Advanced SELECT techniques

Aggregating data over the complete input rowset or aggregating in groups produces aggregated rows only—either one row for the whole input rowset or one row per group. Sometimes you need to return aggregates together with the detail data. One way to achieve this is by using subqueries, queries inside queries.

The following query shows an example of using two subqueries in a single query. In the SELECT clause, a subquery calculates the sum of quantity for each customer; it returns a scalar value. The subquery refers to the customer key from the outer query. The subquery can't execute without the outer query; this is a correlated subquery. There is another subquery in the FROM clause that calculates the overall quantity ...

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