引用父别名时将外部应用转换为左连接

Question

I'm currently trying to turn an outer apply into a left join to save some complexity.

SELECT *
FROM fact_table h
     OUTER APPLY (SELECT TOP 1
                         *
                  FROM dimension mcc WITH (NOLOCK)
                  WHERE h.product = mcc.product
                    AND h.country = mcc.country
                    AND mcc.date IN (SELECT MAX(date)
                                     FROM dimension dd WITH (NOLOCK)
                                     WHERE FORMAT(DATEADD(MONTH, -3, dd.date), 'yyyyMM') <= h.month_in_the_year
                                       AND dd.product = h.product
                                       AND dd.country = h.country)) a;

I basically use it to get the related data from Dimension linked with the latest data point that's earlier than 3 months ago.

I'm trying to turn it into a left join, but it's taking a lot more time since I don't filter the dimension before the join:

SELECT TOP 10
       *
FROM fact_table h
     LEFT JOIN dimension a ON h.product = a.product
                          AND h.country = a.country
                          AND a.pkid = (SELECT TOP 1
                                               pkid
                                        FROM dimension dd
                                        WHERE FORMAT(DATEADD(MONTH, -3, dd.date), 'yyyyMM') <= h.month_in_the_year
                                        ORDER BY date DESC);

Do you have an idea on how to turn it efficiently into a left join?

Answer 1

It looks like you can significantly simplify this query, by simply adding an ORDER BY . I've also modified the date filter in order to leverage indexing properly.

SELECT *
FROM fact_table h
OUTER APPLY (
    SELECT TOP 1 *
    FROM dimension mcc
    WHERE h.product = mcc.product
      AND h.country = mcc.country
      AND mcc.date < DATEADD(MONTH, 2, DATEFROMPARTS(LEFT(h.month_in_the_year, 4), RIGHT(h.month_in_the_year, 2), 1))
    ORDER BY mcc.date DESC
) a;

To transform this into a LEFT JOIN , you need to utilize row-numbering

SELECT *
FROM (
    SELECT *,
      rn = ROW_NUMBER() OVER (PARTITION BY h.PrimaryKeyColumn ORDER BY mcc.date)
    FROM fact_table h
    LEFT JOIN dimension mcc
      ON h.product = mcc.product
        AND h.country = mcc.country
        AND mcc.date < DATEADD(MONTH, 2, DATEFROMPARTS(LEFT(h.month_in_the_year, 4), RIGHT(h.month_in_the_year, 2), 1))
) a
WHERE rn = 1;