Question

Can this MS-Access query be improved?

i.e. the “SELECT TOP 1 col FROM Table2” line is repeated

If there is an index before curindex in Table2,
the query returns the previous value of col from Table2,

otherwise the value of col matching id is returned from Table3.

i.e.

if curindex = 7 value returned = 221 from Table2

if curindex = 5 value returned = 200 from Table3

Table2                               Table3
id index  col                        id    col
1    1    110                        1     100
1    2    120                        2     200
1    3    130                        3     300
1    4    140
2    5    211
2    6    221
2    7    231

PARAMETERS [curindex] Short;
SELECT TOP 1
  IIF (
    ( SELECT TOP 1 col
      FROM Table2
      WHERE index < [curindex]
       AND id =
        ( SELECT id
           FROM Table2
           WHERE index = [curindex])
    ) ,
        ( SELECT TOP 1 col
          FROM Table2
          WHERE index < [curindex]
            AND id =
            ( SELECT id
              FROM Table2
              WHERE index = [curindex])
          ORDER BY index DESC
        ),
        ( SELECT TOP 1 col
          FROM Table3
          WHERE id = 
           ( SELECT id
             FROM Table2
             WHERE index = [curindex])
        )
     ) AS col
FROM Table2
ORDER BY index

Answer 1

You can limit the cost of the iff condition by reducing your result query.

ie

SELECT TOP 1
  IIF (
( SELECT TOP 1 col
  FROM Table2
  WHERE index < [curindex]
   AND id =
    ( SELECT id
       FROM Table2
       WHERE index = [curindex])
) ,
    ( SELECT TOP 1 col
      FROM Table2
      WHERE index = [curindex])
    ),
    ( SELECT TOP 1 col
      FROM Table3
      WHERE id = 
       ( SELECT id
         FROM Table2
         WHERE index = [curindex])
    )
 ) AS col
FROM Table2
ORDER BY index