what is the difference between a join and a link ?

join is a relation ship between a bc and a table, which is not a base table
there are two types of join
1. impicit joins
2. explicit join
where as link is a relationship between two business components i.e 1-m or m-1 or m-m

Join is used to provide M:1 relationship:
e.g: a contact record was created in the DB, the employee that created it is X, all the contacts that been created by X will have a field containing the ID of X. So, in Siebel, you can have a contact record displayed on an Applet, this... a piece of information you might want to provide is who created this record. For this you need to create a join.

The Link is used to provide either 1:M relationship, or M:M relationship:
Looking at the example above, say you want to display all the contacts created by employee X in the employees screen under X's record. For that you need to create a link where Employee business component is the parent, and contact business component is then child, keep your source field empty (default is Id), and the destination field is Created By field. This is 1:M relationship.

The M:M relationship is little more complex. You relate two record via an intersection table. So, if a contact has many addresses, and in on address live many contacts... you create the Address once, relate all contacts to it. And for one contact you associate many addresses.
For this example the S_ADDR_PER table have to foreign keys, one to contact and one to address, and then used to establish the linkage between the record using the Link object.

// Islam