Figure 6-16: Net UNION operations in Software

Encoder QR-Code in Software Figure 6-16: Net UNION operations

Figure 6-16: Net UNION operations
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SELECT * FROM A UNION (SELECT * FROM B UNION SELECT * FROM C) Bill Mary George Fred Sue Julia Harry The parentheses in the query indicate which UNION 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: A UNION (B UNION C) (A UNION B) UNION C (A UNION C) UNION B 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: A UNION ALL (B UNION ALL C) (A UNION ALL B) UNION ALL C (A UNION ALL C) UNION ALL B However, if the unions involve a mixture of UNION and UNION ALL, the order of evaluation matters. If this expression:
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A UNION ALL B UNION C is interpreted as: A UNION ALL (B UNION C) 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: (A UNION ALL B) UNION C then it produces only four rows, because the outer UNION eliminates all duplicate rows. For this reason, it's always a good idea to use parentheses in UNIONs of three or more tables to specify the order of evaluation intended.
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This chapter is the first of four chapters about SQL queries. It described the following query features: The SELECT statement is used to express a SQL query. Every SELECT statement produces a table of query results containing one or more columns and zero or more rows. The FROM clause specifies the table(s) containing the data to be retrieved by a query. The SELECT clause specifies the column(s) of data to be included in the query results, which can be columns of data from the database, or calculated columns. The WHERE clause selects the rows to be included in the query results by applying a search condition to rows of the database. A search condition can select rows by comparing values, by checking a value against a range or set of values, by matching a string pattern, and by checking for NULL values. Simple search conditions can be combined with AND, OR, and NOT to form more complex search conditions. The ORDER BY clause specifies that the query results should be sorted in ascending or descending order, based on the values of one or more columns. The UNION operation can be used within a SELECT statement to combine two or more sets of query results into a single set.
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A Two-Table Query Example
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The best way to understand the facilities that SQL provides for multi-table queries is to start with a simple request that combines data from two different tables: "List all orders, showing the order number and amount, and the name and credit limit of the customer who placed it." The four specific data items requested are clearly stored in two different tables, as shown in Figure 7-1.
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Figure 7-1: A request that spans two tables
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The ORDERS table contains the order number and amount of each order, but doesn't have customer names or credit limits. The CUSTOMERS table contains the customer names and balances, but it lacks any information about orders. There is a link between these two tables, however. In each row of the ORDERS table, the CUST column contains the customer number of the customer who placed the order, which matches the value in the CUST_NUM column in one of the rows in the CUSTOMERS table. Clearly, the SELECT statement that handles the request must somehow use this link between the tables to generate its query results. Before examining the SELECT statement for the query, it's instructive to think about how you would manually handle the request, using paper and pencil. Figure 7-2 shows what you would probably do:
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