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The parentheses in the query indicate which UNION operation should be performed first. In fact, if all of the UNIONs in the statement eliminate duplicate rows, or if all of them retain duplicate rows, the order in which they are performed is unimportant. These three expressions are completely equivalent:

Part II:

Note that the default duplicate row handling for the UNION operation and for the simple SELECT statement is exactly opposite. For the SELECT statement, SELECT ALL (duplicates retained) is the default. To eliminate duplicate rows, you must explicitly specify SELECT DISTINCT. For the UNION operation, UNION (duplicates eliminated) is the default. To retain duplicate rows, you must explicitly specify UNION ALL. Database experts have criticized the handling of duplicate rows in SQL and point to this inconsistency as an example of the problems. The reason for the inconsistency is that the SQL defaults were chosen to produce the correct behavior most of the time: In practice, most simple SELECT statements do not produce duplicate rows, so the default is no duplicate elimination. In practice, most UNION operations would produce unwanted duplicate rows, so the default is duplicate elimination. Eliminating duplicate rows from query results is a very time-consuming process, especially if the query results contain a large number of rows. If you know, based on the individual queries involved, that a UNION operation cannot produce duplicate rows, you should specifically use the UNION ALL operation because the query will execute much more quickly.

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and produce seven rows of query results. Similarly, the following three expressions are completely equivalent and produce twelve rows of query results, because the duplicates are retained:

Many requests for information don't require the level of detail provided by the SQL queries described in the last two chapters. For example, each of the following requests asks for a single value or a small number of values that summarize the contents of the database: What is the total quota for all salespeople What are the smallest and largest assigned quotas How many salespeople have exceeded their quota What is the size of the average order What is the size of the average order for each sales office How many salespeople are assigned to each sales office SQL supports these requests for summary data through column functions and the GROUP BY and HAVING clauses of the SELECT statement, which are described in this chapter.

Unions and Sorting*

However, if the unions involve a mixture of UNION and UNION ALL, the order of evaluation matters. If this expression:

The ORDER BY clause cannot appear in either of the two SELECT statements combined by a UNION operation. It wouldn t make much sense to sort the two sets of query results anyway, because they are fed directly into the UNION operation and are never visible to the user. However, the combined set of query results produced by the UNION operation can be sorted by specifying an ORDER BY clause after the second SELECT statement. Since the columns produced by the UNION operation are not named, the ORDER BY clause specifies the columns by column number. However, many SQL products, including Oracle, SQL Server and MySQL, use the column names from the first SELECT statement, and therefore allow you to use those column names in the ORDER BY clause. Here is the same products query as that shown in Figure 6-14, with the query results sorted by manufacturer and product number: List all the products where the price of the product exceeds $2,000 or where more than $30,000 of the product has been ordered in a single order, sorted by manufacturer and product number.

is interpreted as:

SELECT FROM WHERE UNION SELECT FROM WHERE ORDER ACI ACI IMM REI REI MFR_ID, PRODUCT_ID PRODUCTS PRICE > 2000.00 DISTINCT MFR, PRODUCT ORDERS AMOUNT > 30000.00 BY 1, 2; 4100Y 4100Z 775C 2A44L 2A44R

A UNION ALL (B UNION C)

6:

then it produces ten rows of query results (six from the inner UNION, plus four rows from Table A). However, if it is interpreted as:

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