SQL query select top 5 per group leading Access to freeze on simple code and small data

问题

I was trying to reproduce the select top n from group SQL but access freezes (takes minutes to complete) even on small set of data.

date         ret1         anndate

26-Jul-13   0.999214    25-Jul-13
29-Jul-13   0.982684    25-Jul-13
30-Jul-13   0.947938    25-Jul-13
31-Jul-13   1.024081    25-Jul-13
01-Aug-13   1.017739    25-Jul-13
02-Aug-13   1.001621    25-Jul-13
10-Dec-13   0.965405    09-Dec-13
11-Dec-13   1.009705    09-Dec-13
12-Dec-13   1.025508    09-Dec-13
13-Dec-13   0.994232    09-Dec-13
16-Dec-13   1.009065    09-Dec-13
17-Dec-13   0.984549    09-Dec-13
18-Dec-13   1.007299    09-Dec-13

What I wanted to achieve is to have the five following rows per each 'anndate'. for example, for anndate 25 july 2013, I want the first 5 rows. for anndate 09-dec-2013, I want 5 following rows ended on 16-dec-2013.

my code is:

SELECT *
FROM com
WHERE date in 
(select top 5 date from com where com.date>com.anndate);

but it causes access to crash(correction: should be 'take minutes to complete'). Anyone can help point out where is wrong?

update:

I changed the column name [date] to [cdate], it still doesnt work.

I ran something simple like below and it still takes minutes to complete....

SELECT *
FROM com
WHERE cdate in (select cdate from com )

回答1:

I stored your sample data in a table with both date and anndate as Date/Time data type. Then I built a query with anndate_rank generated from a correlated subquery. The purpose of anndate_rank is a rank number within each anndate group.

Then that entire query became a subquery in a new one which selects anndate_rank <= the top limit --- I chose top 2 instead of top 5.

This is the result set from the query, and the query is below:

date       ret1     anndate   anndate_rank
---------- -------- --------- ------------
7/26/2013  0.999214 7/25/2013            1
7/29/2013  0.982684 7/25/2013            2
12/10/2013 0.965405 12/9/2013            1
12/11/2013 1.009705 12/9/2013            2

SELECT
    sub.date,
    sub.ret1,
    sub.anndate,
    sub.anndate_rank
FROM
    (
        SELECT
            c.date,
            c.ret1,
            c.anndate,
            (
                SELECT Count(*)
                FROM com AS c2
                WHERE
                        c2.anndate=c.anndate
                    AND c2.date<=c.date
            ) AS anndate_rank
        FROM com AS c
    ) AS sub
WHERE sub.anndate_rank<=2;

Note this approach assumes no repeated date values within any anndate group, as in your sample data. If your real data does include duplicate date / anndate pairs, this query will not give you the results you want.

来源：https://stackoverflow.com/questions/26805394/sql-query-select-top-5-per-group-leading-access-to-freeze-on-simple-code-and-sma

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select