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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 5.1, “Processing a Simple Query in JDBC” 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 = conn.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 = conn.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.
Cursor based ResultSets cannot be used in all
situations. There a number of restrictions which will make the driver
silently fall back to fetching the whole ResultSet
at once.
The connection to the server must be using the V3 protocol. This is the default for (and is only supported by) server versions 7.4 and later.
The Connection must not be in autocommit
mode. The backend closes cursors at the end of transactions, so in
autocommit mode the backend will have closed the cursor before anything
can be fetched from it.
The Statement must be created with
a ResultSet type of
ResultSet.TYPE_FORWARD_ONLY.
This is the default, so no code will need to be rewritten to take
advantage of this, but it also means that you cannot scroll
backwards or otherwise jump around in the
ResultSet.
The query given must be a single statement, not multiple statements strung together with semicolons.
Example 5.2. 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).
// make sure autocommit is off
conn.setAutoCommit(false);
Statement st = conn.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);
rs = st.executeQuery("SELECT * FROM mytable");
while (rs.next()) {
System.out.print("many rows were returned.");
}
rs.close();
// Close the statement.
st.close();