mysql的表和約束操作_ZenDei技術網路在線

在創建表是預設為加上數據引擎和字元集，如創建一個student表，代碼如下：插入數據(insert和replace) 1.insert命令直接在表的插入一條記錄。 2.replace命令在插入記錄時判斷主鍵是否相同，相同就修改，否則插入一條新記錄。用select 結果集創建一個表修改表名有兩 ...

在創建表是預設為加上數據引擎和字元集，如創建一個student表，代碼如下：

create table students(id int unsigned zerofill auto_increment primary key,
		      name varchar(20) not null,  --不允許欄位為null
		      sex char(1)
		      );


-----------------------------自動加上數據引擎和字元集------------------------
create table students(id int unsigned zerofill auto_increment primary key, 
　　　　　　　　　　　　name varchar(20) not null, 
　　　　　　　　　　　　sex char(1) 
)engines=innodb default charset=utf8;

插入數據(insert和replace)

1.insert命令直接在表的插入一條記錄。

insert into students(sname,sex) values('orna','男');
insert into students(sname,sex) values('lisi','男');
insert into students(sname,sex) values('wangwu','男');

mysql> select  * from students;
+------------+--------+------+------+
| id         | sname  | sex  | tid  |
+------------+--------+------+------+
| 0000000001 | orna   | 男   | NULL |
| 0000000002 | lisi   | 男   | NULL |
| 0000000003 | wangwu | 男   | NULL |
+------------+--------+------+------+
3 rows in set (0.00 sec)

　　2.replace命令在插入記錄時判斷主鍵是否相同，相同就修改，否則插入一條新記錄。

replace into students(id,sname,sex) values(2,'luscy','女');
replace into students(id,sname,sex) values(20,'zhangsan','男');


mysql> select  * from students;
+------------+----------+------+------+
| id         | sname    | sex  | tid  |
+------------+----------+------+------+
| 0000000001 | orna     | 男   | NULL |
| 0000000002 | luscy    | 女   | NULL |
| 0000000003 | wangwu   | 男   | NULL |
| 0000000020 | zhangsan | 男   | NULL |
+------------+----------+------+------+
4 rows in set (0.00 sec)

用select 結果集創建一個表

mysql> select * from students;
+------------+-------+------+
| id         | name  | sex  |
+------------+-------+------+
| 0000000001 | luscy | 女   |
| 0000000002 | lisi  | 男   |
| 0000000003 | orna  | 男   |
+------------+-------+------+
3 rows in set (0.00 sec)

mysql> create table stu select * from students;
Query OK, 3 rows affected (0.33 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from stu;
+------------+-------+------+
| id         | name  | sex  |
+------------+-------+------+
| 0000000001 | luscy | 女   |
| 0000000002 | lisi  | 男   |
| 0000000003 | orna  | 男   |
+------------+-------+------+
3 rows in set (0.00 sec)

修改表名

有兩種方法可以修改表名。

mysql> rename table stu to st;
Query OK, 0 rows affected (0.17 sec)

mysql> alter table st rename to s;
Query OK, 0 rows affected (0.23 sec)

刪除表

drop table 表名;

drop table 表1,表2,表3.............. 表示可以同時刪除多少個表。

修改表的列名稱

在修改欄位需要註意幾點：

修改欄位寬度只能改大，不能改小；
修改欄位類型時，要相容；

mysql> alter table s change name sname varchar(30);
Query OK, 3 rows affected (1.02 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> select * from s;
+------------+-------+------+
| id         | sname | sex  |
+------------+-------+------+
| 0000000001 | luscy | 女   |
| 0000000002 | lisi  | 男   |
| 0000000003 | orna  | 男   |
+------------+-------+------+
3 rows in set (0.01 sec)

限定欄位的取值範圍

create table students(id int unsigned zerofill auto_increment primary key,
		      name varchar(20) not null,
		      sex enum('男','女')   --指定取值範圍。enum和set關鍵字同意
		      );   
insert into students(name,sex) values('orna','男');  --插入數據成功


------------------------------------插入數據時報錯---------------------------------------
mysql> insert into students(name,sex) values('orna','未知');
ERROR 1265 (01000): Data truncated for column 'sex' at row 1
mysql> select * from students;
+------------+------+------+
| id         | name | sex  |
+------------+------+------+
| 0000000001 | orna | 男   |
+------------+------+------+
1 row in set (0.00 sec)

主建、外建和唯一

一個表只能有一個主鍵，但可以用多個欄位建立一個組合主鍵。可以有多個外鍵和唯一約束。

增加主鍵

創建主鍵的兩種方法：
--指定主鍵名
alter table s add constraint pk  primary key(id);   
--不指定主鍵名
alter table s add constraint primary key(id);

　　2.刪除主鍵

mysql> desc s;
+-------+---------------------------+------+-----+------------+-------+
| Field | Type                      | Null | Key | Default    | Extra |
+-------+---------------------------+------+-----+------------+-------+
| id    | int(10) unsigned zerofill | NO   | PRI | 0000000000 |       |
| sname | varchar(30)               | YES  |     | NULL       |       |
| sex   | set('男','女')            | YES  |     | NULL       |       |
+-------+---------------------------+------+-----+------------+-------+
3 rows in set (0.00 sec)

mysql> alter table s drop primary key;
Query OK, 3 rows affected (0.82 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> desc s;
+-------+---------------------------+------+-----+------------+-------+
| Field | Type                      | Null | Key | Default    | Extra |
+-------+---------------------------+------+-----+------------+-------+
| id    | int(10) unsigned zerofill | NO   |     | 0000000000 |       |
| sname | varchar(30)               | YES  |     | NULL       |       |
| sex   | set('男','女')            | YES  |     | NULL       |       |
+-------+---------------------------+------+-----+------------+-------+
3 rows in set (0.00 sec)

　　3.增加欄位

在增加欄位時，可以用first和after關鍵字，但沒有before關鍵字。

mysql> alter table students add address varchar(50) not null after sex;
Query OK, 0 rows affected (0.49 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc students;
+---------+---------------------------+------+-----+---------+----------------+
| Field   | Type                      | Null | Key | Default | Extra          |
+---------+---------------------------+------+-----+---------+----------------+
| id      | int(10) unsigned zerofill | NO   | PRI | NULL    | auto_increment |
| name    | varchar(20)               | NO   |     | NULL    |                |
| sex     | set('男','女')            | YES  |     | NULL    |                |
| address | varchar(50)               | NO   |     | NULL    |                |
+---------+---------------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

　　4.刪除欄位

mysql> select * from s;
+------------+-------+------+
| id         | sname | sex  |
+------------+-------+------+
| 0000000001 | luscy | 女   |
| 0000000002 | lisi  | 男   |
| 0000000003 | orna  | 男   |
+------------+-------+------+
3 rows in set (0.00 sec)

mysql> alter table s drop column sex;
Query OK, 0 rows affected (0.63 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc s;
+-------+---------------------------+------+-----+------------+-------+
| Field | Type                      | Null | Key | Default    | Extra |
+-------+---------------------------+------+-----+------------+-------+
| id    | int(10) unsigned zerofill | NO   |     | 0000000000 |       |
| sname | varchar(30)               | YES  |     | NULL       |       |
+-------+---------------------------+------+-----+------------+-------+
2 rows in set (0.00 sec)

　　2.增加唯一約束

--創建表時增加唯一約束。
create table students(id int unsigned zerofill auto_increment primary key,
		      sname varchar(20) not null unique,
		      sex set('男','女'),
		      tid int unsigned
		      )engine=innodb default charset=utf8;


--創建表後，再增加唯一約束。
alter table students add constraint uk unique(sname);	


mysql> desc students;
+-------+---------------------------+------+-----+---------+----------------+
| Field | Type                      | Null | Key | Default | Extra          |
+-------+---------------------------+------+-----+---------+----------------+
| id    | int(10) unsigned zerofill | NO   | PRI | NULL    | auto_increment |
| sname | varchar(20)               | NO   | UNI | NULL    |                |
| sex   | set('男','女')            | YES  |     | NULL    |                |
| tid   | int(10) unsigned          | YES  |     | NULL    |                |
+-------+---------------------------+------+-----+---------+----------------+
4 rows in set (0.01 sec)

--刪除唯一約束。

mysql> alter table students drop index uk;
Query OK, 0 rows affected (0.26 sec)
Records: 0 Duplicates: 0 Warnings: 0

　　3.增加外鍵約束

alter table students add constraint fk foreign key(tid) references teachers(id);
--alter table students add constraint fk foreign key(tid) references teachers(id) on delete set null;表示刪除外鍵引用的記錄時把當前記錄的外鍵值修改成空。
--alter table students add constraint fk foreign key(tid) references teachers(id) on delete cascade;表示刪除外鍵引用的記錄時把當前記錄同時刪除。
--alter table students add constraint fk foreign key(tid) references teachers(id) on delete set null on update cascade;表示刪除外鍵引用的記錄時把當前記錄的外鍵值修
--改成空，或者修改引用記錄同時修改外鍵引用欄位。
mysql> desc students;
+-------+---------------------------+------+-----+---------+----------------+
| Field | Type                      | Null | Key | Default | Extra          |
+-------+---------------------------+------+-----+---------+----------------+
| id    | int(10) unsigned zerofill | NO   | PRI | NULL    | auto_increment |
| sname | varchar(20)               | NO   |     | NULL    |                |
| sex   | set('男','女')            | YES  |     | NULL    |                |
| tid   | int(10) unsigned          | YES  | MUL | NULL    |                |
+-------+---------------------------+------+-----+---------+----------------+
4 rows in set (0.00 sec)

　　4.刪除外鍵約束

alter table students drop foreign key fk;
alter table students drop index fk;

　　5.創建索引

mysql> create index sname on students(sname desc);
Query OK, 0 rows affected (0.27 sec)
Records: 0  Duplicates: 0  Warnings: 0
--alter table students add index(sname desc);

　　6.刪除索引

alter table students drop index sname;
--drop index sname on students;