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RETRIEVAL: SOME ADVANCED FEATURES
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9-12) to the subquery expression, because it enhances the readability for both the query itself and for its results.
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Note As with any feature or method of query construction, performance can be better or worse than another method. Always test on production-like configurations and data sets to avoid the surprise of a solution that performs well in development but is utterly unable to scale.
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So far, we have distinguished only single-row queries and subqueries returning any number of rows. At this point, it makes sense to identify a third subquery type, which is a subtype of the single-row subquery type: scalar subqueries. The name indicates an important property of this type of subqueries: the result not only consists of precisely one row, but also with precisely one column value. You can use scalar subqueries almost everywhere in your SQL commands in places where a column expression or literal value is allowed and makes sense. The scalar subquery generates the literal value. In summary, you can say that SQL supports the following subquery hierarchy: Multirow subqueries: No restrictions Single-row subqueries: Result must contain a single row Scalar subqueries: Result must be a single row and a single column
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9.3 Subqueries in the FROM Clause
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The next clause we investigate is the FROM clause. Actually, the FROM clause is one of the most obvious places to allow subqueries in SQL. Instead of specifying real table names, you simply provide subqueries (or table expressions) to take their place as a derived table. Listing 9-16 shows an example of a subquery in the FROM clause. The Oracle documentation refers to these subqueries as inline views, as does this book. The name inline view will become clearer in 10, when we discuss views in general. Listing 9-16. Inline View Example select e.ename, e.init, e.msal from employees e join (select x.deptno , avg(x.msal) avg_sal from employees x group by x.deptno ) g using (deptno) where e.msal > g.avg_sal; ENAME INIT MSAL -------- ----- -------ALLEN JAM 1600
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A big difference between a real table and a subquery is that the real table has a name. Therefore, if you use subqueries in the FROM clause, you must define a tuple variable (or table alias, in Oracle terminology) over the result of the subquery. At the end of line 7 in Listing 9-16, we define tuple variable g. This tuple variable allows us to refer to column expressions from the subquery, as shown by g.AVG_SAL in the last line of the example. By the way, the query in Listing 9-16 is an alternative solution for the query in Listing 9-6. One requirement is that the subquery must be independent of the outer query, it cannot be correlated.
9.4 The WITH Clause
Listing 9-16 showed an example of using a subquery in a FROM clause. We could have written the same query with a slightly different syntax, as shown in Listing 9-17. This construct is called a factored subquery (or subquery factoring). Listing 9-17. WITH Clause Example WITH g AS (select x.deptno , avg(x.msal) avg_sal from employees x group by x.deptno) select e.ename, e.init, e.msal from employees e join g using (deptno) where e.msal > g.avg_sal; ENAME -------ALLEN JONES BLAKE SCOTT KING FORD INIT MSAL ----- -------JAM 1600 JM 2975 R 2850 SCJ 3000 CC 5000 MG 3000
As you can see, we have isolated the subquery definition, in lines 1 through 5, from the actual query in lines 6 through 10. This makes the structure of the main query clearer. Using the WITH clause syntax becomes even more attractive if you refer multiple times to the same subquery from the main query. You can define as many subqueries as you like in a single WITH clause, separated by commas.
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