How to avoid duplicates while extract value from the composite key table?

**I have a table(table A) with composite primary key (3 columns)... i have to use one of the column as foreign key in another table(table B).. Now i want to extr开发者_JAVA百科actthe values from the table A by the foriegn key of table B.. How to avoid the duplicates in table B? **

Try by putting a unique constraint.