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();
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.
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.
