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();
Statement or PreparedStatement InterfaceThe 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.
ResultSet InterfaceThe 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.