:
字段名 数据类型 DEFAULT 默认值
例如:
mysql> CREATE TABLE tb_emp7
-> (
-> id INT(11) PRIMARY KEY,
-> name VARCHAR(25) NOT NULL,
-> deptId INT(11) DEFAULT 1111,
-> salary FLOAT,
-> info VARCHAR(50)
-> );
Query OK, 0 rows affected (0.00 sec)
4.1.7 设置表的属性值自动增加
语法:
字段名 数据类型 AUTO_INCREMENT
例如:
mysql> CREATE TABLE tb_emp8
-> (
-> id INT(11) PRIMARY KEY AUTO_INCREMENT,
-> name VARCHAR(25) NOT NULL,
-> deptId INT(11),
-> salary FLOAT
-> );
Query OK, 0 rows affected (0.00 sec)
插入数据验证:
mysql> INSERT INTO tb_emp8(name,salary)
-> VALUES('lucy',1000),('lura',1200),('kevin',1500);
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0
查看:
mysql> SELECT * FROM tb_emp8;
+----+-------+--------+--------+
| id | name | deptId | salary |
+----+-------+--------+--------+
| 1 | lucy | NULL | 1000 |
| 2 | lura | NULL | 1200 |
| 3 | kevin | NULL | 1500 |
+----+-------+--------+--------+
3 rows in set (0.00 sec)
4.2 查看数据表结构
4.2.1 查看表基本结构语句DESCRIBE
语法:
DESCRIBE 表名;
或
DESC 表名;
例如:
mysql> DESCRIBE tb_dept1;
+----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(22) | NO | | NULL | |
| location | varchar(50) | YES | | NULL | |
+----------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
或者
mysql> DESC tb_dept1;
+----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(22) | NO | | NULL | |
| location | varchar(50) | YES | | NULL | |
+----------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
4.2.2 查看表详细结构语句
语法:
SHOW CREATE TABLE <表名\G>
mysql> SHOW CREATE TABLE tb_emp1\G
*************************** 1. row ***************************
Table: tb_emp1
Create Table: CREATE TABLE `tb_emp1` (
`id` int(11) DEFAULT NULL,
`name` varchar(25) DEFAULT NULL,
`deptId` int(11) DEFAULT NULL,
`salary` float DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
4.3 修改数据表
4.3.1 修改表名
语法:
ALTER TABLE <旧表名> RENAME [TO] <新表名>.
例如:
mysql> SHOW TABLES;
+----------------+
| Tables_in_test |
+----------------+
| tb_dept1 |
| tb_dept3 |
| tb_emp1 |
| tb_emp2 |
| tb_emp3 |
| tb_emp4 |
| tb_emp5 |
| tb_emp7 |
| tb_emp8 |
+----------------+
9 rows in set (0.00 sec)
mysql> ALTER TABLE tb_dept3 RENAME tb_deptment3;
Query OK, 0 rows affected (0.05 sec)
mysql> SHOW TABLES;
+----------------+
| Tables_in_test |
+----------------+
| tb_dept1 |
| tb_deptment3 |
| tb_emp1 |
| tb_emp2 |
| tb_emp3 |
| tb_emp4 |
| tb_emp5 |
| tb_emp7 |
| tb_emp8 |
+----------------+
9 rows in set (0.00 sec)
4.3.2 修改字段的数据类型
语法:
ALTER TABLE <表名> MODIFY <字段名> <数据类型>
例如:
mysql> DESC tb_dept1;
+----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(22) | NO | | NULL | |
| location | varchar(50) | YES | | NULL | |
+----------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql> ALTER TABLE tb_dept1 MODIFY name VARCHAR(30);
Query OK, 0 rows affected (0.08 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> DESC tb_dept1;
+----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(30) | YES | | NULL | |
| location | varchar(50) | YES | | NULL | |
+----------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
4.3.3 修改字段名
语法:
ALTER TABLE <表名> CHANGE<旧字段名><新字段名> <新数据类型>
例如:
mysql> ALTER TABLE tb_dept1 CHANGE location loc VARCHAR(50);
Query OK, 0 rows affected (0.10 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> DESC tb_dept1;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(30) | YES | | NULL | |
| loc | varchar(50) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
4.3.4 添加字段
语法:
ALTER TABLE <表名> ADD <新字段名><数据类型> [约束条件] [FIRST|AFTER 已存在字段名]
例如:
mysql> DESC tb_dept1;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(30) | YES | | NULL | |
| loc | varchar(50) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
mysql> ALTER TABLE tb_dept1 ADD column1 VARCHAR(12) not null;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> DESC tb_dept1;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(30) | YES | | NULL | |
| loc | varchar(50) | YES | | NULL | |
| column1 | varchar(12) | NO | | NULL | |
+---------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
或者
mysql> ALTER TABLE tb_dept1 ADD column2 INT(11) FIRST;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> DESC tb_dept1;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| column2 | int(11) | YES | | NULL | |
| id | int(11) | NO | PRI | NULL | |
| name | varchar(30) | YES | | NULL | |
| loc | varchar(50) | YES | | NULL | |
| column1 | varchar(12) | NO | | NULL | |
+---------+-------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
或者
mysql> ALTER TABLE tb_dept1 ADD column3 INT(11) AFTER name ;
Query OK, 0 rows affected (0.09 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> DESC tb_dept1;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| column2 | int(11) | YES | | NULL | |
| id | int(11) | NO | PRI | NULL | |
| name | varchar(30) | YES | | NULL | |
| column3 | int(11) | YES | | NULL | |
| loc | varchar(50) | YES | | NULL | |
| column1 | varchar(12) | NO | | NULL | |
+---------+-------------+------+-----+---------+-------+
6 rows in set (0.00 sec)
4.3.5 删除字段
语法:
ALTER TABLE <表名> DROP <字段名>
例如:
mysql> ALTER TABLE tb_dept1 DROP column2;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> DESC tb_dept1;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(30) | YES | | NULL | |
| column3 | int(11) | YES | | NULL | |
| loc | varchar(50) | YES | | NULL | |
| column1 | varchar(12) | NO | | NULL | |
+---------+-------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
4.3.6 修改字段的排列位置
语法:
ALTER TABLE <表名> MODIFY <字段名> <数据类型> FIRST | AFTER <字段2>
例如:
mysql> ALTER TABLE tb_dept1 MODIFY column1 VARCHAR(12) FIRST;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> DESC tb_dept1;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| column1 | varchar(12) | YES | | NULL | |
| id | int(11) | NO | PRI | NULL | |
| name | varchar(30) | YES | | NULL | |
| column3 | int(11) | YES | | NULL | |
| loc | varchar(50) | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+
5 rows in set (0.01 sec)
或者
mysql> ALTER TABLE tb_dept1 MODIFY column1 VARCHAR(12) AFTER loc;
Query OK, 0 rows affected (0.06 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> DESC tb_dept1;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(30) | YES | | NULL | |
| column3 | int(11) | YES | | NULL | |
| loc | varchar(50) | YES | | NULL | |
| column1 | varchar(12) | YES | | NULL | |
+---------+-------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
4.3.7 更改表的存储引擎
语法:
ALTER TABLE <表名> ENGINE=<更改后的存储引擎>
例如:
mysql> SHOW CREATE TABLE tb_deptment3\G
*************************** 1. row ***************************
Table: tb_deptment3
Create Table: CREATE TABLE `tb_deptment3` (
`id` int(11) NOT NULL,
`name` varchar(22) DEFAULT NULL,
`location` varchar(50) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `STH` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
mysql> ALTER TABLE tb_deptment3 ENGINE=MyISAM;
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> SHOW CREATE TABLE tb_deptment3\G
*************************** 1. row ***************************
Table: tb_deptment3
Create Table: CREATE TABLE `tb_deptment3` (
`id` int(11) NOT NULL,
`name` varchar(22) DEFAULT NULL,
`location` varchar(50) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `STH` (`name`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
4.3.8 删除表的外键约束
语法:
ALTER TABLE <表名> DROP FOREIGN KEY <外键约束名>
例如:
mysql> CREATE TABLE tb_emp9
-> (
-> id INT(11) PRIMARY KEY,
-> name VARCHAR(25),
-> deptId INT(11),
-> salary FLOAT,
-> CONSTRAINT fk_emp_dept FOREIGN KEY (deptId) REFERENCES tb_dept1(id)
-> );
Query OK, 0 rows affected (0.04 sec)
mysql> SHOW CREATE TABLE tb_emp9\G
*************************** 1. row ***************************
Table: tb_emp9
Create Table: CREATE TABLE `tb_emp9` (
`id` int(11) NOT NULL,
`name` varchar(25) DEFAULT NULL,
`deptId` int(11) DEFAULT NULL,
`salary` float DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `fk_emp_dept` (`deptId`),
CONSTRAINT `fk_emp_dept` FOREIGN KEY (`deptId`) REFERENCES `tb_dept1` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
mysql> ALTER TABLE tb_emp9 DROP FOREIGN KEY fk_emp_dept;
Query OK, 0 rows affected (0.08 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> SHOW CREATE TABLE tb_emp9\G
*************************** 1. row ***************************
Table: tb_emp9
Create Table: CREATE TABLE `tb_emp9` (
`id` int(11) NOT NULL,
`name` varchar(25) DEFAULT NULL,
`deptId` int(11) DEFAULT NULL,
`salary` float DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `fk_emp_dept` (`deptId`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
1 row in set (0.00 sec)
4.4 删除数据表
4.4.1 删除没有被关联的表
语法:
DROP TABLE [IF EXISTS]表1,表2...
例如:
mysql> DROP TABLE IF EXISTS tb_dept2;
Query OK, 0 rows affected, 1 warning (0.00 sec)
mysql> SHOW TABLES;
+----------------+
| Tables_in_test |
+----------------+
| tb_dept1 |
| tb_deptment3 |
| tb_emp1 |
| tb_emp2 |
| tb_emp3 |
| tb_emp4 |
| tb_emp5 |
| tb_emp7 |
| tb_emp8 |
| tb_emp9 |
+----------------+
10 rows in set (0.00 sec)