Table of Contents
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 (see the section called “Getting results based on a cursor” here for how to alter this behaviour). Example 4.1, “Processing a Simple Query in JDBC” illustrates this process.
Example 4.1. 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 issues the same query as before but uses 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();
By default the driver collects all the results for the query at once. This can be inconvenient for large data sets so the JDBC driver provides a means of basing a ResultSet on a database cursor and only fetching a small number of rows.
A small number of rows are cached on the client side of the connection and when exhausted the next block of rows is retrieved by repositioning the cursor.
Example 4.2. Setting fetch size to turn cursors on and off.
Changing code to cursor mode is as simple as setting the fetch size of the Statement to the appropriate size. Setting the fetch size back to 0 will cause all rows to be cached (the default behaviour).
Statement st = db.createStatement();
// Turn use of the cursor on.
st.setFetchSize(50);
ResultSet rs = st.executeQuery("SELECT * FROM mytable");
while (rs.next()) {
System.out.print("a row was returned.");
}
rs.close();
// Turn the cursor off.
st.setFetchSize(0);
ResultSet rs = st.executeQuery("SELECT * FROM mytable");
while (rs.next()) {
System.out.print("many rows were returned.");
}
rs.close();
// Close the statement.
st.close();