Question

I have this SQL query in Access, which works fine:

SELECT TableA.FieldA As [Code],
   Count(TableA.FieldC) AS [Count]
FROM ((MainTable)
   LEFT JOIN TableA ON MainTable.FieldB = TableA.FieldB)
WHERE (((MainTable.DateOf)>=#1/1/2012#))
   AND Clng(TableA.FieldA) >= 119593451
   AND Clng(TableA.FieldA) <= 119593461
GROUP BY TableA.FieldA;

But when I try another left join, like so:

SELECT TableA.FieldA As [Code],
   Count(TableA.FieldC) AS [Count]
FROM ((MainTable)
   LEFT JOIN TableA ON MainTable.FieldB = TableA.FieldB)
   LEFT JOIN TableB ON TableA.FieldD = TableB.FieldD
WHERE (((MainTable.DateOf)>=#1/1/2012#))
   AND Clng(TableA.FieldA) >= 119593451
   AND Clng(TableA.FieldA) <= 119593461
GROUP BY TableA.FieldA;

I am using the parenthesis in the FROM claused based on this: http://nm1m.blogspot.com/2007/10/multiple-left-joins-in-ms-access.html

It gives the error Invalid use of Null, which doesn’t make sense to me as I’m performing no null check etc. What is the problem here? I’m trying to pull a field in TableB to display (but did not put it in the select section yet).

Answer 1

Access likes its parentheses. Add more parentheses around joins.

The CLng function requires a non-null value. You can fix this with CLng(Nz(TableA.FieldA, "0")) >= 119593451. Though, if the field is numeric, why on earth would you make it text to begin with? This is a serious problem that I highly recommend fixing immediately (if at all possible) by changing the data type to a numeric one.

But you have another problem. It is meaningless to LEFT JOIN to a table if you then in the WHERE clause put a condition, as you are doing in your query! Either change to an INNER JOIN or put your conditions on TableA into the ON clause of the LEFT JOIN. If Access won’t allow this syntax, then change to a derived table:

SELECT
   TableA.FieldA As [Code],
   Count(TableA.FieldC) AS [Count]
FROM
   (
      (
         (MainTable)
         LEFT JOIN (
            SELECT * FROM TableA
            WHERE
               Clng(Nz(TableA.FieldA, "0")) >= 119593451
               AND Clng(Nz(TableA.FieldA, "0")) <= 119593461
         ) TableA ON MainTable.FieldB = TableA.FieldB
      )
      LEFT JOIN TableB ON TableA.FieldD = TableB.FieldD
   )
WHERE (((MainTable.DateOf)>=#1/1/2012#))
GROUP BY TableA.FieldA;

Note: adding WHERE TableA.FieldA IS NOT NULL may work but may not work. In Access it could be 100% safe but such a query in SQL Server would NOT be safe because there is no guaranteeing the order of conditions being applied. Obviously, in SQL Server you can convert to an integer even when NULL and there is no CLng function, but the point is still valid: do not get in the habit of relying on some imagined order of WHERE conditions protecting you from invalid type conversion: instead you must handle the invalid type conversion inside the expression that can error–that is best practice.