1. Outer join: An extension of join to deal with missing information.

2. Two relations in Fig. 3.8, with the relation schemas,

• employee (ename, stree, city)
• ft works (ename, bname, salary)

3. A join may miss some informaiton on the non-joinable attributes.

4. Three outer-joins: left outer-join, right outer-join, and full outer-join.

5. left outer-join: takes all tuples in the left relation that did not match with any tuple in the right relation,pads the tuples with null values for all other attributes from the right relation, and adds them to the result of the natural join.

6. Similarly, we define right outer-join and full outer-join.