Question

For an access database that looks like this:
(All text fields)

Co1 Co2 Co3 Co4
A   k   t   N1
B   k   t   N2
A   m   t   N3
B   k   z   N4
A   k   z   N5
C   m   t   N6
C   k   z   N7
C   k   t   N8
A   k   t   N9
C   m   t   N10

I need to create some kind of reports that would do the following:

The results needs to select rows:

1. Ordered by Co1 first
2. then ordered by Co2
3. only include data where Co3 = t or r
4. when Co3 is NOT t nor r, then all other rows with a prior C04 where
   Co1 AND Co2 were the same as the current row are also excluded

1,2 and 3 are extremely easy to do (Order by and Where clause respectively) but
can #4 be done in access SQL?
If it can’t, I’ll make a small C# app that loops through the data but I feel this
must be possible to do in SQL and if it can that would definately be much better.

The actual query and database is more complicated but #4 is where I get stuck.

With the example I gave the result would be:

A   m   t   N3
A   k   t   N9
C   m   t   N6
C   m   t   N10
C   k   t   N8

I believe this cover all possible cases.

Precision, we use the fourth column (C4 in this example) to determine the order
but it is a text field with characters and digits in it.
The field is a letter followed by 2 digits: A01, A02,..A99, B01, etc…
Precision #2, I’m not the one to blame for this database design. 😛

So, to the SQL guru’s among us, can this be done in access SQL? How?

Thank you very much for any help you can provide.

Answer 1

If I read you correctly:

when Co3 is NOT t nor r, then all
other rows with a prior C04 where Co1
AND Co2 were the same as the current
row are also excluded

That is equivalent to excluding rows for which a later row with certain conditions exist. You can do that with a NOT EXISTS clause:

select *
from YourTable cur
where cur.Co3 in ('t','r')
and not exists (
    select *
    from YourTable later
    where cur.Co1 = later.Co1 
    and cur.Co2 = later.Co2
    and later.Co3 not in ('t','r') 
    and CInt(Mid(later.Co4,2)) > CInt(Mid(cur.Co4,2))
)
order by cur.Co1, cur.Co2 desc, CInt(Mid(cur.Co4,2))

Now N10 is not bigger than N5 normally. That’s what the CInt(Mid(…,2)) is for: it turns N10 into the number 10, and the number N5 into 5.