返回行，如果记录不存在LEFT OUTER JOIN

Question

我有2个表如下：

Table_1 
[Group No] [Test No] [Description] 
123  1   [First Test] 
123  2   [Second Test] 
123  3   [Third Test] 

Table_2 
[Sample No] [Test No] [Result Description] 
ABC   1   [Some More Result] 
ABC   3   [Some Result] 
DEF   1   [A Result] 
DEF   2   [Results More] 
DEF   3   [Bad Results] 

这里是我的查询：

SELECT Table_1.[Group No], Table_1.[Test No], Table_1.Description, Table_2.[Result Description] 
FROM Table_1 
LEFT OUTER JOIN Table_2 ON Table_1.[Test No] = Table_2.[Test No] 
WHERE (Table_1.[Group No] = '123') AND (Table_2.[Sample No] = 'ABC') 

djacobson的查询：

SELECT Table_1.[Group No], Table_1.[Test No], Table_1.Description, Table_2.[Result Description] 
FROM Table_1 
LEFT OUTER JOIN Table_2 ON Table_1.[Test No] = Table_2.[Test No] 
WHERE (Table_1.[Group No] = '123') 
    AND (Table_2.[Sample No] IS NULL OR Table_2.[Sample No] = 'ABC') 

这将返回：

[Group No] [Test No] [Description] [Result Description] 
123  1   [First Test] [Some More Result] 
123  3   [Third Test] [Some Result] 

但我真正想要的是：

[Group No] [Test No] [Description] [Result Description] 
123  1   [First Test] [Some More Result] 
123  2   [Second Test] NULL 
123  3   [Third Test] [Some Result] 

这可能吗？我想用测试2号返回记录。但是，如何加入不存在的记录？或者这是不可能的？有什么选择？

Answer 1

尽管正确使用了外部联接，但您仍然将结果集限制为Table_2中存在值的情况，方法是在WHERE子句中包含该表中的一列。如果你想记录中，其中样品编号为ABC，或者出现在TABLE_2没有记录，你需要这样做：

SELECT Table_1.[Group No], Table_1.[Test No], Table_1.Description, Table_2.[Result Description] 
FROM Table_1 
LEFT OUTER JOIN Table_2 ON Table_1.[Test No] = Table_2.[Test No] 
WHERE (Table_1.[Group No] = '123') 
    AND (Table_2.[Sample No] IS NULL OR Table_2.[Sample No] = 'ABC') 

或者，你可以加入到它的时候（从TABLE_2过滤其结果，在这种情况下，读得更干净一点）：

SELECT Table_1.[Group No], Table_1.[Test No], Table_1.Description, Table_2.[Result Description] 
FROM Table_1 
LEFT OUTER JOIN Table_2 ON Table_1.[Test No] = Table_2.[Test No] AND Table_2.[Sample No] = 'ABC' 
WHERE (Table_1.[Group No] = '123') 

这应该完成同样的事情。这里重要的一点是，WHERE子句过滤连接表的结果。如果您正在使用外部连接，但想要在外部连接的表上进行筛选，则必须处理连接较远处不存在任何记录的情况。