使用 Group by 和 Join 进行查询的 COUNT 条件

Question

要求：

表格1：

Id      Key          SiteId ItemId    OrderNumber
1       ABCD           X      BTL        NULL
2       BCDE           X      BTL        ABCD
3       CDEF           X      DDD        BSFE

表 2：

Id  ItemId    Name
1    BTL      B Prd
2    DDD      D Prd

所需结果：

Id  ItemId      Name    AvailableKeys
1     BTL      B Prd      1
2     DDD      D Prd      0

可用密钥计算为表 1 中订单号为 null 的项目数

尝试的查询：

1) 
SELECT ps.ItemId, COUNT(ps.ItemId) AS AvailableKeys, item.Name
FROM TABLE1 AS ps 
LEFT JOIN TABLE2 as item ON item.ItemId = ps.ItemId 
WHERE ps.SiteId = 'X'
GROUP BY ps.ItemId, item.Name

   -- With this query, I am getting a count of the whole number of items but not the ones having order number as null (Count including even the Unavailable Items)

2) 
    SELECT ps.ItemId, COUNT(ps.ItemId) AS AvailableKeys, item.Name
    FROM TABLE1 AS ps 
    LEFT JOIN TABLE2 as item ON item.ItemId = ps.ItemId 
    WHERE ps.OrderNumber IS NULL AND ps.SiteId = 'X'
    GROUP BY ps.ItemId, item.Name

   -- With this query, I am missing the items having **ZERO** available keys

任何人都可以帮助我构建查询以从数据中获取所需的结果集。

提前致谢。

Answer 1

您需要一个条件总和，如下所示：

SUM(CASE WHEN OrderNumber IS NULL THEN 1 ELSE 0 END) AS AvailableKeys

以及完整的查询：

SELECT item.id, ps.ItemId
    , item.Name
    , SUM(CASE WHEN OrderNumber IS NULL THEN 1 ELSE 0 END) AS AvailableKeys
FROM TABLE1 AS ps 
LEFT JOIN TABLE2 as item ON item.ItemId = ps.ItemId 
WHERE ps.SiteId = 'X'
GROUP BY item.id, ps.ItemId, item.Name;

回报：

ID	项目 ID	项目名	可用键
1	BTL	湾区	1
2	DDD	D 珠三角	0

顺便说一句：您也可以使用inner join ，除非您想包含表 2 中没有匹配项的情况。

Answer 2

尽管外部联接的顺序错误，但您已完成第一个查询。 只需在ON子句中为您想要在第二个表上的条件添加一个匹配项：

SELECT item.ItemId, COUNT(ps.ItemId) AS AvailableKeys, item.Name
FROM TABLE2 item LEFT JOIN
     TABLE1 ps
     ON item.ItemId = ps.ItemId AND 
        ps.SiteId = 'X' AND
        ps.OrderNumber IS NULL
GROUP BY item.ItemId, item.Name;

Answer 3

SELECT item.id, ps.ItemId, item.Name,
    SUM(IIF(OrderNumber IS NULL,1,0)) AS AvailableKeys
FROM 
    TABLE1 AS ps 
    LEFT JOIN TABLE2 AS item ON item.ItemId = ps.ItemId 
WHERE ps.SiteId = 'X'
GROUP BY item.id, ps.ItemId, item.Name;