Question

I have the following SQL Query.

SELECT MIN(PLD.available_date) as 'FROM', MAX(PLD.available_date) as 'UNTIL', (
CASE
    WHEN DATEDIFF('2012-04-01', '2012-04-10') = (COUNT(PLD.available_date) - 1)
    THEN 'Yes'
    ELSE 'No'
END) as isAvailable, PL.*
FROM `parking_lot_dates` as PLD
INNER JOIN parking_lots as PL ON PLD.plid = PL.plid
WHERE PLD.available_date BETWEEN '2012-04-01' AND '2012-04-10'
GROUP BY PLD.plid

But is it possible to put that INNER JOIN into a CASE? what i’m trying to accomplish is that when the isAvailable column’s value is Yes then grab the extra information otherwise don’t grab it.

I tried putting the INNER JOIN between a CASE statement but it didn’t work.

Thanks in advance.

Answer 1

You can’t do a conditional join in the way you want, but you could instead:

SELECT PLD.FROM, PLD.UNTIL, IF(PLD.isAvailable, 'Yes', 'No') AS isAvailable, PL.*
FROM (
    SELECT   plid
      ,      MIN(available_date) AS `FROM`
      ,      MAX(available_date) AS `UNTIL`
      ,      DATEDIFF('2012-04-01', '2012-04-10') = COUNT(*) - 1 AS `isAvailable`
    FROM     parking_lot_dates
    WHERE    available_date BETWEEN '2012-04-01' AND '2012-04-10'
    GROUP BY plid
  ) AS PLD LEFT JOIN parking_lots AS PL ON (
    PLD.isAvailable AND PL.plid = PLD.plid
  )
WHERE NOT (PLD.isAvailable AND PL.plid IS NULL)

The subquery performs the grouping operation on the parking_lot_dates table, and we make an outer join between that and the parking_lots table in order to have records even when the join condition is not satisfied. Learn about SQL joins.

The WHERE clause imitates the effect of your INNER JOIN by eliminating any results for which there was not a matching record in parking_lots when one was expected; it can be omitted if you didn’t actually want that behaviour.