MYSQL在分组依据中使用限制

Question

我有这种查询，需要在每个“区域”组中限制3个，但它没有按我的预期运行。 “ row_number”似乎没有相应安排。 一定有一些语法我错过了，或者我不知道。 如果有人可以帮助我在这里转储sql。 MYSQL 5.0版

我的查询：

set @type = '';
set @num = 0;

SELECT locinvaisle.Area as ar,locinvaisle.Region as rg,custlist.CustomerName as cn,custlist.Custtype ct,
       SUM(data2.quantity/1000) as mtcur,
       @num := if(@type = locinvaisle.Region, @num + 1, 1) as dummy_1,
  @type := locinvaisle.Region as dummy_2,
  @num as row_number

FROM data2

  INNER JOIN custlist ON data2.customeracc = custlist.Customeraccount
  INNER JOIN locinvaisle ON data2.location = locinvaisle.Location

WHERE
   date1 >= DATE_FORMAT('2018-06-11', '%Y-01-01') AND date1 <= DATE_FORMAT('2018-06-11', '%Y-%m-31')
  AND
   data2.unit = 'KG'
  AND
   data2.customeracc not in (select Customeraccount from custlist WHERE Custcat = 'bcsb')
  AND
   locinvaisle.Area = 'peninsular'
  AND
   custlist.Custtype = 'others'

GROUP BY locinvaisle.Region,custlist.CustomerName
HAVING row_number < 3
ORDER BY locinvaisle.Region,mtcur desc

结果：

所需结果（来自虚拟数据库）：

Answer 1

在应用group by之前，您必须将限制作为完整的子查询（“派生表”）进行。 遵循以下原则：

SELECT 
       ar, rg, cn, ct, sum(quantity)
FROM (
    SELECT
          @row_num :=IF(@prev_value = locinvaisle.Region, @row_num + 1, 1)AS RowNumber
        , locinvaisle.Area as ar
        , locinvaisle.Region as rg
        , custlist.CustomerName as cn
        , custlist.Custtype ct
        , data2.quantity
        , @prev_value := locinvaisle.Region as dummy_2,
    FROM data2
      INNER JOIN custlist ON data2.customeracc = custlist.Customeraccount
      INNER JOIN locinvaisle ON data2.location = locinvaisle.Location
      CROSS JOIN (SELECT @row_num :=1,  @prev_value :='') vars
    WHERE date1 >= DATE_FORMAT('2018-06-11', '%Y-01-01') AND date1 <= DATE_FORMAT('2018-06-11', '%Y-%m-31')
      AND data2.unit = 'KG'
      AND data2.customeracc not in (select Customeraccount from custlist WHERE Custcat = 'bcsb')
      AND  locinvaisle.Area = 'peninsular'
      AND custlist.Custtype = 'others'
    ORDER BY locinvaisle.Region,mtcur desc
    ) d
WHERE rowNumber <= 3
GROUP BY ar, rg, cn, ct
ORDER BY ar, rg, cn, ct

注意：您需要在子查询中应用ORDER BY来方便行号的计算，但是尽管如此，除非您应用final order by子句，否则不能保证最终输出将按期望的顺序进行。

另外请注意，在将来的MySQL版本（V8及更高版本）中，应该使用row_number()函数和over()子句来代替上面看到的变量。

Answer 2

在MySQL中，使用变量时需要小心。 有两点很重要：

• 变量应在同一表达式中分配和引用。
• 您需要注意结果集的顺序。

因此，尝试这样的事情：

SELECT x.*,
       (@rn := IF(@r = la.Region, @num + 1,
                  IF(@r := la.Region, 1, 1)
                 )
       ) as rn
FROM (SELECT la.Area as ar, la.Region as rg, c.CustomerName as cn, c.Custtype ct,
             SUM(data2.quantity/1000) as mtcur
      FROM data2 d INNER JOIN
           custlist c
           ON d.customeracc = c.Customeraccount  INNER JOIN
           locinvaisle la
           ON d.location = la.Location
      WHERE date1 >= DATE_FORMAT('2018-06-11', '%Y-01-01') AND
            date1 <= DATE_FORMAT('2018-06-11', '%Y-%m-31') AND
            d.unit = 'KG' AND
            d.customeracc not in (select Customeraccount from custlist cl WHERE cl Custcat = 'bcsb') AND
            la.Area = 'peninsular' AND
            c.Custtype = 'others'
      GROUP BY la.Region, c.CustomerName
     ) x CROSS JOIN
     (SELECT @r := 0, @rn := 0) params
HAVING rn <= 3;