Question

上的结构表

查询：

SELECT 
ei.ItemIdn as ItemIdn,
ei.UserIdn as UserIdn,
ifnull(cb.BestPrice,0) as BestPrice,
cv.CountShow as CountShow,
cb.CountBids as CountBids,
ifnull(r.CountReactivation,0) as CountReactivation

FROM items ei

left join (
  SELECT count(*) as CountShow, ItemIdn 
  FROM CountViewes 
  WHERE ItemIdn = ItemIdn) as cv on cv.ItemIdn = ei.ItemIdn

left join (
  SELECT count(*) as CountBids, Max(Bid) as BestPrice, ItemIdn 
  FROM CountBids 
  WHERE ItemIdn = ItemIdn) as cb on cb.ItemIdn = ei.ItemIdn

left join (SELECT count(*) as CountReactivation, ItemIdn 
           FROM Reactivation 
           WHERE ItemIdn = ItemIdn) as r on r.ItemIdn = ei.ItemIdn

WHERE ei.UserIdn = '0000' 
LIMIT 0, 25

问题：如果我使用left join我只获得一行（请参阅sqlfiddle），但表items有4行

请告诉我，如何使用left join计算行数和最大值？

Answer 1

您可以通过在子查询中进行聚合来解决此问题：

left join (SELECT ItemIdn, count(*) as CountBids, Max(Bid) as BestPrice
           FROM CountBids
           GROUP BY ItemIdn
          ) cb
          on cb.ItemIdn = ei.ItemIdn

编辑：

您需要使用ItemIdn = ItemIdn替换每个子查询中的group by ItemIdn。您还需要从外部查询中删除count()：

SELECT 
    ei.ItemIdn as ItemIdn,
    ei.UserIdn as UserIdn,
    ifnull(cb.BestPrice,0) as BestPrice,
    cv.CountShow as CountShow,
    cb.CountBids as CountBids,
    ifnull(r.CountReactivation,0) as CountReactivation

FROM items ei

    left join (
      SELECT count(*) as CountShow, ItemIdn 
      FROM CountViewes 
      group by ItemIdn) as cv on cv.ItemIdn = ei.ItemIdn

    left join (
      SELECT count(*) as CountBids, Max(Bid) as BestPrice, ItemIdn 
      FROM CountBids  cb
      group by ItemIdn ) as cb on cb.ItemIdn = ei.ItemIdn

    left join (SELECT count(*) as CountReactivation, ItemIdn 
               FROM Reactivation
               group by ItemIdn) as r on r.ItemIdn = ei.ItemIdn

WHERE ei.UserIdn = '0000' ;

如果您打算在外层进行聚合，请使用sum()，而不是count()。

Answer 2

这应该有效：

SELECT 
    ei.DateStart as DateStart,
    ei.DateEnd as DateEnd,
    ei.ItemIdn as ItemIdn,
    ei.Title as Title,
    ei.AuctionMinCost as AuctionMinCost,
    ei.Priority as Priority,
    ei.PriceBuyOnly as PriceBuyOnly,
    ei.ItemCount as ItemCount,

    ifnull(MAX(cb.Bid),0) as BestPrice,
    count(cb.ItemIdn) as CountBids

FROM items ei
LEFT JOIN CountBids cb ON cb.ItemIdn = ei.ItemIdn

WHERE ei.User = '0' 
GROUP BY ei.ItemIdn
Order by ei.DateStart ASC 
LIMIT 0, 25

更新：

当连接多个表并需要对每个表进行分组时，您需要使用子查询。每个子查询都需要进行自己的分组，不要在主查询中使用聚合。

SELECT 
    ei.ItemIdn as ItemIdn,
    ei.UserIdn as UserIdn,
    ifnull(cb.BestPrice,0) as BestPrice,
    cv.CountShow as CountShow,
    cb.CountBids as CountBids,
    ifnull(r.CountReactivation,0) as CountReactivation

FROM items ei

    left join (
      SELECT count(*) as CountShow, ItemIdn 
      FROM CountViewes 
      GROUP BY ItemIdn) as cv on cv.ItemIdn = ei.ItemIdn

    left join (
      SELECT count(*) as CountBids, Max(Bid) as BestPrice, ItemIdn 
      FROM CountBids 
      GROUP BY ItemIdn) as cb on cb.ItemIdn = ei.ItemIdn

    left join (SELECT count(*) as CountReactivation, ItemIdn 
               FROM Reactivation 
               GROUP BY ItemIdn) as r on r.ItemIdn = ei.ItemIdn

WHERE ei.UserIdn = '0000' 
LIMIT 0, 25

FIDDLE

顺便说一下，查询中不需要WHERE ItemIdn = ItemIdn。如果查询没有条件，请忽略WHERE子句。

mysql - 如何在表中获取计数行和最大计数？

2 个答案: