php - Join 3 Tables with 3rd Table not associated to Table 2
I am trying to Do the following:-
SELECT t1.a, t1.b, t2.a, t2.b, t3.a, t3.b, t3.c FROM t1 LEFT JOIN t2 ON t1.b = t2.b LEFT JOIN t3 ON t2.b <> t3.b WHERE t3.c = 'someuser'
Result:- I am getting rows of t2 associated in t3
Expected Result:- Get rows of t2 NOT associated in t3
Remember a left join that has non-matching records will result in null data in
t3. I changed the last two lines. Try this:
Although, if you're doing a conditional check on t3, it's always going to be false (when not checking for null)...
Maybe you want this?
The difference between the two queries is: the first one should always return 0 rows since you're expecting t3.b to be null (which MAY indicate that the joined table has no matching records) but then you check t3.c for a value which will never exist in the results. The second query filters on t3.c during the join, and then in your WHERE clause you find null matches, which should hopefully indicate that there was no match (depending on how your data is setup).