即使记录不存在，也返回一行

Question

I have 2 tables below:

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

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

Here's my query:

 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')

This returns:

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

Empty rows....

But what I really want is:

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

Is this possible?

Answer 1

Try it this way.

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'

The way you did it required a record to exist in table_2 because the WHERE said that a column from that table had to equal something.

Answer 2

When you left join you must be careful what you put in the where clause. Try this:

 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'

Putting these conditions in the join allows the left join to include records from the left where the condition is not met because no record exists on the right table matching the criteria.