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5.3. Issuing a Query and Processing the Result

Any time you want to issue SQL statements to the database, you require a Statement or PreparedStatement instance. Once you have a Statement or PreparedStatement, you can use issue a query. This will return a ResultSet instance, which contains the entire result. Example 5-1 illustrates this process.

Example 5-1. Processing a Simple Query in JDBC

This example will issue a simple query and print out the first column of each row using a Statement.

Statement st = db.createStatement();
ResultSet rs = st.executeQuery("SELECT * FROM mytable where columnfoo = 500");
while(rs.next()) {
    System.out.print("Column 1 returned ");
    System.out.println(rs.getString(1));
}
rs.close();
st.close();

This example will issue the same query as before using a PreparedStatement and a bind value in the query.

int foovalue = 500;
PreparedStatement st = db.prepareStatement("SELECT * FROM mytable where columnfoo = ?");
st.setInt(1, foovalue);
ResultSet rs = st.executeQuery();
while(rs.next()) {
    System.out.print("Column 1 returned ");
    System.out.println(rs.getString(1));
}
rs.close();
st.close();

5.3.1. Using the Statement or PreparedStatement Interface

The following must be considered when using the Statement or PreparedStatement interface:

  • 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.

  • If you need to perform a query while processing a ResultSet, you can simply create and use another Statement.

  • If you are using threads, and several are using the database, you must use a separate Statement for each thread. Refer to Section 5.8 if you are thinking of using threads, as it covers some important points.

  • When you are done using the Statement or PreparedStatement you should close it.

5.3.2. Using the ResultSet Interface

The following must be considered when using the ResultSet interface:

  • Before reading any values, you must 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 field only once. It is safest to stick to this rule, although at the current time, the PostgreSQL driver will allow you to access a field as many times as you want.

  • You must close a ResultSet by calling close() once you have finished using it.

  • Once you make another query with the Statement used to create a ResultSet, the currently open ResultSet instance is closed automatically.

  • ResultSet is currently read only. You can not update data through the ResultSet. If you want to update data you need to do it the old fashioned way by issuing a SQL update statement. This is in conformance with the JDBC specification which does not require drivers to provide this functionality.

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