Question

我有两张桌子：

Table_1
[Group No] [Test No] [Description]
123        1         [First Test]

Table_2
[Sample No] [Test No] [Result Description]
DEF         1         [A Result]

这是我的疑问：

 SELECT Table_1.[Group No], Table_1.[Test No], Table_1.Description, Table_2.  [Result Description], A.Table_2.  [Result Description]
FROM Table_1 
LEFT JOIN Table_2 ON Table_1.[Test No] = Table_2.[Test No]
LEFT JOIN Table_2 A ON Table_1.[Test No] = A.Table_2.[Test No]
WHERE Table_1.[Test No] = '1'
AND (Table_2.[Sample No] = 'DEF') 
AND (A.Table_2.[Sample No] = 'ABC')

返回：

[Group No] [Test No] [Description] [Result Description]

空行......

但我真正想要的是：

[Group No] [Test No] [Description] [Result Description] [Result Description]
123        1         [First Test]  [A Result]            NULL

这可能吗？

Answer 1

以这种方式试试。

SELECT Table_1.[Group No], Table_1.[Test No], Table_1.Description, Table_2.  [Result Description], A.Table_2.  [Result Description]
FROM Table_1 
LEFT JOIN Table_2 ON Table_1.[Test No] = Table_2.[Test No] AND Table_2.[Sample No] = 'DEF'
LEFT JOIN Table_2 A ON Table_1.[Test No] = A.Table_2.[Test No] AND A.Table_2.[Sample No] = 'ABC'
WHERE Table_1.[Test No] = '1'

你这样做的方式需要在table_2中存在一条记录，因为WHERE表示该表中的一列必须等于某事。

Answer 2

当你离开加入时，你必须小心你在where子句中放置的内容。试试这个：

 SELECT Table_1.[Group No], Table_1.[Test No], Table_1.Description, Table_2.  [Result Description], A.Table_2.  [Result Description]
FROM Table_1 
LEFT JOIN Table_2 ON Table_1.[Test No] = Table_2.[Test No] 
                   and (Table_2.[Sample No] = 'DEF') 
LEFT JOIN Table_2 A ON Table_1.[Test No] = A.Table_2.[Test No]
                            AND (A.Table_2.[Sample No] = 'ABC')
WHERE Table_1.[Test No] = '1'

将这些条件放在连接中允许左连接包括左边不满足条件的记录，因为右表上没有符合条件的记录。

即使记录不存在也返回行2在一个表中过滤

2 个答案: