Table 1
a+b+c -PK
Table 2
a+b+d - PK
How I can relation this tables with use (a+b)?
ps
Sorry for my english

I would create a parent table with A and B as elements of its primary key. I would create table 1 as a dependant child of the parent with C as part of its PK, and table 2 as a dependant child of the parent with D as part of its Pk.

-PatP

yes... do look for the possibility that when designing databases, the physical model doesn't necessary follow from the logical model...

in some cases, a mapping/relation/linking table (as shown by Pat) is necessary in the physical model [to fulfill the relation requirements in the logical model]

cheers!