Summary of database problems

left join on and vs where

Scene description

Querying business data is , There are the following requirements . Need from a Remove from table b The data contained in the table . My first thought is a left join b on a.id = b.aid and b.aid is null, Then the data came out . But I made a few discoveries ,a There is ,b There are also . So I was lost in thought .

Reappear

To build the data

CREATE TABLE `join_1` (
  `id` int(11) DEFAULT NULL,
  `address` varchar(255) DEFAULT NULL,
  `name` varchar(255) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `join_2` (
  `id` int(11) DEFAULT NULL,
  `phone` varchar(255) DEFAULT NULL,
  `thing` varchar(255) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Inquire about

select * from join_1 left join join_2 on join_1.id = join_2.id and join_2.phone  is not  null;

With and The query results are as follows

It's obvious here that , I obviously don't want my mobile number to be null Of , But there is still something in the query results .. I have clearly added the conditions for judging the air ..

select * from join_1 left join join_2 on join_1.id = join_2.id where join_2.phone  is not  null;

With where The query results are as follows

The query result here is exactly what I want . however and and where What's the difference .

analysis

where Statement occurs in join after , It's right join Filtering of results
on、and sentence , It happened in join Before , The data is in join Is filtered in the process .
Here I use and The query has a null value because ,join Before join_2 There is data in the table , Back left join And then it came out null Data .

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