从两个表中选择不匹配的记录，并在第二个表上使用过滤器

Question

I have 2 mysql tables a and b. I need to show all records in a that are not in b using a common column 'ID'. Normally this is pretty straight forward but my problem here is this: I need to put a where clause on table b because I'm not interested in all unmatched records just the ones that meet the table 2 where clause criterion:

SELECT a.ID, a.Description 
FROM a 
LEFT JOIN b ON a.ID = b.ID 
WHERE a.Inactive = 0 
AND b.Room = '101' 
AND b.ID Is Null

This returns nothing. However if I remove the AND b.Room = '101' part, it displays the expected results. But I need that condition because I need only unmatched records specific to a 'room' and not all unmatched records

Answer 1

Move the conditions involving table b from WHERE to ON clause:

SELECT a.ID, a.Description 
FROM a 
LEFT JOIN b ON a.ID = b.ID AND b.Room = '101' 
WHERE a.Inactive = 0 
AND b.ID Is Null

It'll find rows where a does not have a match in b (id matches and room number = 101).