JDBC - Result Sets

The SQL statements that read data from a database query return the data in a result set. The SELECT statement is the standard way to select rows from a database and view them in a result set. The java.sql.ResultSet interface represents the result set of a database query.

A ResultSet object maintains a cursor that points to the current row in the result set. The term "result set" refers to the row and column data contained in a ResultSet object.

The methods of the ResultSet interface can be broken down into three categories:

1. Navigational methods: used to move the cursor around.
2. Get methods: used to view the data in the columns of the current row being pointed to by the cursor.
3. Update methods: used to update the data in the columns of the current row. The updates can then be updated in the underlying database as well.

The cursor is movable based on the properties of the ResultSet. These properties are designated when the corresponding Statement that generated the ResultSet is created.

JDBC provides following connection methods to create statements with desired ResultSet:

1. createStatement(int RSType, int RSConcurrency);
2. prepareStatement(String SQL, int RSType, int RSConcurrency);
3. prepareCall(String sql, int RSType, int RSConcurrency);

The first argument indicate the type of a ResultSet object and the second argument is one of two ResultSet constants for specifying whether a result set is read-only or updatable.

Type of ResultSet:

The possible RSType are given below, If you do not specify any ResultSet type, you will automatically get one that is TYPE_FORWARD_ONLY.

Type	Description
ResultSet.TYPE_FORWARD_ONLY	The cursor can only move forward in the result set.
ResultSet.TYPE_SCROLL_INSENSITIVE	The cursor can scroll forwards and backwards, and the result set is not sensitive to changes made by others to the database that occur after the result set was created.
ResultSet.TYPE_SCROLL_SENSITIVE.	The cursor can scroll forwards and backwards, and the result set is sensitive to changes made by others to the database that occur after the result set was created.

Concurrency of ResultSet:

The possible RSConcurrency are given below, If you do not specify any Concurrency type, you will automatically get one that is CONCUR_READ_ONLY.

Concurrency	Description
ResultSet.CONCUR_READ_ONLY	Creates a read-only result set. This is the default
ResultSet.CONCUR_UPDATABLE	Creates an updateable result set.

Our all the examples written so far can be written as follows which initializes a Statement object to create a forward-only, read only ResultSet object:

try { Statement stmt = conn.createStatement( ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY); } catch(Exception ex) { .... } finally { .... }

Navigating a Result Set:

There are several methods in the ResultSet interface that involve moving the cursor, including:

S.N.	Methods & Description
1	public void beforeFirst() throws SQLException Moves the cursor to just before the first row
2	public void afterLast() throws SQLException Moves the cursor to just after the last row
3	public boolean first() throws SQLException Moves the cursor to the first row
4	public void last() throws SQLException Moves the cursor to the last row.
5	public boolean absolute(int row) throws SQLException Moves the cursor to the specified row
6	public boolean relative(int row) throws SQLException Moves the cursor the given number of rows forward or backwards from where it currently is pointing.
7	public boolean previous() throws SQLException Moves the cursor to the previous row. This method returns false if the previous row is off the result set
8	public boolean next() throws SQLException Moves the cursor to the next row. This method returns false if there are no more rows in the result set
9	public int getRow() throws SQLException Returns the row number that the cursor is pointing to.
10	public void moveToInsertRow() throws SQLException Moves the cursor to a special row in the result set that can be used to insert a new row into the database. The current cursor location is remembered.
11	public void moveToCurrentRow() throws SQLException Moves the cursor back to the current row if the cursor is currently at the insert row; otherwise, this method does nothing

Viewing a Result Set:

The ResultSet interface contains dozens of methods for getting the data of the current row.

There is a get method for each of the possible data types, and each get method has two versions:

1. One that takes in a column name.
2. One that takes in a column index.

For example, if the column you are interested in viewing contains an int, you need to use one of the getInt() methods of ResultSet:

S.N.	Methods & Description
1	public int getInt(String columnName) throws SQLException Returns the int in the current row in the column named columnName
2	public int getInt(int columnIndex) throws SQLException Returns the int in the current row in the specified column index. The column index starts at 1, meaning the first column of a row is 1, the second column of a row is 2, and so on.

Similarly there are get methods in the ResultSet interface for each of the eight Java primitive types, as well as common types such as java.lang.String, java.lang.Object, and java.net.URL

There are also methods for getting SQL data types java.sql.Date, java.sql.Time, java.sql.TimeStamp, java.sql.Clob, and java.sql.Blob. Check the documentation for more information about using these SQL data types.

Updating a Result Set:

The ResultSet interface contains a collection of update methods for updating the data of a result set.

As with the get methods, there are two update methods for each data type:

1. One that takes in a column name.
2. One that takes in a column index.

For example, to update a String column of the current row of a result set, you would use one of the following updateString() methods:

S.N.	Methods & Description
1	public void updateString(int columnIndex, String s) throws SQLException Changes the String in the specified column to the value of s.
2	public void updateString(String columnName, String s) throws SQLException Similar to the previous method, except that the column is specified by its name instead of its index.

There are update methods for the eight primitive data types, as well as String, Object, URL, and the SQL data types in the java.sql package.

Updating a row in the result set changes the columns of the current row in the ResultSet object, but not in the underlying database. To update your changes to the row in the database, you need to invoke one of the following methods.

S.N.	Methods & Description
1	public void updateRow() Updates the current row by updating the corresponding row in the database.
2	public void deleteRow() Deletes the current row from the database
3	public void refreshRow() Refreshes the data in the result set to reflect any recent changes in the database.
4	public void cancelRowUpdates() Cancels any updates made on the current row.
5	public void insertRow() Inserts a row into the database. This method can only be invoked when the cursor is pointing to the insert row.