Question

I’m new to SQL and I want to implement the following query:

I’ve got two tables, LicenseTbl and UnlockTbl:

LicenseTbl contains information about a purchased software license:

LicenseID, ProgramID, Owner, Location, OrderNo, BlockTime

UnlockTbl contains information about a specific software registration:

UnlockID, LicenseID (foreign key into LicenseTbl), Timestamp, SerialNo, Key, UninstallTime

where BlockTime and UninstallTime contain a timestamp if the license was blocked or the software uninstalled and NULL otherwise.

I want to devise a query that gives me ALL LicenseIDs for which the following conditions hold:

• belongs to a given customer,
• is not blocked,
• is either not listed in the UnlockTbl or there are < X different SerialNo’s in lines which are not marked as uninstalled.

I have written this, but I’m not sure if it is absolutely correct (it’s one of my first SQL queries ever):

SELECT LicenseID FROM LicenseTbl
JOIN UnlockTbl
   ON (LicenseTbl.LicenseID = UnlockTbl.LicenseID) 
WHERE  LicenseTbl.OrderNo   = '$givenOrderNo'
   AND LicenseTbl.Owner     = '$givenOwner'
   AND LicenseTbl.Location  = '$givenLocation'
   AND LicenseTbl.BlockTime IS NULL
   AND UnlockTbl.UninstallTime IS NULL
GROUP BY LicenseTbl.LicenseID, UnlockTbl.Key
HAVING COUNT(*) < $X

(which is supposed to mean, list all licenses which have only been used less than X times simultaneously. I would prefer those that have been used the least first but don’t know how to sort like that.)

Answer 1

This is a good start, but I would change the query to the following…

SELECT
  LicenseID
FROM
  LicenseTbl
LEFT JOIN
  UnlockTbl
    ON  UnlockTbl.LicenseID = LicenseTbl.LicenseID
    AND UnlockTbl.UninstallTime IS NULL
WHERE
      LicenseTbl.OrderNo   = '$givenOrderNo'
  AND LicenseTbl.Owner     = '$givenOwner'
  AND LicenseTbl.Location  = '$givenLocation'
  AND LicenseTbl.BlockTime IS NULL
GROUP BY
  LicenseTbl.LicenseID
HAVING
  COUNT(DISTINCT UnlockTbl.SerialNo) < $X
ORDER BY
  COUNT(DISTINCT UnlockTbl.SerialNo)

1). LEFT JOIN

A LEFT JOIN ensures that all rows in LicenseTbl are returned, even if there are no matches in the UnlockTbl table. (If there are no matches, the UnlockTbl table’s values are all represented as NULL.)

2). UnlockTbl.UninstallTime IS NULL in the JOIN and not the WHERE

The WHERE clause is applied after the JOIN. This means that any records in UnlockTbl where UninstallTime have a real value (NOT NULL) get joined and then get filtered out. This in turn means that if all the relevant records in UnlockTbl have a non-NULL value in UninstallTime, all the rows for that License will get filtered.

3). GROUP BY on just the license, not the Key.

Simply, I don’t know why you had it there, and it doesn’t appear in the English description of what you want.

As you want a list of LicenseIDs, grouping by only that field ensures that you get one record per LicenseID.

4). HAVING clause modified to look at COUNT(DISTINCT SerialNo)

COUNT(*) counts all records. Even if there was no match (All the UnlockTbl values appearing as NULL), this would return 1.

COUNT(SerialNo) counts only records where SerialNo is NOT NULL. If there was no match (All the UnlockTbl values appearing as NULL), this would return 0.

COUNT(DISTINCT SerialNo) also counts only records where SerialNo is NOT NULL, but treats duplicates of the sme value as just 1 entry.

5). ORDER BY COUNT(DISTINCT SerialNo)

Takes the same value as is being filtered in the HAVING clause, and orders by it.