在有条件的三张桌子上进行内联接

Question

I am trying to fetch all the photos detail from a tag slug (URL for the tag) , the database has three tables:

|-----------------------|
|==> photo              |
|   -> id               |
|   -> custom_id        |
|   -> title            |
|-----------------------|
|==> tags               |
|   -> id               |
|   -> slug             |
|-----------------------|
|==> tags_relation      |
|   -> tid              | <-- this is the tags.id
|   -> pid              | <-- this is the photo.custom_id
|-----------------------|

here is my mysql code to INNER JOIN all the tables and to get 20 photos from a tag:

SELECT photo.*, tags.*, tags_relation.*, 
FROM tags WHERE tags.slug = 'people' 
INNER JOIN tags_relation ON = tags_relation.tid = tags.id 
INNER JOIN photo ON photo.custom_id = tags_relation.pid
LIMIT 20  
ORDER BY photo.date DESC

The query is not correct anyway, and i can't understand how the INNER JOIN should work here, any idea? Thanks

Answer 1

SQL has a specific ordering of clauses. In your case:

• SELECT
• FROM
• WHERE
• GROUP BY
• ORDER BY
• LIMIT

This is always the ordering within a query. Note that JOIN expressions are not "clauses". They are part of the FROM clause (and in MySQL, the update and delete clauses as well).

Applied to your query:

SELECT p.*, t.*, tr.*
FROM tags t INNER JOIN
     tags_relation tr
     ON tr.tid = t.id INNER JOIN
     photo p
     ON p.custom_id = tr.pid
WHERE t.slug = 'people' 
ORDER BY p.date DESC
LIMIT 20  

You will note that the indentation highlights the clauses which are a fundamental part of the language.

I also added table aliases, which make the query easier to write and to read. And fixed some minor things, such as misplaced commas.

I note you are pulling too many columns out of the data. You should just list the columns you want (probably p.* ).

Answer 2

try this..

SELECT photo.*, tags.*, tags_relation.* 
    FROM tags WHERE tags.slug = 'people' 
    INNER JOIN tags_relation ON(tags_relation.tid = tags.id) 
    INNER JOIN photo ON (photo.custom_id = tags_relation.pid)
    ORDER BY photo.date DESC
    LIMIT 20