常用sql

2019独角兽企业重金招聘Python工程师标准>>>

1、添加/删除 主键约束

alter table user add CONSTRAINT PRIMARY KEY(id); alter table user drop PRIMARY key;

2、添加/删除 唯一性约束

alter table user add CONSTRAINT UNIQUE(username); alter table user drop INDEX username;

3、添加/删除非空约束

alter table user modify age varchar(11) not null; alter table user modify age varchar(11);

4、添加外键约束

alter table student add foreign key(tab_id) references tab(id);

5、查看mysql隔离级别

show variables like '%isolation%';

6、设置mysql隔离级别

set session transaction isolation level read uncommitted/read committed/repeatable read/serializable;

7、字符串拼接

SELECT empno,'姓名：'||ename FROM emp;

转载于:https://my.oschina.net/bob1900/blog/1552661