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SQL2 row-value constructor syntax diagram
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Subqueries and Query Expressions
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WHERE clause (to the left of the equal sign) generates a two-column row, containing the manufacturer code and the product number for the current order being considered. The second row-value constructor (to the right of the equal sign) generates a twocolumn row, containing the (literal) manufacturer code ACI and product number 41002. The equal sign is now comparing two rows of values, not two scalar values. The SQL2 standard defines this type of row-valued comparison for equality, which is processed by comparing, pairwise, each of the columns in the two rows. The result of the comparison is TRUE only if all of the pairwise column comparisons are TRUE. Of course, it s possible to write the query without the row-value constructors, like this:
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List the order number, quantity, and amount of all orders for ACI-41002 widgets.
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SELECT ORDER_NUM, QTY, AMOUNT FROM ORDERS WHERE (MFR = 'ACI') AND (PRODUCT = '41002')
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and in this simple example, the meaning of the query is probably equally clear with either form. However, row-value constructors can be very useful in simplifying the appearance of more complex queries, and they become even more useful when combined with rowvalued subqueries.
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Row-Valued Subqueries (SQL2)
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As described throughout the earlier parts of this chapter, the SQL1 standard provides a subquery capability for expressing more complex database queries. The subquery takes the same form as a SQL query (that is, a SELECT statement), but a SQL1 subquery must be scalar-valued that is, it must produce a single data value as its query results. The value generated by the subquery is then used as part of an expression within the main SQL statement that contains the subquery. This use of subqueries is supported by the major enterprise-class relational database systems today. The SQL2 standard dramatically expands the subquery facility, including support for row-valued subqueries. A row-valued subquery returns not just a single data item, but a row of data items, which can be used in SQL2 expressions and compared to other rows. For example, suppose you wanted to show the order numbers and dates for all of the orders placed against the highest-priced product in the sample database. A logical way to start building the appropriate SQL query is to find an expression that will give you the identity (manufacturer ID and product ID) of the high-priced product in question. Here is a query that finds the right product: Find the manufacturer ID and product ID of the product with the highest unit price.
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SELECT MFR_ID, PRODUCT_ID FROM PRODUCTS WHERE PRICE = (SELECT MAX(PRICE) FROM PRODUCTS)
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Ignoring the possibility of a tie for the most expensive product for a moment, this query will generate a single row of query results, consisting of two columns. Using SQL2 s row-valued subquery capability, you can embed this entire query as a subquery within a SELECT statement to retrieve the order information: List the order numbers and dates of all orders placed for the highest-priced product.
SELECT ORDER_NUM, ORDER_DATE FROM ORDERS WHERE (MFR, PRODUCT) = (SELECT MFR_ID, PRODUCT_ID FROM PRODUCTS WHERE PRICE = (SELECT MAX(PRICE) FROM PRODUCTS))
The top-level WHERE clause in this query contains a row-valued comparison. On the left side of the equal sign is a row-value constructor consisting of two column names. Each time the WHERE clause is examined to carry out the top-level query, the value of this row-valued expression is a manufacturer-ID/product-ID pair from a row of the ORDERS table. On the right side of the equal sign is the subquery that generates the identity of the product with the highest dollar value. The result of this subquery is again a row value, with two colu mns, whose data types match those of the row-valued expression on the left side of the equal sign. It s possible to express this query without the row-valued subquery, but the resulting query will be much less straightforward: List the order numbers and dates of all orders placed for the highest-priced product.
SELECT ORDER_NUM, ORDER_DATE FROM ORDERS WHERE (MFR = (SELECT MFR_ID FROM PRODUCTS WHERE PRICE = (SELECT MAX(PRICE) FROM PRODUCTS))) AND (PRODUCT = (SELECT PRODUCT_ID FROM PRODUCTS WHERE PRICE = (SELECT MAX(PRICE) FROM PRODUCTS)))
Instead of a single row-valued comparison in the WHERE clause, the resulting query has two separate scalar-valued comparisons, one for the manufacturer ID and one for the
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