sql - 上一个值的 ms 访问查询

Question

这个 MS-Access 查询可以改进吗？

即重复“SELECT TOP 1 col FROM Table2”行

如果Table2中curindex之前有索引，则查询返回Table2中col之前的值，

否则从 Table3 中返回 col 匹配 id 的值。

IE

如果 curindex = 7 返回值 = 来自 Table2 的 221

如果 curindex = 5 返回值 = 200 来自 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

您可以通过减少结果查询来限制 iff 条件的成本。

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