Chapter 3. Statement

Index:

  1. Creating a Statement Instance
  2. Issuing a Query
  3. Performing Updates
  4. Creating and Modifying Database Objects

Creating a Statement Instance

Once a Connection is established, it can be used to create Statements and PreparedStatements. Any time you want to issue SQL statements to the database, you require a Statement or PreparedStatement instance. To get a Statement object, you call the createStatement() method on the Connection object you have retrieved via the DriverManager.getConnection() method. Once you have a Statement object, you can execute a SELECT query by calling the executeQuery(String SQL) method with the SQL you want to use. To update data in the database use the executeUpdate(String SQL) method. This method returns the number of rows affected by the update statement. If you don't know ahead of time whether the SQL statement will be a SELECT or an UPDATE/INSERT, then you can use the execute(String SQL) method. This method will return true if the SQL query was a SELECT, or false if an UPDATE/INSERT/DELETE query. If the query was a SELECT query, you can retrieve the results by calling the getResultSet() method. If the query was an UPDATE/INSERT/DELETE query, you can retrieve the affected rows count by calling getUpdateCount() on the Statement instance. This is explained in the following sections.

Issuing a Query

A simple sample can illustrates more than some words:
            String sql = "select * from test where int1>0";

            Connection con = DriverManager.getConnection(url, "", "");

            Statement stmt = con.createStatement();
			
            ResultSet rs = stmt.executeQuery(sql);

            ResultSetMetaData resultSetMetaData = rs.getMetaData();
            int iNumCols = resultSetMetaData.getColumnCount();
            for (int i = 1; i <= iNumCols; i++) {
                System.out.println(resultSetMetaData.getColumnLabel(i)
                                   + "  " +
                                   resultSetMetaData.getColumnTypeName(i));
            }

            Object colval;
            while (rs.next()) {
                for (int i = 1; i <= iNumCols; i++) {
                    colval = rs.getObject(i);
                    System.out.print(colval + "  ");
                }
                System.out.println();
            }

            rs.close();
            stmt.close();
            con.close();
This example issues the same query as before but uses a PreparedStatement and a bind value in the query.
            String sql = "select * from test where int1>?";

            Connection con = DriverManager.getConnection(url, "", "");

            PreparedStatement stmt = con.prepareStatement(sql);
			
            stmt.setInt(1, 0);

            ResultSet rs = stmt.executeQuery();

            ResultSetMetaData resultSetMetaData = rs.getMetaData();
            int iNumCols = resultSetMetaData.getColumnCount();
            for (int i = 1; i <= iNumCols; i++) {
                System.out.println(resultSetMetaData.getColumnLabel(i)
                                   + "  " +
                                   resultSetMetaData.getColumnTypeName(i));
            }

            Object colval;
            while (rs.next()) {
                for (int i = 1; i <= iNumCols; i++) {
                    colval = rs.getObject(i);
                    System.out.print(colval + "  ");
                }
                System.out.println();
            }

            rs.close();
            stmt.close();
            con.close();

You can use a single Statement instance as many times as you want. You could create one as soon as you open the connection and use it for the connection's lifetime. But you have to remember that only one ResultSet can exist per Statement or PreparedStatement at a given time. When you are done using the Statement or PreparedStatement, you should close it.

Before reading any values from ResultSet, you have to call next(). This returns true if there is a result, but more importantly, it prepares the row for processing. Under the JDBC specification, you should access a column only once. It is safest to stick to this rule, although the HXTT Cobol driver will allow you to access a column as many times as you want. You should close a ResultSet by calling close() once you have finished using it too. Once you make another query with the Statement used to create a ResultSet, the currently open ResultSet instance is closed automatically. The HXTT Cobol driver supports updatable result sets, but an updatable query can only span one table (i.e. no joins).

 

Performing Updates

To change data (perform an INSERT, UPDATE, or DELETE), you should use the executeUpdate() method. This method is similar to the method executeQuery() used to issue a SELECT statement, but it doesn't return a ResultSet; instead it returns the number of rows affected by the INSERT, UPDATE, or DELETE statement. This example will issue a simple UPDATE statement and print out the number of rows updated.

            String sql="update test set boolean1=not boolean1 where recno()%5=0";

            Connection con = DriverManager.getConnection(url, "", "");

            Statement stmt=con.createStatement();

            int updateCount=stmt.executeUpdate(sql);
            System.out.println(sql+" : "+updateCount);

            stmt.close();
            con.close();

Creating and Modifying Database Objects

To create, modify or drop a database object like a table, index, or view, you should use the execute() method. This method is similar to the method executeUpdate(), but it doesn't return a result. This example will drop a table.

            String sql="drop table test";

            Connection con = DriverManager.getConnection(url, "", "");

            Statement stmt=con.createStatement();

            stmt.execute(sql);
			
            stmt.close();
            con.close();

The HXTT Cobol driver can create, modify or drop a database object like a table, index, or view through executeUpdate(), but the returned result is unexpectable. For instance, dropping a table can return 1(only one table file), 2(two table files, or one table files and one index file), 3, 4, and so on. The returned result of executeUpdate() is valuable when it creates something with IF NOT EXISTS clause, or drops something with IF EXISTS clause. This example will drop a test table if that table exists.

            String sql="drop table if exists test";

            Connection con = DriverManager.getConnection(url, "", "");

            Statement stmt=con.createStatement();

            boolean droppedFlag=stmt.executeUpdate(sql)!=0;
			
            stmt.close();
            con.close();
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