free barcode generator using vb.net Figure 6-15: Using UNION to combine query results in Software

Maker QR Code in Software Figure 6-15: Using UNION to combine query results

Figure 6-15: Using UNION to combine query results
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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. The first part of the request can be satisfied with the top query in the figure: List all the products whose price exceeds $2,000.
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SELECT MFR_ID, PRODUCT_ID FROM PRODUCTS WHERE PRICE > 2000.00 MFR_ID -----ACI REI ACI REI PRODUCT_ID ---------4100Y 2A44L 4100Z 2A44R
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Similarly, the second part of the request can be satisfied with the bottom query in the figure: List all the products where more than $30,000 of the product has been ordered in a single order. SELECT DISTINCT MFR, PRODUCT FROM ORDERS WHERE AMOUNT > 30000.00 MFR --IMM REI REI PRODUCT ------775C 2A44L 2A44R
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As shown in Figure 6-15, the UNION operation produces a single table of query results that combines the rows of the top query results with the rows of the bottom query results. The SELECT statement that specifies the UNION operation looks like this: 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. SELECT MFR_ID, PRODUCT_ID FROM PRODUCTS WHERE PRICE > 2000.00 UNION SELECT DISTINCT MFR, PRODUCT FROM ORDERS WHERE AMOUNT > 30000.00 ACI ACI IMM REI REI 4100Y 4100Z 775C 2A44L 2A44R
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There are severe restrictions on the tables that can be combined by a UNION operation: The two tables must contain the same number of columns. The data type of each column in the first table must be the same as the data type of
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the corresponding column in the second table. Neither of the two tables can be sorted with the ORDER BY clause. However, the combined query results can be sorted, as described in the following section. Note that the column names of the two queries combined by a UNION do not have to be identical. In the preceding example, the first table of query results has columns named MFR_ID and PRODUCT_ID, while the second table of query results has columns named MFR and PRODUCT. Because the columns in the two tables can have different names, the columns of query results produced by the UNION operation are unnamed. The ANSI/ISO SQL standard specifies a further restriction on a SELECT statement that participates in a UNION. It permits only column names or an "all columns" specification (SELECT *) in the select list and prohibits expressions in the select list. Most commercial SQL implementations relax this restriction and permit simple expressions in the select list. However, many SQL implementations do not allow the SELECT statements to include the GROUP BY or HAVING clauses, and some do not allow column functions in the select list (prohibiting summary queries as described in 8). In fact, some SQL implementations (including SQL Server) do not support the UNION operation at all.
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Unions and Duplicate Rows *
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Because the UNION operation combines the rows from two sets of query results, it would tend to produce query results containing duplicate rows. For example, in the query of Figure 6-15, product REI-2A44L sells for $4,500.00, so it appears in the top set of query results. There is also an order for $31,500.00 worth of this product in the ORDERS table, so it also appears in the bottom set of query results. By default, the UNION operation eliminates duplicate rows as part of its processing. Thus, the combined set of query results contains only one row for product REI-2A44L. If you want to retain duplicate rows in a UNION operation, you can specify the ALL keyword immediately following the word "UNION." This form of the query produces two duplicate rows for product REI-2A44L: 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. SELECT MFR_ID, PRODUCT_ID FROM PRODUCTS WHERE PRICE > 2000.00 UNION ALL SELECT DISTINCT MFR, PRODUCT FROM ORDERS WHERE AMOUNT > 30000.00 ACI REI ACI REI IMM REI REI 4100Y 2A44L 4100Z 2A44R 775C 2A44L 2A44R
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
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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.
Unions and Sorting *
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 must specify the columns by column number. Here is the same products query as that shown in Figure 6-15, 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. SELECT MFR_ID, PRODUCT_ID FROM PRODUCTS WHERE PRICE > 2000.00 UNION SELECT DISTINCT MFR, PRODUCT FROM ORDERS WHERE AMOUNT > 30000.00 ORDER BY 1, 2 ACI ACI IMM REI REI 4100Y 4100Z 775C 2A44L 2A44R
Multiple UNIONs *
The UNION operation can be used repeatedly to combine three or more sets of query results, as shown in Figure 6-16. The union of Table B and Table C in the figure produces a single, combined table. This table is then combined with Table A in another
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UNION operation. The query in the figure is written this way:
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