alar subqueries can be similarlytranslated by allowing nested relational algebra expressions to appear inscalar expressions. An alternative approach to handling such subqueriesused in some database systems, such as MicrosoftSQL Server, introduces anewrelational algebra operator called the Apply operator;see Chapter 30,page 1230-1231 for details. Without suchextensions, translating subqueriesinto standard relational algebra can be plicated.1The case of outer joins with theusingclause is a little plicated; with a right outer join it is possible thatR1.A1 is null, butR2.A1 is not, and the output should contain the non-null value. TheSQLcoalescefunction canbe used, replacingSbycoalesce(R1.A1,R2.A1),coalesce(R1.A2,R2.A2), . . .coalesce(R1.An,R2.An), followed by theother attributes ofR1 andR2.
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