在 DB2 SQL 中，如何使用 FETCH FIRST (n) ROWS ONLY.. return 可变数量的行？

Question

如何在 DB2 SQL 中 return 具有 FETCH FIRST (n) ROWS ONLY 的可变 (n) 行数？这是 DB2/400 v7r3.

documentation for FETCH 确实明确表示无法完成，但我无法想象另一种方法...

...fetch-row-count must not contain a scalar-fullselect, a column reference, a table reference, a user-defined function reference, or a built-in scalar function...

...我的意思是它必须是常量，例如 "10"，不能是变量或列名，不幸的是，这正是我想要做的。

这个有效：

SELECT      PREFILTER.*
FROM        PREFILTER
INNER JOIN  GT1 ON FILTERED.GTAMT=GT1.LOSTAMT
ORDER BY    GTDATE DESC
FETCH FIRST 10 ROWS ONLY

这不起作用：

SELECT      PREFILTER.*
FROM        PREFILTER
INNER JOIN  GT1 ON PREFILTER.GTAMT=GT1.LOSTAMT
ORDER BY    GTDATE DESC
FETCH FIRST (GT1.LOSTAMT) ROWS ONLY    <=== changed here

收到以下错误：

SQL Error [428H7]: [SQ20467] Expression containing LOSTAMT must calculate a constant value.

此查询旨在查找在较大的 table 中孤立或键入不正确的金融交易记录，这些记录具有相当松散的规范化和参照完整性（或缺乏）。

PREFILTER 是一个查询，它 return 是主要 table 的一个子集，而 GT1 是另一个查询，它计算这些记录的更小和更复杂的子集.然后 JOIN 它们，以及 return (n) 列 LOSTQTY 指定的行，按日期降序排列。所以它应该只有 return (n) 个最近的记录。

请注意，我确实认识到我对 FETCH 的放置是不正确的，并且它（或它变形的任何东西）可能必须移动到 CTE 查询之一，例如 GT1。

此外，在这个早期阶段，我似乎得到了一些笛卡尔结果，但一旦 FETCH 问题得到解决，这可能会得到解决。

作为参考，这里是项目的完整 SQL：

WITH  --SET THE INITIAL ACCOUNT & DATE RANGE  
        PREFILTER    AS  (   SELECT      *
                            FROM        GLTRANT
                            WHERE       GTDATE > 20170000
                            AND         GTACCT=112068
                        ),
      --CREATE LIST OF ALL POSITIVE VALUES
        POSVALS     AS  (   SELECT      GTAMT      AS POSAMT, COUNT(GTAMT) AS POSC
                            FROM        PREFILTER 
                            WHERE       GTAMT > 0
                            GROUP BY    GTAMT
                        ),
      --CREATE LIST OF ALL NEGATIVE VALUES, WITH SIGN DROPPED
        NEGVALS     AS  (   SELECT      ABS(GTAMT) AS NEGAMT, COUNT(GTAMT) AS NEGC
                            FROM        PREFILTER 
                            WHERE       GTAMT < 0
                            GROUP BY    ABS(GTAMT)
                        ),
      --CALCULATE DISCREPANCIES BETWEEN THE TWO LISTS. SUBTRACT THE TWO AND MULTIPLY THE SIGN BY THE ABSOLUTE VALUE 
      --OF THE DIFFERENCE. THEN TO RESTORE THE SIGN, MULTIPLY THE AMOUNT BY THE SIGN OF THE DIFFERENCE. 
        FOJ         AS  (   SELECT      SIGN(COALESCE(POSC,0)-COALESCE(NEGC,0))*COALESCE(POSAMT,NEGAMT) AS LOSTAMT, 
                                        ABS (COALESCE(POSC,0)-COALESCE(NEGC,0))                         AS LOSTQTY
                            FROM        POSVALS 
                            FULL OUTER JOIN NEGVALS ON POSAMT=NEGAMT
                            WHERE       COALESCE(POSC,0)-COALESCE(NEGC,0) <> 0
                        ),
      --GET DISCREPANCIES WITH COUNT >1 
        GT1         AS  (   SELECT      *
                            FROM        FOJ
                            WHERE       LOSTQTY>1 
                        )

--SEARCH PREFILTER FOR EACH AMOUNT (LOSTAMT) IN GT1 AND RETURN THE MOST RECENT (LOSTQTY) RECORDS 
SELECT      PREFILTER.*
FROM        PREFILTER
INNER JOIN  GT1 ON PREFILTER.GTAMT=GT1.LOSTAMT
ORDER BY    GTDATE DESC
FETCH FIRST (GT1.LOSTQTY) ROWS ONLY --DOES NOT WORK

Answer 1

我不确定您能否在一条语句中完成您想做的事情。但是，您应该能够组合一个存储过程或 SQL 函数来执行此操作。在存储过程中，将查询放在一起，然后用游标打开它。然后游标可以获取所需的行数，将其放入结果集中，然后 return 来自过程的结果集。来自 IBM 的最新文档位于：https://www.ibm.com/support/knowledgecenter/en/ssw_ibm_i_73/sqlp/rbafyresultsets.htm

Answer 2

您应该可以为此使用 window function ROW_NUMBER：

WITH  --SET THE INITIAL ACCOUNT & DATE RANGE  
        PREFILTER    AS  (  
          SELECT  T.*,
                  ROW_NUMBER() OVER (ORDER BY T.GTDATE DESC) AS RN
          FROM (
                            SELECT      *
                            FROM        GLTRANT
                            WHERE       GTDATE > 20170000
                            AND         GTACCT=112068
          ) T
        ),
...
--SEARCH PREFILTER FOR EACH AMOUNT (LOSTAMT) IN GT1 AND RETURN THE MOST RECENT (LOSTQTY) RECORDS 
SELECT      PREFILTER.*
FROM        PREFILTER
INNER JOIN  GT1 ON PREFILTER.GTAMT=GT1.LOSTAMT
WHERE RN <= GT1.LOSTQTY
ORDER BY    GTDATE DESC

它按照GTDATE的降序为PREFILTER中的每一行分配连续的数字，您稍后可以使用它来限制结果集。

Answer 3

好的，吸收后 Mustaccio's previous answer and banging my head on the IBM documentation all day, I was able to work this out. Much thanks to Mustaccio 为我指明了正确的方向。

两个重要的位：ROW_NUMBER() 的位置必须进一步向下移动；它在原来的地方不起作用。此外，它还需要添加 PARTITION BY 参数，以便每组新值重新开始编号。所以它变成了 1-2-1-2-1-2-3 而不是 1-2-3-4-5-6-7。这对于 WHERE RN <= LOSTQTY 的工作至关重要。

完整结果如下：

WITH  --SET THE INITIAL ACCOUNT & DATE RANGE  
        PREFILTER   AS  (   SELECT  GTCOMP, GTACCT, GTDATE, GTSRCE, GTREF#, GTENT#, GTAMT, GTDESC, "GTPO#", 
                                    "GTCHK#", "GTINV#", GTCKAC, GT1099, GTXXX1, GTAFLG, GTVEND, "GTBAT#"
                            FROM        F_CERTOB.GLTRANT
                            WHERE       GTDATE > 20180000
                            AND         GTACCT=112068
                        ),
      --CREATE LIST OF ALL POSITIVE VALUES
        POSVALS     AS  (   SELECT      GTAMT      AS POSAMT, COUNT(GTAMT) AS POSC
                            FROM        PREFILTER 
                            WHERE       GTAMT > 0
                            GROUP BY    GTAMT
                        ),
      --CREATE LIST OF ALL NEGATIVE VALUES, WITH SIGN DROPPED
        NEGVALS     AS  (   SELECT      ABS(GTAMT) AS NEGAMT, COUNT(GTAMT) AS NEGC
                            FROM        PREFILTER 
                            WHERE       GTAMT < 0
                            GROUP BY    ABS(GTAMT)
                        ),
      --CALCULATE DISCREPANCIES BETWEEN THE TWO LISTS. SUBTRACT THE "DE-SIGNED" NEGATIVE FROM THE POSITIVE AND 
      --MULTIPLY THE SIGN BY THE ABSOLUTE VALUE OF THE DIFFERENCE. THEN TO RESTORE THE SIGN, MULTIPLY THE AMOUNT 
      --BY THE SIGN OF THE DIFFERENCE. THIS IS A FULL OUTER JOIN, SO NULLS ARE A GIVEN, AND COALESCE() IS USED 
      --TO FILL IN THE HOLES.  
        FOJ         AS  (   SELECT      SIGN(COALESCE(POSC,0)-COALESCE(NEGC,0))*COALESCE(POSAMT,NEGAMT) AS LOSTAMT, 
                                        ABS (COALESCE(POSC,0)-COALESCE(NEGC,0))                         AS LOSTQTY
                            FROM        POSVALS 
                            FULL OUTER JOIN NEGVALS ON POSAMT=NEGAMT
                            WHERE       COALESCE(POSC,0)-COALESCE(NEGC,0) <> 0
                        ),
      --THIS IS AN EXTRA KNOB TO CONTROL THE NUMBER OF RESULTS DURING DEVELOPMENT. IF SET TO >1, IT WILL SHOW
      --ONLY THE DATA THAT WOULD RETURN TWO OR MORE ROWS. USEFUL WHEN 99% OF THE DATA WOULD BE ONE ROW. THIS
      --WAS NEEDED TO DEVELEOP & TEST "ROW_NUMBER() OVER (PARTITION BY GTAMT ORDER BY GTDATE DESC) AS RN" AND
      --IF THE NUMBERING WAS RESTARTING PROPERLY FOR EACH GROUP OF VALUES.
        GT          AS  (   SELECT      *
                            FROM        FOJ
                            WHERE       LOSTQTY>0
                        ),
      --RETRIEVE THE ITEMS, RANK THEM BY AMT & DATE. USE PARTITON-BY TO RESTART ROW NUMBERING FOR EACH GROUP 
      --OF AMOUNTS. USE ORDER-BY TO NUMBER BY DATE IN DESCENDING ORDER. 
        GROUPED     AS  (   SELECT      PREFILTER.*, LOSTQTY, 
                                        ROW_NUMBER() OVER (PARTITION BY GTAMT ORDER BY GTDATE DESC) AS RN
                            FROM        PREFILTER
                            INNER JOIN  GT ON GTAMT=LOSTAMT
                        ),
      --NARROW IT DOWN TO ONLY THE TOP (n) ITEMS
        RECENT      AS  (   SELECT      *
                            FROM        GROUPED
                            WHERE       RN <= LOSTQTY
                        )

SELECT      *
FROM        RECENT
ORDER BY    GTAMT

Answer 4

参考页确实说您不能像您尝试的那样使用列引用作为行数的表达式 return。

它允许的是一个表达式。有关于它所说的不允许的规则，但这些规则不排除使用主机变量。

    FETCH FIRST :xqty ROWS ONLY

在 DB2 SQL 中，如何使用 FETCH FIRST (n) ROWS ONLY.. return 可变数量的行？

In DB2 SQL, how can I return a variable number of rows with FETCH FIRST (n) ROWS ONLY..?

db2

db2-400