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A subquery is a query within a query. The results of the subquery are used by the DBMS to determine the results of the higher-level query that contains the subquery. In the simplest forms of a subquery, the subquery appears within the WHERE or HAVING clause of another SQL statement. Subqueries provide an efficient, natural way to handle query requests that are themselves expressed in terms of the results of other queries. Here is an example of such a request: List the offices where the sales target for the office exceeds the sum of the individual salespeople s quotas. The request asks for a list of offices from the OFFICES table, where the value of the TARGET column meets some condition. It seems reasonable that the SELECT statement that expresses the query should look something like this:
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SELECT CITY FROM OFFICES WHERE TARGET >
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The value needs to be filled in and should be equal to the sum of the quotas of the salespeople assigned to the office in question. How can you specify that value in the query From 8, you know that the sum of the quotas for a specific office (say, office number 21) can be obtained with this query:
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Subqueries and Query Expressions
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SELECT SUM(QUOTA) FROM SALESREPS WHERE REP_OFFICE = 21
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But it would be inefficient to have to type in this query, write down the results, and then type in the previous query with the correct amount. How can you put the results of this query into the earlier query in place of the question marks It would seem reasonable to start with the first query and replace the with the second query, as follows:
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SELECT CITY FROM OFFICES WHERE TARGET > (SELECT SUM(QUOTA) FROM SALESREPS WHERE REP_OFFICE = OFFICE) RETRIEVING DATA
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In fact, this is a correctly formed SQL query. For each office, the inner query (the subquery) calculates the sum of the quotas for the salespeople working in that office. The outer query (the main query) compares the office s target to the calculated total and decides whether to add the office to the main query results. Working together, the main query and the subquery express the original request and retrieve the requested data from the database. SQL subqueries typically appear as part of the WHERE clause or the HAVING clause. In the WHERE clause, they help to select the individual rows that appear in the query results. In the HAVING clause, they help to select the row groups that appear in the query results.
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What Is a Subquery
Figure 9-1 shows the form of a SQL subquery. The subquery is enclosed in parentheses, but otherwise it has the familiar form of a SELECT statement, with a FROM clause and optional WHERE, GROUP BY, and HAVING clauses. The form of these clauses in a subquery is identical to that in a SELECT statement, and they perform their normal functions when used within a subquery. There are, however, a few differences between a subquery and an actual SELECT statement: I In the most common uses, a subquery must produce a single column of data as its query results. This means that a subquery almost always has a single select item in its SELECT clause. I The ORDER BY clause cannot be specified in a subquery. The subquery results are used internally by the main query and are never visible to the user, so it makes little sense to sort them anyway.
SQL: The Complete Reference
Figure 9-1.
Basic subquery syntax diagram
I Column names appearing in a subquery may refer to columns of tables in the main query. These outer references are described in detail later in the Outer References section. I In most implementations, a subquery cannot be the UNION of several different SELECT statements; only a single SELECT is allowed. (The SQL2 standard allows much more powerful query expressions and relaxes this restriction, as described later in the section Advanced Queries in SQL2. )
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