Mysql：仅当列为空时才加入

Question

I am trying to write a query in MySQL to fetch a list of images. The problem is that, I want to join another table only when a column is not null.

images:

|id|image                                                               |user_id|circle_id|is_deleted|
------------------------------------------------------------------------------------------------------
|1 |cdb10bee27262c597619fdef25bae0eb673f5d5d1b9b33697e40906ad851a25b.jpg|30     |1        |0         |
|2 |9e3213276225b900a3e19262a7761ec3fdf608b95441aa8fbc012f246514d394.jpg|30     |1        |0         |
|3 |a51e104c82de33a804ae0a09e719f3cd7aa205a0830eab4e1e4aa26fd8a6313d.jpg|30     |NULL     |0         |

circles_users:

|id|circle_id|user_id|
----------------------
|1 |1        |31     |

users:

|id|username|
-------------
|30|elk     |
|31|moose   |

Initially, I came up with this, but after inspection, I realised it would not work.

SELECT images.*
FROM images
    LEFT JOIN circles_users ON circles_users.circle_id = images.circle_id
WHERE (circles_users.user_id = '31' OR images.circle_id IS NULL)
    AND images.user_id = '30'
    AND images.is_deleted = '0'
LIMIT 16

I can't seem to get it right anyhow, so I decided to write some pseudo sql to illustrate what I wish to achieve

SELECT images.*
    FROM images

        CASE
            WHEN images.circle_id IS NOT NULL 
            THEN LEFT JOIN circles_users ON circles_users.circle_id = images.circle_id AND circles_users.user_id = '32'
        END CASE

    WHERE images.user_id = '30'
    AND images.is_deleted = '0'
    LIMIT 16

Hope you get what I mean. Thanks for the help.

Basically, I want to fetch all images posted by a user (poster) has if another user is in the circle shared by the poster or if the circle_id is null

Answer 1

如果我理解你的问题，你应该使用INNER JOIN 。

Answer 2

Then Change your query to something like

SELECT im.*
FROM images im
    LEFT JOIN circles_users cu ON cu.circle_id = im.circle_id AND cu.user_id = '31'
WHERE im.user_id = '30'
    AND im.is_deleted = '0'
LIMIT 16

Answer 3

My test data was flawed and my trust in my sql knowledge ahaky. The original query is correct and works as expected.

SELECT images.* FROM images LEFT JOIN circles_users ON circles_users.circle_id = images.circle_id WHERE (circles_users.user_id = '31' OR images.circle_id IS NULL) AND images.user_id = '30' AND images.is_deleted = '0' LIMIT 16

EXPLANATION When the query is run, it fetches all images with user_id = 30. If circle_id is not null, it fetches the corresponding circles_users row else nothing as it is a left join The Where statement then checks if the circles_users.user_id (if it exists) = 31 else it checks if images.circle_id is null effectively achieving the What I aimed to.