Construct a relational algebra query that produces the same result as the outer join r outer cond s using only these operators: union, di?erence, Cartesian product, projection, general join (not outer join). You can also use constant relations, that is, relations with ?xed content (e.g., one that has tuples ?lled with nulls or other prede?ned constants).
Let nullR be the relation over R that contains only one tuple, which entirely consists of NULLs. Similarly, let nulls be a relation over S that has a single NULL-tuple.
Then r outer conds is:
```
r cond s
? (r ? ?R(r conds)) × nullS
? nullR × (s ? ?S(r conds))
```
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