ms访问查询以前的值

Question

是否可以改善此MS-Access查询？

即重复“ SELECT TOP 1 col FROM Table2”行

如果表2中的curindex之前有一个索引，则查询返回表2中col的先前值，

否则，从表3返回col匹配id的值。

即

如果curindex = 7从表2返回的值= 221

如果curindex = 5从表3返回的值= 200

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

您可以通过减少结果查询来限制iff条件的成本。

即

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