如何在表格中找到最近的12个日期？

Question

I have a table containing sales information by customer account and posting month. The posting month is a text field formatted as YYMM. I can easily write a query to return the distinct posting months in descending order, but how do I pick the 12th row from that? I want to find the posting month 12 months prior to the most recent recorded, ideally as a scalar subquery so that it can be used as a selection criterion. At the moment this parameter is supplied by a UDF but I suspect performance would be better from a subquery.

Answer 1

You can get the 12th by doing:

select distinct month
from t
order by month desc
offset 11
fetch first 1 row only;

This is ANSI SQL, but it will not work in all databases. However, most databases support similar functionality.

Answer 2

You could use a "window function" to identify the 12th row, as follows...

    ;WITH cteExampleData
        (
        PostingMonth,
        Data
        )
        AS
        (
        SELECT '1611', 'Test1'
        UNION ALL SELECT '1612', 'Test2'
        UNION ALL SELECT '1701', 'Test3'
        UNION ALL SELECT '1601', 'Test4'
        UNION ALL SELECT '1602', 'Test5'
        UNION ALL SELECT '1604', 'Test6'
        UNION ALL SELECT '1605', 'Test7'
        UNION ALL SELECT '1501', 'Test8'
        UNION ALL SELECT '1502', 'Test9'
        UNION ALL SELECT '1503', 'Test10'
        UNION ALL SELECT '1401', 'Test11'
        UNION ALL SELECT '1403', 'Test12'
        UNION ALL SELECT '1404', 'Test13'
        UNION ALL SELECT '1405', 'Test14'
        UNION ALL SELECT '1406', 'Test15'
        )
        SELECT *
            INTO #ExampleData
            FROM cteExampleData;

    ;WITH cteFind12thRow
        AS
        (
        SELECT RowNumber = ROW_NUMBER() OVER (ORDER BY PostingMonth),
            *
            FROM #ExampleData
        )
        SELECT * FROM cteFind12thRow
            WHERE RowNumber = 12