【MySQL-手记】数据表的修改(添加/删除数据表，添加/删除约束)

落日映苍穹つ 2022-06-16 00:15 237阅读 0赞

**![20170430143347802][]**

**【添加/删除数据表】**

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ALTER TABLE users ADD age TINYINT UNSIGNED NOT NULL DEFAULT 10;//添加单列

![Center][]

ALTER TABLE users ADD password VARCHAR(30) NOT NULL AFTER username ;//添加有序单列

![Center 1][]

**ALTER TABLE users ADD truename VARCHAR(30) NOT NULL FIRST ;//添加有序单列**

**![Center 2][]**

ALTER TABLE users DROP truename;      //删除单列

**![20170430131410020][]**

**ALTER TABLE users DROP password,DROP age;      //删除多列**

![20170430131659680][]

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**【添加/删除约束】**

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# 添加主键约束 #

**创建一个users2表格。**

CREATE TABLE users2(
    username VARCHAR(20) NOT NULL,
    pid SMALLINT UNSIGNED
    );

![20170430132500699][]

**先增加一个新列： ALTER TABLE users2 ADD id SMALLINT UNSIGNED FIRST;**

**![20170430132858962][]**

**为新增ID添加主键约束：ALTER TABLE users2 ADD CONSTRAINT PK\_users2\_id PRIMARY KEY (id);![20170430133508111][]**

**删除主键约束：**

**ALTER TABLE users2 DROP PRIMARY KEY;**

**![20170430140432923][]**

# 添加唯一约束 #

为usename添加唯一约束： ALTER TABLE users2 ADD UNIQUE (username);

![20170430134107740][]

**删除唯一约束前先查看，哪些字段有约束，删除的是约束而不是字段！！！**

SHOW INDEXES FROM users2;//查看索引

**![20170430141316660][]**

ALTER TABLE users2 DROP INDEX  username;

![20170430141536052][]

# 添加外键约束 #

给pid添加外键约束，已知外键列父表的结构：![20170430134459937][]

**ALTER TABLE user2 ADD FOREIGN KEY (pid) REFERENCES provinces(id);**

**![20170430135201918][]**

**删除外键约束，先查看：SHOW CREATE TABLE users2;**

**![20170430141952065][]**

ALTER TABLE users2 DROP FOREIGN  KEY users2\_ibfk\_1;    //删除之后，查看下

![20170430142339025][]

# 添加／删除默认约束 #

新建一个字段ａｇｅ，给ａｇｅ添加默认约束

![20170430135736029][]

**ALTER TABLE users2 ALTER age SET DEFAULT 15;**

**![20170430135946749][]**

**删除默认约束：**

**ALTER TABLE users2 ALTER age DROP DEFAULT;**

**![20170430140149219][]**

**——————————————————————————————————————————**

**//添加唯一约束  
ALTER TABLE getUserImage ADD UNIQUE (openID );  
//修改数据表字段的名称  
alter table getUserImage change province region varchar(60);  
  
//修改数据库格式  
alter database weixin\_getUserImage character set utf8;   
//修改数据表编码  
 ALTER TABLE  getUserImage DEFAULT CHARACTER SET utf8;  
//该命令用于将表中字段的编码方式改为utf8  
ALTER TABLE getUserImage CHANGE sex sex VARCHAR(10) CHARACTER SET utf8 NOT NULL; **

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[Center]: /images/20220616/c20f9c21fb2c4fc28d3f80722d42c904.png
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[Center 2]: /images/20220616/afabaecada344e24bb587a5e7c49fb73.png
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