查找数据

﹏ヽ暗。殇╰゛Y 2022-05-19 02:41 135阅读 0赞

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只查找指定的字段：

select id ,usersname from users;

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select users.id,users.username from users;针对多表查询

换名查找

select id as userid,username as uname from user;

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select id as usersname from users;

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条件查找

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select sex from users group by sex ;

select sex from users group by 1 ;

对sex栏所有数据结果进行分组；

也可以对对指定的记录进行分组：

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select sex，age  from users group by 1 having age>35,在所有记录中，在age大于当前条件下，进行分组

主义age也要在select语句里；

select sex from users group by 1 having count(id) >=2;意思是  sex的值为2的有几个？

此处的id出现在聚合函数内

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select \* from users order by id desc,通过降序排列

select \* from users order by age ,age默认升序从小到大排列，

select \* from users order by age ，id desc;

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limit3,2的意思是从下标为3（下标从0开始）的记录开始返回，返回2条数据；

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虽然是降序排列，但是limit里的第一个数和id没有关系，只与数据集里的排列顺序有关

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