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/* Prompt user for tablename and column name * / printf("Enter name of table to be updated: gets(tblname); ");
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Using the PREPARE and EXECUTE statements
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printf("Enter name of column to be searched: "); gets(searchcol); printf("Enter name of column to be updated: "); gets(updatecol); /* Build SQL statement in buffer; ask DBMS to compile it */ sprintf(stmtbuf, "update %s set %s = where %s = ", tblname, searchcol, updatecol); exec sql prepare mystmt from :stmtbuf; if (sqlca.sqlcode) { printf("PREPARE error: %ld\n", sqlca.sqlcode); exit();
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} /* Loop prompting user for parameters and performing updates */ for ( ; ; ) { printf("\nEnter search value for %s: ", searchcol); scanf("%f", &search_value); printf("Enter new value for %s: ", updatecol); scanf("%f", &new_value); /* Ask the DBMS to execute the UPDATE statement */ execute mystmt using :search_value, :new_value; if (sqlca.sqlcode) { printf("EXECUTE error: %ld\n", sqlca.sqlcode); exit(); } /*Ask user if there is another update */ printf("Another (y/n) "); gets(yes_no); if (yes_no[0] == 'n') break; } printf("\nUpdates complete.\n"); exit(); }
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Using the PREPARE and EXECUTE statements (continued)
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Dynamic SQL *
user for a table name and two column names, and constructs an UPDATE statement for the table that looks like this:
update table-name set second-column-name = where first-column-name =
The user s input thus determines the table to be updated, the column to be updated, and the search condition to be used. The search comparison value and the updated data value are specified as parameters, to be supplied later when the UPDATE statement is actually executed. After building the UPDATE statement text in its buffer, the program asks the DBMS to compile it with the PREPARE statement. The program then enters a loop, prompting the user to enter pairs of parameter values to perform a sequence of table updates. This user dialog shows how you could use the program in Figure 18-4 to update the quotas for selected salespeople:
Enter name of table to be updated: staff Enter name of column to be searched: empl_num Enter name of column to be updated: quota Enter search value for empl_num: 106 Enter new value for quota: 150000.00 Another (y/n) y Enter search value for empl_num: 102 Enter new value for quota: 225000.00 Another (y/n) y Enter search value for empl_num: 107 Enter new value for quota: 215000.00 Another (y/n) n Updates complete.
This program is a good example of a situation where two-step dynamic execution is appropriate. The DBMS compiles the dynamic UPDATE statement only once but executes it three times, once for each set of parameter values entered by the user. If the program had been written using EXECUTE IMMEDIATE instead, the dynamic UPDATE statement would have been compiled three times and executed three times. Thus, the two-step dynamic execution of PREPARE and EXECUTE helps to eliminate some of the performance disadvantage of dynamic SQL. As mentioned earlier, this same two-step approach is used by all of the callable SQL APIs described in 19.
PROGRAMMING WITH SQL
SQL: The Complete Reference
The PREPARE Statement
The PREPARE statement, shown in Figure 18-5, is unique to dynamic SQL. It accepts a host variable containing a SQL statement string and passes the statement to the DBMS. The DBMS compiles the statement text and prepares it for execution by generating an application plan. The DBMS sets the SQLCODE/SQLSTATE variables to indicate any errors detected in the statement text. As described previously, the statement string can contain a parameter marker, indicated by a question mark, anywhere that a constant can appear. The parameter marker signals the DBMS that a value for the parameter will be supplied later, when the statement is actually executed. As a result of the PREPARE statement, the DBMS assigns the specified statement name to the prepared statement. The statement name is a SQL identifier, like a cursor name. You specify the statement name in subsequent EXECUTE statements when you want to execute the statement. DBMS brands differ in how long they retain the prepared statement and the associated statement name. For some brands, the prepared statement can be reexecuted only until the end of the current transaction (that is, until the next COMMIT or ROLLBACK statement). If you want to execute the same dynamic statement later during another transaction, you must prepare it again. Other brands relax this restriction and retain the prepared statement throughout the current session with the DBMS. The ANSI/ISO SQL2 standard acknowledges these differences and explicitly says that the validity of a prepared statement outside of the current transaction is implementation dependent. The PREPARE statement can be used to prepare almost any executable DML or DDL statement, including the SELECT statement. Embedded SQL statements that are actually precompiler directives (such as the WHENEVER or DECLARE CURSOR statements) cannot be prepared, of course, because they are not executable.
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