MySQL Join表并获取不存在的结果

Question

I have two tables, one that has a foreign key from the other. I want to get all records that don't exist in the foreign table, based on certain criteria.

Here are the tables I have:

item_setting

• setting_id
• category_id

item

• item_id
• setting_id
• name
• expired_dt

Here's the query I'm using now:

SELECT 
    iset.setting_id
FROM
    item_settings iset
LEFT OUTER JOIN
    item i ON i.setting_id = iset.setting_id
WHERE
    iset.category_id = '5' AND i.setting_id is null

This query works in providing any setting_id's that do not have a record in the item's table within a specific category.

However, now I want to include cases where the expired_dt less than than time() (meaning it's past expired). In otherwords, I would think to add this:

WHERE
    iset.category_id = '5' AND (i.setting_id is null OR i.expired_dt < '".time()."')

However, this doesn't work, it returns all the records.

Any suggestions? Maybe I'm completely over complicating this.... I just want to return the setting_id's from the item_settings table, where the expired_dt associated in the item table is expired or if it does not even exist in the item table.

Thank you!

Answer 1

Try moving the timestamp condition into the join clause. Something like

item_settings iset
LEFT OUTER JOIN
    item i ON i.setting_id = iset.setting_id and i.expired_dt > time()