SQL LEFT JOIN不返回正确的数据

Question

Im tryoing to build a SQL join table, but it is not returning the correct data that i want. So i have a sliders table with structure like below.

So the table has a foreign key that referes to an item. Items or sliders have a same column called: published, that tells if a sliders/items is or is't published.

I'm trying to build a query that returns all sliders that are published AND items where they belonge are published, Joined with the sliders where item_id is nll.

So according to the image above the query should return only 3 rows with id (2-3-4), because item with id 34 (item_id=34) on table items is not published therefore it shouldnt be on the returned value. The query that i have build so far is this:

SELECT *
FROM sliders
LEFT JOIN items ON sliders.item_id = items.id
WHERE sliders.published =1 AND items.published =1

But the query returns only 2 rows. rows with id 2 and 3 which correspond to item with id 23, 28.

The returned result is as follow:

So it returned 2 rows instead of 3, the row missing is the row with id 3, where item_id=null.

I have been trying all day to figure this out but it seems i am stuck here...

Any help would be very appreciated..

Answer 1

If you use a LEFT JOIN and there isn't a match, the right hand table ( items ) will have NULL as the values of its columns. You therefore can't have items.published = 1 in your WHERE clause and expect those rows to show up because NULL doesn't equal 1 .

Assuming I've not misunderstood the question, you'll need to change that to AND (items.published IS NULL OR items.published = 1) - you could probably also get away with just doing AND items.published <> 0 . That essentially checks that the items are published if they exist .

Answer 2

This is the Final query that returns the correct Data.

SELECT *
 FROM sliders
  LEFT JOIN items 
    ON sliders.item_id = items.id
WHERE sliders.published =1
 AND (
   items.published IS NULL
   OR items.published =1
)