Question

If you use a LEFT JOIN and also have a WHERE clause, are all the conditions in the WHERE clause being disregarded if the table that you tried to join does not exist?

In other words, do I have to specifically compare against the id from the LEFT JOIN?

SELECT distinct(watchedItems.id) 
FROM globalItems, watchedItems 
  LEFT JOIN bidGroups ON bidGroups.bidGroupID = watchedItems.bidGroupID

WHERE    
  watchedItems.aid = globalItems.aid 
  AND watchedItems.processRunning = 0
  (watchedItems.bidGroupID IS NULL 
  OR (watchedItems.bidGroupID IS NOT NULL AND bidGroups.bidGroupQty > 0))

Could I write instead of the entire last bit just

AND bidGroups.bidGroupQty > 0

and it will not be tested because bidGroups does not exist if the LEFT JOIN fails? I know that without the LEFT JOIN it will definitely test against it all the time, which means if this test fails, the entire statement is not executed. But I want it to be executed in any case (with and without bidGroups.)

Answer 1

If the table doesn’t exist your statement won’t parse and will result in a SQL exception.

EDIT

I’d write your query this way, for readability:

SELECT distinct(watchedItems.id)
FROM globalItems INNER JOIN
     watchedItems ON globalItems.aid = watchedItems.aid LEFT JOIN
     bidGroups ON bidGroups.bidGroupID = watchedItems.bidGroupID
WHERE
    watchedItems.processRunning = 0
AND (watchedItems.bidGroupID IS NULL
     OR (watchedItems.bidGroupID IS NOT NULL
     AND bidGroups.bidGroupQty > 0)
);

The answer to whether you need to check watchedItems.bigGroupID IS NOT NULL is no, you do not have to as the join condition already covers that.