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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:

Navigational methods: Used to move the cursor around.
Get methods: Used to view the data in the columns of the current row being pointed by the cursor.
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 generates the ResultSet is created.

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

createStatement(int RSType, int RSConcurrency);
prepareStatement(String SQL, int RSType, int RSConcurrency);
prepareCall(String sql, int RSType, int RSConcurrency);

The first argument indicates 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 forward and backward, 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 forward and backward, and the result set is sensitive to changes made by others to the database that occur after the result set was created.

Concurrency of Result Set

The possible RSConcurrency are given below. If you do not specify any Concurrency type, you will automatically get one that is CONCUR_READ_ONLY.
Creates a read-only result set. This is the defaultCreates an updateable result set.

Concurrency	Description
ResultSet.CONCUR_READ_ONLY
ResultSet.CONCUR_UPDATABLE

All our 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 just before the first row.
2	public void afterLast() throws SQLException Moves the cursor 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 backward, from where it is currently 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.

Navigate - Example Code

Following is the example, which makes use of few navigation methods described in the Result Set tutorial.

This sample code has been written based on the environment and database setup done in the previous chapters.

Copy and past the following example in JDBCExample.java, compile and run as follows:

//STEP 1. Import required packages
import java.sql.*;
public class JDBCExample {
// JDBC driver name and database URL
static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";
static final String DB_URL = "jdbc:mysql://localhost/EMP";
// Database credentials
static final String USER = "username";
static final String PASS = "password";
public static void main(String[] args) {
Connection conn = null;
Statement stmt = null;
try{
//STEP 2: Register JDBC driver
Class.forName("com.mysql.jdbc.Driver");
//STEP 3: Open a connection
System.out.println("Connecting to database...");
conn = DriverManager.getConnection(DB_URL,USER,PASS);
//STEP 4: Execute a query to create statment with
// required arguments for RS example.
System.out.println("Creating statement...");
stmt = conn.createStatement(
ResultSet.TYPE_SCROLL_INSENSITIVE,
ResultSet.CONCUR_READ_ONLY);
String sql;
sql = "SELECT id, first, last, age FROM Employees";
ResultSet rs = stmt.executeQuery(sql);
// Move cursor to the last row.
System.out.println("Moving cursor to the last...");
rs.last();
//STEP 5: Extract data from result set
System.out.println("Displaying record...");
//Retrieve by column name
int id = rs.getInt("id");
int age = rs.getInt("age");
String first = rs.getString("first");
String last = rs.getString("last");
//Display values
System.out.print("ID: " + id);
System.out.print(", Age: " + age);
System.out.print(", First: " + first);
System.out.println(", Last: " + last);
// Move cursor to the first row.
System.out.println("Moving cursor to the first row...");
rs.first();
//STEP 6: Extract data from result set
System.out.println("Displaying record...");
//Retrieve by column name
id = rs.getInt("id");
age = rs.getInt("age");
first = rs.getString("first");
last = rs.getString("last");
//Display values
System.out.print("ID: " + id);
System.out.print(", Age: " + age);
System.out.print(", First: " + first);
System.out.println(", Last: " + last);
// Move cursor to the first row.
System.out.println("Moving cursor to the next row...");
rs.next();
//STEP 7: Extract data from result set
System.out.println("Displaying record...");
id = rs.getInt("id");
age = rs.getInt("age");
first = rs.getString("first");
last = rs.getString("last");
//Display values
System.out.print("ID: " + id);
System.out.print(", Age: " + age);
System.out.print(", First: " + first);
System.out.println(", Last: " + last);
//STEP 8: Clean-up environment
rs.close();
stmt.close();
conn.close();
}catch(SQLException se){
//Handle errors for JDBC
se.printStackTrace();
}catch(Exception e){
//Handle errors for Class.forName
e.printStackTrace();
}finally{
//finally block used to close resources
try{
if(stmt!=null)
stmt.close();
}catch(SQLException se2){
}// nothing we can do
try{
if(conn!=null)
conn.close();
}catch(SQLException se){
se.printStackTrace();
}//end finally try
}//end try
System.out.println("Goodbye!");
}//end main
}//end JDBCExample

Now let us compile the above example as follows:

C:\>javac JDBCExample.java
C:\>

When you run JDBCExample, it produces the following result:

C:\>java JDBCExample
Connecting to database...
Creating statement...
Moving cursor to the last...
Displaying record...
ID: 103, Age: 30, First: Sumit, Last: Mittal
Moving cursor to the first row...
Displaying record...
ID: 100, Age: 18, First: Zara, Last: Ali
Moving cursor to the next row...
Displaying record...
ID: 101, Age: 25, First: Mahnaz, Last: Fatma
Goodbye!
C:\>

Viewing a Result

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:

One that takes in a column name.
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.

For a better understanding, let us study the Viewing - Example Code as discussed below.

Viewing - Example Code

Following is the example, which makes use of few getInt and getString methods described in the Result Set chapter. This example is very similar to previous example explained in the Navigation Result Set Section.

This sample code has been written based on the environment and the database setup done in the previous chapters.

Copy and past the following example in JDBCExample.java, compile and run as follows:

Now let us compile the above example as follows:

C:\>javac JDBCExample.java
C:\>

When you run JDBCExample, it produces the following result:

Updating a Result

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:

One that takes in a column name.
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.

For a better understanding, let us study the Updating - Example Code as discussed below.

Updating - Example Code

Following is the example, which makes use of the ResultSet.CONCUR_UPDATABLE and ResultSet.TYPE_SCROLL_INSENSITIVE described in the Result Set tutorial. This example would explain INSERT, UPDATE and DELETE operation on a table.

It should be noted that tables you are working on should have Primary Key set properly.

This sample code has been written based on the environment and database setup done in the previous chapters.

Copy and past the following example in JDBCExample.java, compile and run as follows:

//STEP 1. Import required packages
import java.sql.*;
public class JDBCExample {
// JDBC driver name and database URL
static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";
static final String DB_URL = "jdbc:mysql://localhost/EMP";
// Database credentials
static final String USER = "username";
static final String PASS = "password";
public static void main(String[] args) {
Connection conn = null;
try{
//STEP 2: Register JDBC driver
Class.forName("com.mysql.jdbc.Driver");
//STEP 3: Open a connection
System.out.println("Connecting to database...");
conn = DriverManager.getConnection(DB_URL,USER,PASS);
//STEP 4: Execute a query to create statment with
// required arguments for RS example.
System.out.println("Creating statement...");
Statement stmt = conn.createStatement(
ResultSet.TYPE_SCROLL_INSENSITIVE,
ResultSet.CONCUR_UPDATABLE);
//STEP 5: Execute a query
String sql = "SELECT id, first, last, age FROM Employees";
ResultSet rs = stmt.executeQuery(sql);
System.out.println("List result set for reference....");
printRs(rs);
//STEP 6: Loop through result set and add 5 in age
//Move to BFR postion so while-loop works properly
rs.beforeFirst();
//STEP 7: Extract data from result set
while(rs.next()){
//Retrieve by column name
int newAge = rs.getInt("age") + 5;
rs.updateDouble( "age", newAge );
rs.updateRow();
}
System.out.println("List result set showing new ages...");
printRs(rs);
// Insert a record into the table.
//Move to insert row and add column data with updateXXX()
System.out.println("Inserting a new record...");
rs.moveToInsertRow();
rs.updateInt("id",104);
rs.updateString("first","John");
rs.updateString("last","Paul");
rs.updateInt("age",40);
//Commit row
rs.insertRow();
System.out.println("List result set showing new set...");
printRs(rs);
// Delete second record from the table.
// Set position to second record first
rs.absolute( 2 );
System.out.println("List the record before deleting...");
//Retrieve by column name
int id = rs.getInt("id");
int age = rs.getInt("age");
String first = rs.getString("first");
String last = rs.getString("last");
//Display values
System.out.print("ID: " + id);
System.out.print(", Age: " + age);
System.out.print(", First: " + first);
System.out.println(", Last: " + last);
//Delete row
rs.deleteRow();
System.out.println("List result set after \
deleting one records...");
printRs(rs);
//STEP 8: Clean-up environment
rs.close();
stmt.close();
conn.close();
}catch(SQLException se){
//Handle errors for JDBC
se.printStackTrace();
}catch(Exception e){
//Handle errors for Class.forName
e.printStackTrace();
}finally{
//finally block used to close resources
try{
if(conn!=null)
conn.close();
}catch(SQLException se){
se.printStackTrace();
}//end finally try
}//end try
System.out.println("Goodbye!");
}//end main
public static void printRs(ResultSet rs) throws SQLException{
//Ensure we start with first row
rs.beforeFirst();
while(rs.next()){
//Retrieve by column name
int id = rs.getInt("id");
int age = rs.getInt("age");
String first = rs.getString("first");
String last = rs.getString("last");
//Display values
System.out.print("ID: " + id);
System.out.print(", Age: " + age);
System.out.print(", First: " + first);
System.out.println(", Last: " + last);
}
System.out.println();
}//end printRs()
}//end JDBCExample

Now let us compile the above example as follows:

C:\>javac JDBCExample.java C:\>

When you run JDBCExample, it produces the following result:

C:\>java JDBCExample
Connecting to database...
Creating statement...
List result set for reference.
... ID: 100, Age: 33, First: Zara, Last: Ali
ID: 101, Age: 40, First: Mahnaz, Last: Fatma
ID: 102, Age: 50, First: Zaid, Last: Khan
ID: 103, Age: 45, First: Sumit, Last: Mittal
List result set showing new ages...
ID: 100, Age: 38, First: Zara, Last: Ali
ID: 101, Age: 45, First: Mahnaz, Last: Fatma
ID: 102, Age: 55, First: Zaid, Last: Khan
ID: 103, Age: 50, First: Sumit, Last: Mittal
Inserting a new record...
List result set showing new set...
ID: 100, Age: 38, First: Zara, Last: Ali
ID: 101, Age: 45, First: Mahnaz, Last: Fatma
ID: 102, Age: 55, First: Zaid, Last: Khan
ID: 103, Age: 50, First: Sumit, Last: Mittal
ID: 104, Age: 40, First: John, Last: Paul

List the record before deleting...
ID: 101, Age: 45, First: Mahnaz, Last: Fatma
List result set after deleting one records...
ID: 100, Age: 38, First: Zara, Last: Ali
ID: 102, Age: 55, First: Zaid, Last: Khan
ID: 103, Age: 50, First: Sumit, Last: Mittal
ID: 104, Age: 40, First: John, Last: Paul
Goodbye!
C:\>

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