OUTER JOIN

Outer join is an extended form of the regular or inner join operation. It differs from the inner join in that tuples in one relation having no counterpart in the other appear in, the result with nulls in the other attribute positions, instead of simply being ignored as they normally are. It is not a primitive operation; for example, the following expression could be used to construct the outer join of suppliers and shipments on supplier numbers.

In fig. 3, the top portion shows some sample data values for relation variables S and SP, the middle portion shows the regular inner join. and the bottom portion shows the corresponding outer join. As the figure indicates, the inner join "loses information" for suppliers who supply no parts (supplier S5, in the example), whereas the outer join “preserves” such information; indeed, exactly that distinction is the whole point of outer join.

Figure 3 – Inner vs. outer join (example)

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