do i need to join one dimension table to another dimension table

[inu]

HI
Is it necessary to join one dimension table to another dimension table, if so, then why.
can we retrieve the data from both the tables without joing, ?

Why we need to join a dimension table to a fact table, without joining , can we retrieve the data back from database?

pls elaborate the things with example.

Regards
Inam

[bdbits]

No, you don't normally join dimensions to each other. That makes it a non-star schema, and Cognos loves star (or snowflaked star) schemas.

Cognos uses the joins to determine whether tables from which you are extracting data are dimensions or facts, and to generate the SQL. You can create a model without joins, but good luck on getting anything useful out of it.

If you want to understand dimensional modeling, a good starting point is the FM user guide/help. There are plenty of books and lots of reading you can do with some simple Google searches. Or take a class.

[inu]

No, you don't normally join dimensions to each other. That makes it a non-star schema, and Cognos loves star (or snowflaked star) schemas.

Cognos uses the joins to determine whether tables from which you are extracting data are dimensions or facts, and to generate the SQL. You can create a model without joins, but good luck on getting anything useful out of it.

If you want to understand dimensional modeling, a good starting point is the FM user guide/help. There are plenty of books and lots of reading you can do with some simple Google searches. Or take a class.

Hi bdbits,
IF i m writing sql statement for retrieving data from two tables, and both are dimension tables. In this case do i need to join the tables, if they are inter related.
Second
IF i m writing sql statement for retrieving data from two tables, and one is dimension and other is fact table. In this case do i need to join the tables, if they are inter related. Without joining these two, can we get the data.


Cheers!
Inam

[charlie]

Looks like you dont have much experience writing SQLs :).. as bdbits suggested get your familiarised by researching on the web.

While retrieving data from any two tables (fact or dimension) creating a join is essential. Otherwise you end up getting a cross product. Check this out:
http://en.wikipedia.org/wiki/Join_(SQL)