MySQL数据表基本操作

数据表基本操作

数据表是数据库的重要组成部分，每一个数据库都是由若干个数据表组成。没有数据表就没法在数据库中存放数据。

1.创建数据表

创建数据表是指在已经创建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。

CREATE  TABLE [IF NOT EXISTS] tb1_name

案例1：

创建基本表：

创建如下要求的表tb_emp1

字段名称	数据类型	备注
id	int(11)	员工编号
name	varchar(25)	员工名称
deptid	int(11)	所在部门编号
salary	float	工资

##创建test数据库
mysql> create database test;
Query OK, 1 row affected (0.01 sec)

##切换数据库
mysql> use test;
Database changed

##在test数据库中创建表
mysql> create table tb_emp1(id int(11),name varchar(25),deptid int(11),salary float);
Query OK, 0 rows affected, 2 warnings (0.06 sec)

##查看当前数据库中所有表

mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| tb_emp1        |
| test_table     |
+----------------+
2 rows in set (0.02 sec)

##查询表结构

mysql> desc tb_emp1;
+--------+-------------+------+-----+---------+-------+
| Field  | Type        | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id     | int         | YES  |     | NULL    |       |
| name   | varchar(25) | YES  |     | NULL    |       |
| deptid | int         | YES  |     | NULL    |       |
| salary | float       | YES  |     | NULL    |       |
+--------+-------------+------+-----+---------+-------+
4 rows in set (0.01 sec)

##查询表定义

mysql> show create table tb_emp1\G;
*************************** 1. row ***************************
       Table: tb_emp1
Create Table: CREATE TABLE `tb_emp1` (
  `id` int DEFAULT NULL,
  `name` varchar(25) DEFAULT NULL,
  `deptid` int DEFAULT NULL,
  `salary` float DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.01 sec)

案例2：

创建临时表：

临时表不生成物理文件，会花内有效。

mysql> create temporary table new_tb_emp1 select * from tb_emp1;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0


mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| tb_emp1        |
| test_table     |
+----------------+
2 rows in set (0.00 sec)


mysql> show create table new_tb_emp1;
+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table       | Create Table

 |
+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| new_tb_emp1 | CREATE TEMPORARY TABLE `new_tb_emp1` (
  `id` int DEFAULT NULL,
  `name` varchar(25) DEFAULT NULL,
  `deptid` int DEFAULT NULL,
  `salary` float DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci |
+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

2.修改数据表

ALTER TABLE命令可以修改表结构，比如增加和删除索引，修改已经存在字段的数据类型，重命名字段或表名称，也可以修改表的存储引擎等特性。

ALTER TABLE tb_name .....

案例1（增，删）

mysql> create table t1(a int,b char(10));
Query OK, 0 rows affected (0.03 sec)

mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| t1             |
| tb_emp1        |
| test_table     |
+----------------+
3 rows in set (0.00 sec)

##修改表名
mysql> alter table t1 rename t2;
Query OK, 0 rows affected (0.02 sec)


mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| t2             |
| tb_emp1        |
| test_table     |
+----------------+
3 rows in set (0.00 sec)

mysql> desc t2;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| a     | int      | YES  |     | NULL    |       |
| b     | char(10) | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+
2 rows in set (0.00 sec)


##增加字段
mysql> alter table t2 add d timestamp;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0


##修改数据类型和字段名称
mysql> alter table t2 modify a tinyint not null,change b c char(20);
Query OK, 0 rows affected (0.07 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc t2;
+-------+-----------+------+-----+---------+-------+
| Field | Type      | Null | Key | Default | Extra |
+-------+-----------+------+-----+---------+-------+
| a     | tinyint   | NO   |     | NULL    |       |
| c     | char(20)  | YES  |     | NULL    |       |
| d     | timestamp | YES  |     | NULL    |       |
+-------+-----------+------+-----+---------+-------+
3 rows


##删除字段
mysql> alter table t2 drop c;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc t2;
+-------+-----------+------+-----+---------+-------+
| Field | Type      | Null | Key | Default | Extra |
+-------+-----------+------+-----+---------+-------+
| a     | tinyint   | NO   |     | NULL    |       |
| d     | timestamp | YES  |     | NULL    |       |
+-------+-----------+------+-----+---------+-------+
2 rows in set (0.00 sec)


##增加自增新字段
mysql> alter table t2 add c int unsigned not null auto_increment,add primary key (c);
Query OK, 0 rows affected (0.07 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc t2;
+-------+--------------+------+-----+---------+----------------+
| Field | Type         | Null | Key | Default | Extra          |
+-------+--------------+------+-----+---------+----------------+
| a     | tinyint      | NO   |     | NULL    |                |
| d     | timestamp    | YES  |     | NULL    |                |
| c     | int unsigned | NO   | PRI | NULL    | auto_increment |
+-------+--------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)


##指定字段最左增加字段
mysql> alter table t2 add first_col int(11) first;
Query OK, 0 rows affected, 1 warning (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 1

mysql> desc t2;
+-----------+--------------+------+-----+---------+----------------+
| Field     | Type         | Null | Key | Default | Extra          |
+-----------+--------------+------+-----+---------+----------------+
| first_col | int          | YES  |     | NULL    |                |
| a         | tinyint      | NO   |     | NULL    |                |
| d         | timestamp    | YES  |     | NULL    |                |
| c         | int unsigned | NO   | PRI | NULL    | auto_increment |
+-----------+--------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)


##指定相对位置增加字段

mysql> alter table t2 add second_col int(11) after d;
Query OK, 0 rows affected, 1 warning (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 1

mysql> desc t2;
+------------+--------------+------+-----+---------+----------------+
| Field      | Type         | Null | Key | Default | Extra          |
+------------+--------------+------+-----+---------+----------------+
| first_col  | int          | YES  |     | NULL    |                |
| a          | tinyint      | NO   |     | NULL    |                |
| d          | timestamp    | YES  |     | NULL    |                |
| second_col | int          | YES  |     | NULL    |                |
| c          | int unsigned | NO   | PRI | NULL    | auto_increment |
+------------+--------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

##修改字段位置

mysql> alter table t2 modify c int unsigned first;
Query OK, 0 rows affected (0.07 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc t2;
+------------+--------------+------+-----+---------+-------+
| Field      | Type         | Null | Key | Default | Extra |
+------------+--------------+------+-----+---------+-------+
| c          | int unsigned | NO   | PRI | NULL    |       |
| first_col  | int          | YES  |     | NULL    |       |
| a          | tinyint      | NO   |     | NULL    |       |
| d          | timestamp    | YES  |     | NULL    |       |
| second_col | int          | YES  |     | NULL    |       |
+------------+--------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

3.复制数据表

复制指定表的表结构，如列定义和索引定义等，但不复制内容，指定的表不能为视图。

案列：

##为表插入数据
mysql> insert into tb_emp1(id,name,deptid,salary) value(1,'zhangsan',10,1000);
Query OK, 1 row affected (0.01 sec)

mysql> select * from tb_emp1;
+------+----------+--------+--------+
| id   | name     | deptid | salary |
+------+----------+--------+--------+
|    1 | zhangsan |     10 |   1000 |
+------+----------+--------+--------+
1 row in set (0.00 sec)

#复制数据表，但不复制数据
mysql> create table like_tb_emp1 like tb_emp1;
Query OK, 0 rows affected (0.03 sec)

mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| like_tb_emp1   |
| t2             |
| tb_emp1        |
| test_table     |
+----------------+
4 rows in set (0.00 sec)

mysql> desc like_tb_emp1;
+--------+-------------+------+-----+---------+-------+
| Field  | Type        | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id     | int         | YES  |     | NULL    |       |
| name   | varchar(25) | YES  |     | NULL    |       |
| deptid | int         | YES  |     | NULL    |       |
| salary | float       | YES  |     | NULL    |       |
+--------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

mysql> select *from like_tb_emp1;
Empty set (0.00 sec)



###要复制表数据，可以通过查询指定数据创建新表
mysql> create table select_tb_emp1 select *from tb_emp1;
Query OK, 1 row affected (0.03 sec)
Records: 1  Duplicates: 0  Warnings: 0


mysql> select * from select_tb_emp1;
+------+----------+--------+--------+
| id   | name     | deptid | salary |
+------+----------+--------+--------+
|    1 | zhangsan |     10 |   1000 |
+------+----------+--------+--------+
1 row in set (0.00 sec)

4.删除数据表

在MySQL数据库中，对于不再需要的数据表，可以将其从数据库中删除。

在删除表的同时，表的结构和表中所有的数据都会被删除，因此在删除数据表之前最好先备份，以免造成无法挽回的损失。

DROP TABLE [IF NOT EXISTS] 表名......

案例：

mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| like_tb_emp1   |
| select_tb_emp1 |
| t2             |
| tb_emp1        |
| test_table     |
+----------------+
5 rows in set (0.00 sec)

mysql> drop table t2;
Query OK, 0 rows affected (0.02 sec)

mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| like_tb_emp1   |
| select_tb_emp1 |
| tb_emp1        |
| test_table     |
+----------------+
4 rows in set (0.00 sec)

mysql> desc t2;
ERROR 1146 (42S02): Table 'test.t2' doesn't exist
mysql> select * from t2;
ERROR 1146 (42S02): Table 'test.t2' doesn't exist