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数据库约束对mysql数据库，共四种约束：unique，not null，primary key，foreign key。1.使用列级约束语法建立约束：create table test( test_id int primary key, test_name varchar(64) unique, test_sex varchar(2) not null );2.使用表级约束

单表查询select column1,column2,... from 表名 [where condition];#从查询结果中去掉重复行 select distinct column1,column2... from 表名 [where condition];**sql中特殊的比较运算符**1.expr1 between expr2 and expr3; 2.expr1 in (expr2,exp

http://blog.youkuaiyun.com/mhmyqn/article/details/17043921 http://bbs.youkuaiyun.com/topics/391851148 https://jingyan.baidu.com/article/f3ad7d0ffc061a09c3345bf0.html http://www.jianshu.com/p/890b050898c6