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Rules for Single-Table Query Processing
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Single-table queries are generally simple, and it s usually easy to understand the meaning of a query just by reading the SELECT statement. As queries become more complex, however, it s important to have a more precise definition of the query results that will be produced by a given SELECT statement. The following steps describe the procedure for generating the results of a SQL query that includes the clauses described in this chapter. As these steps show, the query results produced by a SELECT statement are specified by applying each of its clauses, one by one. The FROM clause is applied first (selecting the table containing data to be retrieved). The WHERE clause is applied next (selecting specific rows from the table). The SELECT clause is applied next (generating the specific columns of query results and eliminating duplicate rows, if requested). Finally, the ORDER BY clause is applied to sort the query results.
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Simple Queries
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To generate the query results for a SELECT statement, follow these steps: 1. Start with the table named in the FROM clause. 2. If there is a WHERE clause, apply its search condition to each row of the table, retaining those rows for which the search condition is TRUE, and discarding those rows for which it is FALSE or NULL. 3. For each remaining row, calculate the value of each item in the select list to produce a single row of query results. For each column reference, use the value of the column in the current row.
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4. If SELECT DISTINCT is specified, eliminate any duplicate rows of query results that were produced. 5. If there is an ORDER BY clause, sort the query results as specified. The rows generated by this procedure comprise the query results. These rules for SQL query processing will be expanded several times in the next three chapters to include the remaining clauses of the SELECT statement.
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Combining Query Results (UNION) *
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Occasionally, it s convenient to combine the results of two or more queries into a single table of query results. SQL supports this capability through the UNION feature of the SELECT statement. Figure 6-15 illustrates how to use the UNION operation to satisfy the following request: 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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SQL: The Complete Reference
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
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 FROM WHERE UNION SELECT FROM WHERE ACI ACI IMM REI REI MFR_ID, PRODUCT_ID PRODUCTS PRICE > 2000.00 DISTINCT MFR, PRODUCT ORDERS AMOUNT > 30000.00 4100Y 4100Z 775C 2A44L 2A44R
There are severe restrictions on the tables that can be combined by a UNION operation: I The two tables must contain the same number of columns. I The data type of each column in the first table must be the same as the data type of the corresponding column in the second table. I 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.
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