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This prohibition exists for the same reason that the NULL keyword is not allowed in the search condition it makes no sense to test whether QUOTA and NULL are equal, because the answer will always be NULL (unknown). Instead of using the indicator variable, you must use an explicit IS NULL test. This pair of embedded SQL statements accomplishes the intended task of the preceding illegal statement:
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if (amount_ind < 0) { exec sql delete from salesreps where quota is null; } else { exec sql delete from salesreps where quota = :amount; }
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Indicator variables are especially useful when you are retrieving data from the database into your program and the retrieved data values may be NULL. This use of indicator variables is described later in the section Retrieving NULL Values.
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Data Retrieval in Embedded SQL
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Using the embedded SQL features described thus far, you can embed any interactive SQL statement except the SELECT statement in an application program. Retrieving data with an embedded SQL program requires some special extensions to the SELECT statement. The reason for these extensions is that there is a fundamental mismatch between the SQL language and programming languages such as C and COBOL: a SQL query produces an entire table of query results, but most programming languages can manipulate only individual data items or individual records (rows) of data. Embedded SQL must build a bridge between the table-level logic of the SQL SELECT statement and the row-by-row processing of C, COBOL, and other host programming languages. For this reason, embedded SQL divides SQL queries into two groups: Single-row queries You expect the query results to contain a single row of data. Looking up a customer s credit limit or retrieving the sales and quota for a particular salesperson are examples of this type of query. Multirow queries You expect that the query results may contain zero, one, or many rows of data. Listing the orders with amounts over $20,000 or retrieving the names of all salespeople who are over quota are examples of this type of query. Interactive SQL does not distinguish between these two types of queries; the same interactive SELECT statement handles them both. In embedded SQL, however, the two types of queries are handled very differently. Single-row queries are simpler to handle and are discussed in the next section. Multirow queries are discussed shortly.
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Single-Row Queries
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Many useful SQL queries return a single row of query results. Single-row queries are especially common in transaction-processing programs, where a user enters a customer number or an order number, and the program retrieves relevant data about the customer or order.
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Programming with SQL
In embedded SQL, single-row queries are handled by the singleton SELECT statement, shown in Figure 17-19. The singleton SELECT statement has syntax much like that of the interactive SELECT statement. It has a SELECT clause, a FROM clause, and an optional WHERE clause. Because the singleton SELECT statement returns a single row of data, there is no need for a GROUP BY, HAVING, or ORDER BY clause. The INTO clause specifies the host variables that are to receive the data retrieved by the statement. Figure 17-20 shows a simple program with a singleton SELECT statement. The program prompts the user for an employee number and then retrieves the name, quota, and sales of the corresponding salesperson. The DBMS places the three retrieved data items into the host variables repname, repquota, and repsales, respectively. Recall that the host variables used in the INSERT, DELETE, and UPDATE statements in the previous examples were input host variables. In contrast, the host variables specified in the INTO clause of the singleton SELECT statement are output host variables. Each host variable named in the INTO clause receives a single column from the row of query results. The select list items and the corresponding host variables are paired in sequence, as they appear in their respective clauses, and the number of query results columns must be the same as the number of host variables. In addition, the data type of each host variable must be compatible with the data type of the corresponding column of query results. Most DBMS brands will automatically handle reasonable conversions between DBMS data types and the data types supported by the programming language. For example, most DBMS products will convert DECIMAL data retrieved from the database into packed decimal (COMP-3) data before storing it in a COBOL variable, or into floating point data before storing it in a C variable. The precompiler uses its knowledge of the host variable s data type to handle the conversion correctly.
FIGURE 17-19
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