Sql Query with unique column value

Question

My goal is to get a query written. I have three tables, A, B and C. The tables are written such that A.bID = B.bID, and B.cID = C.cID. This basically allows me to write a query where I link a record from a to b, and link the b record to a record from c. So far so good, simple query.

What my problem is... one of the columns included in the query (let's call it C.col3) has to have unique values; the values in this column can only show up once in the query result, but other columns from the other tables do not have this requirement.

Can anybody help me write this query?

Thanks...

Update 1:

Here is the table layout (sorry, I have to use generic names)

Table A
aID, bID, aCol1, aCol2, aCol3 ... aCol10

Table B
bID, cID, bCol1, bCol2, bCol3 ... bCol10

Table C
cID, cCol1, cCol2, col3, cCol4 ... cCol10

Without the unique value constraint in col3, I would write the query like this:

SELECT
    A.aID, A.bID, A.aCol1 ... A.aCol10,
    B.bID, B.cID, B.bCol1 ... B.bCol10,
    C.cID, C.cCol1, C.cCol2, C.col3 ... C.cCol10
FROM
    A, B, C
WHERE 
    A.bID = B.bID AND B.cID = C.cID

... but of course that doesn't make sure the that values in C.col3 are unique.

Update 2: More info...
Table A and Table B have a one to many relationship; A is the "header", B is the "item".
Table B and Table C have a one to one relationship.

These tables are part of a caching mechanism, so lots of data that looks similar, but is still different in some cols.

Since A is the header, most of the duplicate values will be found in A.

I first need to order the rows by A.aID, but then after that I only need the first rows returned, where the value for C.col3 does not appear in a previous row for that col.

Does that make things a little clearer, or am I still not making any sense? :)

Final Update:

I chose Bartosz Klimek's answer as it was the closest to what I needed; I just had to modify the nested join clause in the middle.

Thank you all for your help!

Answer 1

select distinct c.col3 from c inner join b on c.cID = b.cID inner join a on b.bID = a.bID

Answer 2

If you need other values from the tables as well, use:

select max(a.col1), sum(b.col2), col3 from a, b, c
where A.bID = B.bID, and B.cID = C.cID
group by C.col3

On all columns not grouped by you need to use aggregate functions such as

• AVG: Average of the column.
• COUNT: Number of records.
• MAX: Maximum of the column.
• MIN: Minimum of the column.
• SUM: Sum of the column.

Answer 3

I started to post another answer, but after rethinking it I deleted it. If I am reading the question correctly, I think this is an impossible/illogical question. Let me explain with an example. if I read this wrong, please clarify the question with an exampe of what you are looking for.

Table A

BID COL1
1   Value1
2   Value1
3   Value2

Table B

BID CID COL 2
1   4   ValueX
2   5   ValueY
3   6   ValueZ

Table C

CID COL3
4   Value#
5   Value@
6   Value~

Expected Result

A.Col1   A.BID B.BID B.CID B.COL2 C.CID C.COL3
Value1?? 1     1     4     ValueX 4     Value#
Value1?? 2     1     5     ValueY 5     Value@ 
Value2   3     3     6     ValyeZ 6     Value~

Per the question you don't want value1 repeated in the first column, but what do you propose goes into the second row where it would normally be repeated if you didn't have the unique constraint?

Answer 4

SELECT A.*, B.*, C.*
  FROM C
    JOIN B ON B.cID = C.cID
    JOIN A ON A.bID = B.bID
    JOIN
    (
      SELECT id = min(aID)
        FROM C
          JOIN B ON B.cID = C.cID
          JOIN A ON A.bID = B.bID
        GROUP BY col3
    ) D ON D.id = A.aID

Note that the subquery at the end assures that for each col3 value you will have at most one record in the final resultset. The record selected is the one with the minimal aID. Obviously I assume that aID, bID and cID are primary keys of A, B and C, respectively.

Answer 5

I'm going to quickly make a little example of what you're trying to do and hopefully this will help clarify why what you are asking (currently) is impossible.

If you had a Customer Table [CustomerID, CustomerName] and an Orders Table [OrderID, CustomerID, DollarAmount]

If you wanted all orders for customers:

SELECT CustomerName, OrderID, DollarAmount
FROM Customer, Orders
WHERE Customer.CustomerID = Orders.CustomerID

it would return

 "Acme Corp.", 1, $2300
 "Acme Corp.", 2, $3022
 "A company",  3, $1234

Everything is good.

But the equivalent of your question is asking for this query, but with unique CustomerNames. What would you display for OrderID and DollarAmount beside "Acme Corp"?

You could use aggregates to display something,

SELECT CustomerName, MAX(OrderID), SUM(DollarAmount)
FROM Customer, Orders
WHERE Customer.CustomerID = Orders.CustomerID
GROUP BY Orders.CustomerID

But I believe that you mentioned that you do not want to use aggregates.

Does this explain the issue clearly?