自我加入+左加入Sql Server

Question

我有一个表可以存储不同的测量值，类型作为关联列来存储测量类型。

以下是带有数据的样本表

我可以自己加入表格，根据日期获得ht和wt值。我的要求是，我还需要所有日期的wt值，即使表中没有该日期的ht。

示例结果

编辑：我尝试了以下查询，但它只返回第一行。

SELECT 
  ta.uid, ta.value as 'wt', lta.value as 'ht', ta.date,ta.id as lftid, lta.id as rtid FROM 
  [test].[dbo].[tbl2] ta
     LEFT JOIN [test].[dbo].[tbl2] Lta 
        ON ta.[date] = Lta.[date]
       AND ta.[uid] = 11 WHERE 
      ta.type = 'wt'
  AND Lta.type ='ht'

Answer 1

您在where子句中引用正确的别名这一事实有效地将您的left join更改为inner join。 Read this answer to find out why.
为避免这种情况，请将where子句中的条件移至on子句：

SELECT ta.uid, 
       ta.value as 'wt', 
       lta.value as 'ht', 
       ta.date,
       ta.id as lftid, 
       lta.id as rtid 
FROM [test].[dbo].[tbl2] ta
LEFT JOIN [test].[dbo].[tbl2] Lta 
ON ta.[date] = Lta.[date]
AND ta.type = 'wt'
AND Lta.type ='ht'           
WHERE ta.[uid] = 11 

Answer 2

只需对您的查询进行一些更改即可：

SELECT 
  ta.uid, ta.value as 'wt', lta.value as 'ht', ta.date,ta.id as lftid, lta.id as rtid FROM 
  [test].[dbo].[tbl2] ta
     LEFT JOIN [test].[dbo].[tbl2] Lta 
        ON ta.[date] = Lta.[date]
       AND ta.[uid] = 11  and ta.type<>Lta.type
       WHERE 
      ta.type = 'wt'
  AND isnull(Lta.type,'ht') ='ht'