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Subqueries and Query Expressions
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I No decision making within queries. Suppose you wanted to generate a two-column report from the sample database showing the name of each sales office and either its annual sales target or its year-to-date sales, whichever is larger. With standard SQL query features, this is hard to do. Or suppose you had a database that kept track of sales by quarter (four columns of data for each office) and wanted to write a program that displayed offices and their sales for
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To generate the query results for a SELECT statement:
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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. Form the product of the tables named in the FROM clause. If the FROM clause names a single table, the product is that table. 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. 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. 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. 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 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. If SELECT DISTINCT is specified, eliminate any duplicate rows of query results that were produced. 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. 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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SQL query processing rules (final version)
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a specific (user-supplied) quarter. Again, this program is more difficult to write using standard SQL queries. You must include four separate SQL queries (one for each quarter), and the program logic must select which query to run, based on user input. This simple case isn t too difficult, but in a more general case, the program could become much more complex. I Limited use of subqueries. The simplest example of this limitation is the SQL1 restriction that a subquery can appear only on the right side of a comparison test in a WHERE clause. The database request list the offices where the sum of the salesperson s quotas is greater than the office target is most directly expressed as this query:
SELECT OFFICE FROM OFFICES WHERE (SELECT SUM(QUOTA) FROM SALESREPS WHERE REP_OFFICE = OFFICE) > TARGET
But this isn t a legal SQL1 statement. Instead, you must turn the inequality around:
SELECT OFFICE FROM OFFICES WHERE TARGET > (SELECT SUM(QUOTA) FROM SALESREPS WHERE REP_OFFICE = OFFICE)
In this simple example, it isn t hard to turn the logic around, but the restriction is a nuisance at best, and it does prevent you from comparing the results of two subqueries, for example. I Limited-row expressions. Suppose you wanted to list the suppliers, item numbers, and prices for a set of products that are substitutes for one another. Conceptually, these are a set of products whose identification (a manufacturerID/product-ID pair) matches one of a set of values, and it would be natural to write the query using a set membership test as:
SELECT MFR_ID, PRODUCT_ID, PRICE FROM PRODUCTS WHERE (MFR_ID, PRODUCT_ID) IN (('ACI',41003),('BIC',41089), )
The SQL1 standard doesn t permit this kind of set membership test. Instead, you must construct the query as a long set of individual comparisons, connected by ANDs and ORs.
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