SQL在存在時選擇一個值，否則多次填充空值

Question

我想選擇兩個綁定到表的值，如果它們不存在，則填充空值。但是，我只希望每條記錄都有一行。如果我運行下面的查詢，CASE語句會創建重複的行。

SELECT DISTINCT YT.TITLE, YT.DETAILS, 
CASE WHEN XT.ACOLUMN = 'CATEGORY' THEN XT.BCOLUMN END AS CATEGORY, 
CASE WHEN XT.ACOLUMN = 'DIVISION' THEN XT.BCOLUMN END AS DIVISION 
FROM DB.YTABLE YT 
LEFT OUTER JOIN DB.XTABLE XT 
ON YT.ID_NUM = XT.ID_NUM 

數據我想：

TitleExample1，DetailsExample1，組別，Division1

TitleExample2，DetailsExample2，空，Division2

數據我得到：

TitleExample1，DetailsExample1，組別，空

TitleExample1，DetailsExample1，空，Division1

TitleExample1，DetailsExample1，NULL，NULL

TitleExample2，DetailsExample2，空，Division2

TitleExample2 ，DetailsExample2，null，null

我也試圖做一個連接使用多個表身份爲同一個表：

SELECT DISTINCT YT.TITLE, YT.DETAILS, XT1.BCOLUMN AS CATEGORY1, 
     XT2.BCOLUMN AS DIVISION 
LEFT OUTER JOIN DB.XTABLE XT1 
ON YT.ID_NUM = XT.ID_NUM 
LEFT OUTER JOIN DB.XTABLE XT2 
ON YT.ID_NUM = XT.ID_NUM 
WHERE XT1.ACOLUMN = 'CATEGORY' AND 
XT2.ACOLUMN = 'DIVISION' 

這種運作良好，如果填充兩個類別和分裂值。但是，如果標題和詳細信息記錄沒有關聯的類別或部門，則查詢將不會選擇該記錄。

數據我想：

TitleExample1，DetailsExample1，組別，Division1

TitleExample2，DetailsExample2，空，Division2

數據我得到：

TitleExample1，DetailsExample1，組別，Division1

Answer 1

SELECT YT.TITLE, YT.DETAILS, 
Min(CASE WHEN XT.ACOLUMN = 'CATEGORY' THEN XT.BCOLUMN END) AS CATEGORY, 
Min(CASE WHEN XT.ACOLUMN = 'DIVISION' THEN XT.BCOLUMN END) AS DIVISION 
FROM DB.YTABLE YT 
LEFT OUTER JOIN DB.XTABLE XT 
ON YT.ID_NUM = XT.ID_NUM 
Group By YT.TITLE, YT.DETAILS 

Answer 2

你的第二個方法是更好的，但你必須在外部的ON語句添加您的病情JOIN

SELECT DISTINCT YT.TITLE, YT.DETAILS, XT1.BCOLUMN AS CATEGORY1, XT2.BCOLUMN AS DIVISION 
FROM DB.YTABLE YT 
LEFT OUTER JOIN DB.XTABLE XT1 ON (YT.ID_NUM = XT1.ID_NUM AND XT1.ACOLUMN = 'CATEGORY') 
LEFT OUTER JOIN DB.XTABLE XT2 ON (YT.ID_NUM = XT2.ID_NUM AND XT2.ACOLUMN = 'DIVISION')