sql没有结果，因为左连接为空

Question

So I have this query:

SELECT 
    AED.aId, CJ.* 
FROM 
    AED
        LEFT JOIN 
            Cronjob as CJ 
                ON CJ.aID = AED.aId 
WHERE 
                AED.aStatus = '1'
        AND 
                (
                        CJ.cjDatum < CURRENT_DATE - INTERVAL 14 DAY 
                AND
                        AED.aRegistratie > CURRENT_DATE - INTERVAL 10 YEAR
                )
        OR
                ( 
                        CJ.cjStatus = '9' 
                OR 
                        CJ.cjStatus = '2'
                )

The problem is, the Cronjob table is empty, and if it's empty is still want to give all the Id's from AED with the status 1

I couldn't find anything use full, so I hope you guys can help!

Answer 1

You should move all the CJ criteria into the join's ON clause.

ON (
  CJ.aID = AED.aId 
  AND (cjStatus in ('2','9') OR cjDatum < CURRENT_DATE - INTERVAL 14 DAY ))

Option two would be to leave them in WHERE, but make provisions for the case that cjStatus and friends are NULL (which they will be if no match is found).

OR cjStatus IS NULL

Answer 2

When there is no CronJob associated, the filter CJ.cjStatus = '9' (for example) return false, since CJ.cjStatus is null. That's what a LEFT JOIN do, it returns null field when there is no correspondance.

To add filter on the table you want to LEFT JOIN with, the filter clause must be in the join clauses like this:

SELECT AED.aId
     , CJ.* 
FROM AED
    LEFT JOIN Cronjob as CJ 
        ON CJ.aID = AED.aId 
        AND (CJ.cjDatum < CURRENT_DATE - INTERVAL 14 DAY 
             AND AED.aRegistratie > CURRENT_DATE - INTERVAL 10 YEAR
             )
        OR (CJ.cjStatus = '9' OR CJ.cjStatus = '2')
WHERE AED.aStatus = '1'

Answer 3

Add OR (CJ.aid is null) to your AND part of condition:

SELECT 
    AED.aId, CJ.* 
FROM 
    AED
        LEFT JOIN 
            Cronjob as CJ 
                ON CJ.aID = AED.aId 
WHERE 
                AED.aStatus = '1'
        AND 
             (  
              (
                        CJ.cjDatum < CURRENT_DATE - INTERVAL 14 DAY 
                AND
                        AED.aRegistratie > CURRENT_DATE - INTERVAL 10 YEAR
                ) 
              OR (CJ.aid is null)
              ) 


        OR
                ( 
                        CJ.cjStatus = '9' 
                OR 
                        CJ.cjStatus = '2'
                )