Question

我有两张桌子：

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]

这是我的疑问：

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')

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2号测试返回记录。但是，如何加入不存在的记录？或者这根本不可能？有哪些替代方案？

Answer 1

尽管正确使用了外连接，但您通过在WHERE子句中包含该表中的列，将结果集限制为表2中存在值的情况。如果您想要样品编号为ABC的记录，或者表2中没有记录，则需要执行以下操作：

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，您可以在连接到Table_2时过滤Table_2的结果（在这种情况下，读取更干净一点）：

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')

那应该完成同样的事情。这里重要的一点是WHERE子句过滤了连接表的结果。如果您正在使用外连接但希望过滤外连接表，则必须处理连接远端不存在记录的情况。

即使记录不存在也返回行LEFT OUTER JOIN

1 个答案: