Question

On the project I am working on, I am stuck with the table structure from Hades. Two things to keep in mind:

1. I can’t change the table structure right now. I’m stuck with it for the time being.
2. The queries are dynamically generated and not hard coded. So, while I am asking for a query that can pull this data, what I am really working toward is an algorithm that will generate the query I need.

Hopefully, I can explain the problem without making your eyes glaze over and your brain implode.

We have an instance table that looks (simplified) along these lines:

Instances
InstanceID    active
1              Y
2              Y
3              Y
4              N
5              Y
6              Y

Then, there are multiple data tables along these lines:

Table1

InstanceID    field1   reference_field2
1             John      5
2             Sally        NULL
3             Fred          6
4             Joe          NULL


Table2
InstanceID    field3 
5              1
6              1


Table3
InstanceID    fieldID    field4
5               1        Howard
5               2        James
6               2        Betty

Please note that reference_field2 in Table1 contains a reference to another instance.
Field3 in Table2 is a bit more complicated. It contains a fieldID for Table 3.

What I need is a query that will get me a list as follows:

InstanceID    field1     field4     
1              John      Howard
2              Sally
3              Fred

The problem is, in the query I currently have, I do not get Fred because there is no entry in Table3 for fieldID 1 and InstanceID 6. So, the very best list I have been able to get thus far is

InstanceID    field1     field4     
1              John      Howard
2              Sally

In essence, if there is an entry in Table1 for Field 2, and there is not an entry in Table 3 that has the instanceID contained in field2 and the field ID contained in field3, I don’t get the data from field1.

I have looked at joins till I’m blue in the face, and I can’t see a way to handle the case when table3 has no entry.

Answer 1

LEFT JOIN…

SELECT a.InstanceID, b.field1, d.field4
FROM instances AS a 
    JOIN Table1 AS b ON a.InstanceID = b.InstanceID
    LEFT JOIN Table2 AS c ON b.reference_field2 = c.InstanceID
    LEFT JOIN Table3 AS d ON (c.InstanceID = d.InstanceID AND c.field3 = d.fieldId)
WHERE a.active = 'Y'

The two left joins should handle the case where there are no other rows…