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Query Expressions in the FROM Clause
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SQL2 query expressions provide a much more powerful and flexible method for generating and combining tables of query results than the simple subquery and UNION operations provided by the SQL1 standard. To make query expressions even more useful and more general-purpose, the SQL2 standard allows them to appear almost anywhere that a table reference could appear in a SQL1 query. In particular, a query expression can appear in place of a table name in the FROM clause. Here is a simple example of a SQL2 query for the sample database that uses this feature: Show the names and total outstanding orders of all customers with credit limits over $50,000. (SELECT COMPANY, TOT_ORDERS FROM CUSTOMER, (SELECT CUST, SUM(AMOUNT) AS TOT_ORDERS FROM ORDERS GROUP BY CUST), WHERE (CREDIT_LIMIT > 50000.00) AND (CUST_NUM = CUST) The second "table name" in the FROM clause of the main query is not a table name at all, but a full-blown query expression. In fact, the expression could have been much more complex, involving UNION or JOIN operations. When a query expression appears in the FROM clause, as it does here, the DBMS conceptually carries it out first, before any other processing of the query, and creates a "temporary table" of the query results generated by the query expression. In this case, this "temporary table" consists of two columns, listing each customer number and the total of orders for that customer number. This temporary table then acts as one of the source tables for the main query. In this example, its contents are joined to the CUSTOMER table to obtain the company name and generate the answer to the main question. There are many other ways in which this query could be written. The entire query could be written as one top-level grouped query that joins the CUSTOMER and ORDERS table. The join operation could be made explicit with a SQL2 JOIN operator, and then the results of the join could be grouped in the top-level query. As this example shows, one of the benefits of the SQL2 query expression capabilities is that they typically provide several different ways to obtain the same query results. The general philosophy behind the SQL2 capabilities in this area was that the SQL language should provide the flexibility to express a query in the most natural form. The underlying DBMS must be able to take the query, however expressed, break it down into its fundamentals, and then determine the most efficient way to carry out the query. This "internal query execution plan" may be quite different than the apparent plan called for by the actual SQL statement, but so long as it produces the same query results, the net effect is to shift the "optimization" workload from the human user or programmer to the DBMS.
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This concludes the discussion of the SQL queries and the SELECT statement that began in 6. As described in the last three chapters, the clauses of the SELECT statement provide a powerful, flexible set of capabilities for retrieving data from the database. Each clause plays a specific role in data retrieval: The FROM clause specifies the source tables that contribute data to the query results. Every column name in the body of the SELECT statement must unambiguously identify a column from one of these tables, or it must be an outer reference to a column from a source table of an outer query.
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The WHERE clause, if present, selects individual combinations of rows from the source tables to participate in the query results. Subqueries in the WHERE clause are evaluated for each individual row. The GROUP BY clause, if present, groups the individual rows selected by the WHERE clause into row groups. The HAVING clause, if present, selects row groups to participate in the query results. Subqueries in the HAVING clause are evaluated for each row group. The SELECT clause determines which data values actually appear as columns in the final query results. The DISTINCT keyword, if present, eliminates duplicate rows of query results. The UNION operator, if present, merges the query results produced by individual SELECT statements into a single set of query results. The ORDER BY clause, if present, sorts the final query results based on one or more columns. The SQL2 query expression capabilities add row-valued and table-valued expressions and INTERSECT and EXCEPT operations to the SQL1 capabilities. The fundamental flow of query processing is not changed, but the ability to express "queries within queries" is greatly enhanced. Figure 9-16 shows the final version of the rules for SQL query processing, extended to include subqueries. It provides a complete definition of the query results produced by a SELECT statement.
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To generate the query results for a SELECT statement: 1. If the statement is a UNION of SELECT statements, apply Steps 2 through 7 to each of the statements to generate their individual query results. 2. Form the product of the tables named in the FROM clause. If the FROM clause names a single table, the product is that table. 3. If there is a WHERE clause, apply its search condition to each row of the product table, retaining those rows for which the search condition is TRUE (and discarding those for which it is FALSE or NULL). If the HAVING clause contains a subquery, the subquery is performed for each row as it is tested. 4. If there is a GROUP BY clause, arrange the remaining rows of the product table into row groups, so that the rows in each group have identical values in all of the grouping columns. 5. If there is a HAVING clause, apply its search condition to each row group, retaining those groups for which the search condition is TRUE (and discarding those for which it is FALSE or NULL). If the HAVING clause contains a subquery, the subquery is performed for each row group as it is tested. 6. For each remaining row (or row group), calculate the value of each item in the select list to produce a single row of query results. For a simple column reference, use the
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value of the column in the current row (or row group). For a column function, use the current row group as its argument if GROUP BY is specified; otherwise, use the entire set of rows. 7. If SELECT DISTINCT is specified, eliminate any duplicate rows of query results that were produced. 8. If the statement is a UNION of SELECT statements, merge the query results for the individual statements into a single table of query results. Eliminate duplicate rows unless UNION ALL is specified. 9. If there is an ORDER BY clause, sort the query results as specified. The rows generated by this procedure comprise the query results.
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Figure 9-16: SQL query processing rules (final version)
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