Question

I have 3 different transaction tables, which look very similar, but have slight differences. This comes from the fact that there are 3 different transaction types; depending on the transaction types the columns change, so to get them in 3NF I need to have them in separate tables (right?).

As an example:
t1:
date,user,amount

t2:
date,user,who,amount

t3:
date,user,what,amount

Now I need a query who is going to get me all transactions in each table for the same user, something like

select * from t1,t2,t3 where user=’me’;
(which of course doesn’t work).

I am studying JOIN statements but haven’t got around the right way to do this. Thanks.

EDIT: Actually I need then all of the columns from every table, not just the ones who are the same.

---

EDIT #2: Yeah,having transaction_type doesn’t break 3NF, of course – so maybe my design is utterly wrong. Here is what really happens (it’s an alternative currency system):
– Transactions are between users, like mutual credit. So units get swapped between users.

– Inventarizations are physical stuff brought into the system; a user gets units for this.
– Consumations are physical stuff consumed; a user has to pay units for this.

|--------------------------------------------------------------------------|
|  type     |  transactions       |  inventarizations  |  consumations     |
|--------------------------------------------------------------------------|
|  columns  |  date               |  date              |  date             |
|           |  creditor(FK user)  |  creditor(FK user) |                   |
|           |  debitor(FK user)   |                    |  debitor(FK user) |
|           |  service(FK service)|                    |                   |
|           |                     |  asset(FK asset)   |  asset(FK asset)  |
|           |  amount             |  amount            |  amount           |
|           |                     |                    |  price            |
|--------------------------------------------------------------------------|

(Note that ‘amount’ is in different units;these are the entries and calculations are made on those amounts. Outside the scope to explain why, but these are the fields).
So the question changes to “Can/should this be in one table or be multiple tables (as I have it for now)?”
I need the previously described SQL statement to display running balances.

(Should this now become a new question altogether or is that OK to EDIT?).

EDIT #3: As EDIT #2 actually transforms this to a new question, I also decided to post a new question. (I hope this is ok?).

Answer 1

You can supply defaults as constants in the select statements for columns where you have no data;

so

SELECT Date, User, Amount, 'NotApplicable' as Who, 'NotApplicable' as What from t1 where user = 'me'
UNION
SELECT Date, User, Amount, Who, 'NotApplicable' from t2 where user = 'me'
UNION
SELECT Date, User, Amount, 'NotApplicable', What from t3 where user = 'me'

which assumes that Who And What are string type columns. You could use Null as well, but some kind of placeholder is needed.

I think that placing your additional information in a separate table and keeping all transactions in a single table will work better for you though, unless there is some other detail I’ve missed.