Re: Which column is taken in JOIN with multiple columns and different content ?

On Mar 2, 11:07 am, mer..._at_gmx.net (Tobias Merler) wrote:
> Assume I join two tables.
> Both of these tables contain a column with name "col1". This column ist not he primary key.
> However this column contains different values which value is taken ?
>
> Lets see the following example:
>
> Select *
> FROM tablea a, tableb b
> Where a.id = b.id
>
> tablea:
>
> id col1 col2
>
> 1 aaa ddd
> 2 bbb eee
> 3 ccc fff
>
> tableb:
>
> id col1 col3
>
> 1 zzz uuu
> 2 xxx vvv
> 3 yyy www
>
> The result table will look like:
>
> id col1 col2 col3
>
> 1 ??? ddd uuu
> 2 ??? eee vvv
> 3 ??? fff www
>
> Which values will be for the questions marks ?
>
> Ok, I know the data model is not perfect and the two tables could be merged.
> But keep in mind that the sample is only a simplified version of a more complex
> SQL statement. So lets concentrate on my question.
>
> Thank you
>
> Toby

First, note that this is not an ORACLE specific question. you could have asked this just as easily in comp.databases

Why don't you try it and see? Try to think about what "SELECT *" means. How many columns should you get in the result of joining tablea and tableb? what would you expect if col1 in tableb was named arthur? What does your textbook say?

Who said anything about the two tables having to be merged? If the entities you are modelling are two separate things then two separate tables should exist. (example one table PERSON and one table COMPANY. Both have phone numbers. You would not put them both in the same table just because of that common attribute.)

You need to concentrate on your question. It is easy when you understand what SQL queries really do. Think about it. experiment. You cannot hurt anything with a SELECT statement.

It would be nice if you also let us know how you did.

   Ed Received on Tue Mar 06 2007 - 07:40:03 CST