如何在PHP中运行三个表INNER JOIN？

Question

I need to query several tables for "product tags", based on the producttags table, and subcategorytags table.

Then to produce products where a tag is "shirt".

If I remove the first inner join, and the " WHERE t.producttag = 'shirt' ", it works. Same for the subcategorytags section.

But when I try it with both, it doesn't error, but doesn't product anything.

SELECT p.datebackinstock, p.datepricedrop, p.id, p.catid, p.subid, p.catname, p.subname, 
       p.title, p.price, p.photoprimary, p.comingsoon, p.rcstock, p.preorder, 
       p.bundleroman1, p.bundleroman2, p.bundleroman3, p.bundleroman4, p.bundleroman5, 
       p.pricedrop  FROM products AS p 

INNER JOIN producttags AS t ON p.id = t.prodid

INNER JOIN subcategorytags AS s ON p.subid = s.subid 

WHERE t.producttag = 'shirt' OR s.tag = 'shirt'

Answer 1

You will have to use left joins for this as you do not know wich tags are set. In your query the row will only be returned if a producttag and a subcategorytag is found. A left join will return the row even if no row in another table is found. Because of this you will also need to do an extra check to make sure the values are not null.

SELECT p.datebackinstock, p.datepricedrop, p.id, p.catid, p.subid, p.catname, p.subname, 
       p.title, p.price, p.photoprimary, p.comingsoon, p.rcstock, p.preorder, 
       p.bundleroman1, p.bundleroman2, p.bundleroman3, p.bundleroman4, p.bundleroman5, 
       p.pricedrop  FROM products AS p 

LEFT JOIN producttags AS t ON p.id = t.prodid

LEFT JOIN subcategorytags AS s ON p.subid = s.subid 

WHERE (t.producttag NOT NULL AND t.producttag = 'shirt') OR (s.tag NOT NULL AND s.tag = 'shirt')

More information about right and left joins can be found in the mysql docs .

Answer 2

I'd use left outer join or rather, I would try, I think like:

select
    p.`datebackinstock`, 
    p.`datepricedrop`, 
    p.`id`, 
    p.`catid`, 
    p.`subid`, 
    p.`catname`, 
    p.`subname`, 
    p.`title`, 
    p.`price`, 
    p.`photoprimary`, 
    p.`comingsoon`, 
    p.`rcstock`, 
    p.`preorder`, 
    p.`bundleroman1`, 
    p.`bundleroman2`, 
    p.`bundleroman3`, 
    p.`bundleroman4`, 
    p.`bundleroman5`, 
    p.`pricedrop` 
    from `products` p 
    left outer join `producttags` t on p.`id` = t.`prodid`
    left outer join `subcategorytags` s on p.`subid` = s.`subid`
    where t.`producttag` = 'shirt' or s.`tag` = 'shirt'