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queries that applications can perform on the database. For most application programmers, a call to a simple, predefined function that checks an account balance, given a customer number, or one that adds an order, given a customer number, quantity, and product-id, is easier to understand than the corresponding SQL statements. I Business rules enforcement. The conditional processing capabilities of stored procedures are often used to place business rules into the database. For example, a stored procedure used to add an order to the database might contain logic to check the credit of the customer placing the order and check whether there is enough inventory on hand to fill the order, and reject the order if these conditions cannot be met. A large company could quite easily have several different ways in which orders are taken and entered into the corporate database one program for use by direct salespeople, one for people in the telesales department, another that accepts orders placed via the World Wide Web, and so on. Each of these would typically have its own order-acceptance program, usually written by different programmers at different times. But if all of the programs are forced to use the same stored procedure to add an order, the company can be assured that the business rules in that procedure are being uniformly enforced, no matter where the order originated.

In addition to scrollable result sets, later versions of the JDBC specification added support for updateable result sets. This capability corresponds to the UPDATE WHERE CURRENT OF capability in embedded SQL. It allows an update to specific columns of this row, which is indicated by the current position of a cursor. Updateable result sets also allow new rows of data to be inserted into a table via a result set.

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The JDBC interface provides objects and methods for retrieving metadata about databases, query results, and parameterized statements. A JDBC Connection object provides access to metadata about the database that it represents. Invoking its getMetaData() method returns a DatabaseMetaData object, described in Table 19-22. Each method listed in the table returns a result set containing information about a type of database entity: tables, columns, primary keys, and so on. The result set can be processed using the normal JDBC query results processing routines. Other metadata access methods provide information about the database product name supported on this connection, its version number, and similar information. Metadata information about query results can also be very useful. A ResultSet object provides a getMetaData method that can be invoked to obtain a description of its query results. The method returns a ResultSetMetaData object, described in Table 19-23. The methods let you determine how many columns are in the query results, and the name and data type of each column, identified by their ordinal position within the query results.

Different DBMS brands vary in the way they actually implement stored procedures. In several brands, the stored procedure text is stored within the database and is interpreted when the procedure is executed. This has the advantage of creating a very flexible stored procedure language, but it creates significant runtime overhead for complex stored procedures. The DBMS must read the statements that make up the stored procedure at runtime, parse and analyze them, and determine what to do on the fly. Because of the overhead in the interpreted approach, some DBMS brands compile stored procedures into an intermediate form that is much more efficient to execute. Compilation may be automatic when the stored procedure is created, or the DBMS may provide the ability for the user to request stored procedure compilation. The disadvantage of compiled stored procedures is that the exact technique used to carry out the stored procedure is fixed when the procedure is compiled. Suppose, for example, that a stored procedure is created and compiled soon after a database is first created, and later some useful indexes are defined on the data. The compiled queries in the stored procedure won t take advantage of these indexes, and as a result, they may run much more slowly than if they were recompiled. To deal with stale compiled procedures, some DBMS brands automatically mark any compiled procedures that may be affected by subsequent database changes as being in need of recompilation. The next time the procedure is called, the DBMS notices the mark and recompiles the procedure before executing it. Normally, this

Function getColumnCount() getColumnName() getColumnType()

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20:

TABLE 19-23

approach provides the best of both worlds the performance benefits of precompilation while keeping the compiled procedure up to date. Its disadvantage is that it can yield unpredictable stored procedure execution times. When no recompile is necessary, the stored procedure may execute quickly; when a recompile is activated, it may produce a significant delay; and in most cases, the recompile delay is much longer than the disadvantage of using the old compiled version. To determine the stored procedure compilation capabilities of a particular DBMS, you can examine its CREATE PROCEDURE and EXECUTE PROCEDURE statement options or look for other procedure management statements such as ALTER PROCEDURE.

Description Returns number of query results columns Retrieves name of specified results column Retrieves data type of specified results column

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