When we have a many to many relationship between 2 tables, why do we create 
a 3rd table between known as association table ? Don't seem to understand. 
Please assist.

Thanks 

On Wed, 4 Aug 2010 20:53:41 -0700, "Raj" <raj@hotmail.com> wrote:

>When we have a many to many relationship between 2 tables, why do we create 
>a 3rd table between known as association table ? Don't seem to understand. 
>Please assist.

     To establish what links to what.

     Were it 1-to-many, each row in the many table can have an FK into
the 1 table.  How can you do it with many-to-many?  Where would you
put the values?  With the association table, every valid combination
has a row in the association table with FKs into the two many tables.

Sincerely,

Gene Wirchenko

>> When we have a many to many relationship between 2 tables, why do we create a 3rd table between known as association table? <<

The third table is a relationship and not an "association table"; I
have no idea where you got that term.

How else would you do it? Think about it.

A table models either a set of entities or a relationship or an
auxiliary set of data. I have several examples of how to use UNIQUE
constraints set the degree and membership of a relation. If you don't
know those terms, you need to read a book on basic data modeling.