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with 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.

I used a UNIQUE constraint in these SQL statements only as a way to demonstrate how the constraint can be used in a table with a primary key. Most likely, you would not want to use a UNIQUE constraint for the ARTIST_NAME column because it is possible for two artists to share the same name. (For example, two different blues artists, both of whom lived in the earlier part of the last century, went by the name of Sonny Boy Williamson.)

SELECT MFR_ID, PRODUCT_ID FROM PRODUCTS WHERE PRICE = (SELECT MAX(PRICE) FROM PRODUCTS);

Q: A:

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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 SQL s row-valued subquery capability, you can embed this entire query as a subquery within a SELECT statement to retrieve the order information, as shown in the next example. List the order numbers and dates of all orders placed for the highest-priced product.

MFR_ID, PRODUCT_ID PRODUCTS PRICE = (SELECT MAX(PRICE) PRODUCTS));

Can the columns in a table belong to both a UNIQUE constraint and a PRIMARY KEY constraint Yes, as long as they re not the exact same columns. For example, suppose you have a table that includes three columns: ARTIST_ID, ARTIST_NAME, and PLACE_OF_BIRTH. You can define a PRIMARY KEY constraint that includes the ARTIST_ID and ARTIST_ NAME columns, which would ensure unique value pairs in those two columns, but values within the individual columns could still be duplicated. However, you can then define a UNIQUE constraint that includes only the ARTIST_NAME column to ensure that those values are unique as well. (This certainly isn t the best design, but it illustrates my point.) You can also create a UNIQUE constraint that includes the ARTIST_NAME and PLACE_ OF_BIRTH columns to ensure unique value pairs in those two columns. The only thing you can t do is create a UNIQUE constraint that includes the exact same columns as an existing PRIMARY KEY constraint and vice versa. You state that a column that is included in a PRIMARY KEY constraint will not accept null values. What happens if that column is configured with a NOT NULL constraint as well Nothing different happens. The table is still created in the same way. A column definition that includes PRIMARY KEY is saying the same thing as a column definition that includes NOT NULL PRIMARY KEY. In fact, prior to SQL-92, the NOT NULL keywords were required on all columns included in a PRIMARY KEY constraint.

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 columns, 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(s).

SELECT ORDER_NUM, ORDER_DATE FROM ORDERS WHERE (MFR IN (SELECT MFR_ID FROM PRODUCTS WHERE PRICE = (SELECT MAX(PRICE) FROM PRODUCTS))) AND (PRODUCT IN (SELECT PRODUCT_ID FROM PRODUCTS WHERE PRICE = (SELECT MAX(PRICE) FROM PRODUCTS)));

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Q: A:

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 product ID. Because the comparison must be split, the lower-level subquery to find the

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maximum price must be repeated twice as well. Overall, the form of the query using the row-valued expression is a more direct translation of the English-language request, and it s easier to read and understand.

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