qr barcode generator vb.net PREPARE statement-name ATTRIBUTES attributes-variable FROM host-variable in Software

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PREPARE statement-name ATTRIBUTES attributes-variable FROM host-variable
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FIGURE 18-5
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The PREPARE statement syntax diagram
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Programming with SQL
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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 SQL 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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The EXECUTE Statement
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The EXECUTE statement, shown in Figure 18-6, is unique to dynamic SQL. It asks the DBMS to execute a statement previously prepared with the PREPARE statement. You can execute any statement that can be prepared, with one exception. Like the EXECUTE IMMEDIATE statement, the EXECUTE statement cannot be used to execute a SELECT statement, because it lacks a mechanism for handling query results. If the dynamic statement to be executed contains one or more parameter markers, the EXECUTE statement must provide a value for each of the parameters. The values can be provided in two different ways, described in the next two sections. The ANSI/ISO SQL standard includes both of these methods.
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EXECUTE with Host Variables
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The easiest way to pass parameter values to the EXECUTE statement is by specifying a list of host variables in the USING clause. The EXECUTE statement substitutes the values of the host variables, in sequence, for the parameter markers in the prepared statement text. The host variables thus serve as input host variables for the dynamically executed statement. This technique was used in the program shown in Figure 18-4. It is supported by all of the popular DBMS brands that support dynamic SQL and is included in the ANSI/ISO SQL standard for dynamic SQL.
EXECUTE statement-name USING host-variable , DESCRIPTOR descriptor-name
FIGURE 18-6
The EXECUTE statement syntax diagram
18:
Dynamic SQL*
The number of host variables in the USING clause must match the number of parameter markers in the dynamic statement, and the data type of each host variable must be compatible with the data type required for the corresponding parameter. Each host variable in the list may also have a companion host indicator variable. If the indicator variable contains a negative value when the EXECUTE statement is processed, the corresponding parameter marker is assigned the NULL value.
EXECUTE with SQLDA
The second way to pass parameters to the EXECUTE statement is with a special dynamic SQL data structure called a SQL Data Area (SQLDA). You must use a SQLDA to pass parameters when you don t know the number of parameters to be passed and their data types at the time that you write the program. For example, suppose you wanted to modify the general-purpose update program in Figure 18-4 so that the user could select more than one column to be updated. You could easily modify the program to generate an UPDATE statement with a variable number of assignments, but the list of host variables in the EXECUTE statement poses a problem; it must be replaced with a variable-length list. The SQLDA provides a way to specify such a variable-length parameter list. Figure 18-7 shows the layout of the SQLDA used by the IBM databases, including DB2, which set the de facto standard for dynamic SQL. Most other DBMS products also use this IBM SQLDA format or one very similar to it. The ANSI/ISO SQL standard provides a similar structure, called a SQL Descriptor Area. The types of information contained in the ANSI/ISO SQL Descriptor Area and the DB2-style SQLDA are the same, and both structures play the same role in dynamic SQL processing. However, the details of use how program locations are associated with SQL statement parameters, how information is placed into the descriptor area and retrieved from it, and so on are quite different. In practice, the DB2style SQLDA is the more important, because dynamic SQL support appeared in most major DBMS brands, modeled on the DB2 implementation, long before dynamic SQL was written into the SQL standard.
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