A subquery can contain a reference to an object defined in a parent statement. This is called an outer reference. A subquery that contains an outer reference is called a correlated subquery. Correlated subqueries cannot be evaluated independently of the outer query because the subquery uses the values of the parent statement. That is, the subquery is performed for each row in the parent statement. Thus, results of the subquery are dependent upon the active row being evaluated in the parent statement.
For example, the subquery in the statement below returns a value dependent upon the active row in the Products table:
SELECT Name, Description FROM Products WHERE Quantity < 2 * ( SELECT AVG( Quantity ) FROM SalesOrderItems WHERE Products.ID=SalesOrderItems.ProductID );
In this example, the Products.ID column in this subquery is the outer reference. The query extracts the names and descriptions of the products whose in-stock quantities are less than double the average ordered quantity of that product—specifically, the product being tested by the WHERE clause in the main query. The subquery does this by scanning the SalesOrderItems table. But the Products.ID column in the WHERE clause of the subquery refers to a column in the table named in the FROM clause of the main query—not the subquery. As the database server moves through each row of the Products table, it uses the ID value of the current row when it evaluates the WHERE clause of the subquery.
A subquery that does not contain references to objects in a parent statement is called an uncorrelated subquery. In the example below, the subquery calculates exactly one value: the average quantity from the SalesOrderItems table. In evaluating the query, the database server computes this value once, and compares each value in the Quantity field of the Products table to it to determine whether to select the corresponding row.
SELECT Name, Description FROM Products WHERE Quantity < 2 * ( SELECT AVG( Quantity ) FROM SalesOrderItems );
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