The WHERE clause is used in the SELECT, DELETE or, UPDATE statements to specify the rows on which the operation needs to be carried out. Usually, this clause is followed by a condition or expression which returns a Boolean value, the Select, delete or, update operations are performed only on the rows which satisfy the given condition.
ij> SELECT * from table_name WHERE condition; or, ij> DELETE from table_name WHERE condition; or, ij> UPDATE table_name SET column_name = value WHERE condition;
The WHERE clause can use the comparison operators such as =,!=, <, >, <=, and >=, as well as the BETWEEN and LIKE operators.
Example
Let us assume we have a table named Employees in the database with 7 records as shown below −
ID |NAME |SALARY |LOCATION ----------------------------------------------------------------------------- 1 |Amit |30000 |Hyderabad 2 |Kalyan |40000 |Vishakhapatnam 3 |Renuka |50000 |Delhi 4 |Archana |15000 |Mumbai 5 |Trupthi |45000 |Kochin 6 |Suchatra|33000 |Pune 7 |Rahul |39000 |Lucknow
The following SQL DELETE statement fetches the records of the employees whose salary is more than 35000 −
ij> SELECT * FROM Employees WHERE Salary>35000;
This will produce the following output −
ID |NAME |SALARY |LOCATION --------------------------------------------------- 2 |Kalyan |40000 |Vishakhapatnam 3 |Renuka |50000 |Delhi 5 |Trupthi|45000 |Kochin 7 |Rahul |39000 |Lucknow 4 rows selected
Similarly, you can also delete and update records using this clause.
Following example updates the location of those whose salary is less than 30000.
ij> UPDATE Employees SET Location = 'Vijayawada' WHERE Salary<35000; 3 rows inserted/updated/deleted
If you verify the contents of the table, you can see the updated table as shown below −
ij> SELECT * FROM Employees; ID |NAME |SALARY |LOCATION ------------------------------------------------------------------------------ 1 |Amit |30000 |Vijayawada 2 |Kalyan |40000 |Vishakhapatnam 3 |Renuka |50000 |Delhi 4 |Archana |15000 |Vijayawada 5 |Trupthi |45000 |Kochin 6 |Suchatra|33000 |Vijayawada 7 |Rahul |39000 |Lucknow 7 rows selected
Where clause JDBC example
This section teaches you how to use WHERE clause and perform CURD operations on a table in Apache Derby database using JDBC application.
If you want to request the Derby network server using network client, make sure that the server is up and running. The class name for the Network client driver is org.apache.derby.jdbc.ClientDriver and the URL is jdbc:derby://localhost:1527/DATABASE_NAME;create=true;user=USER_NAME; password=PASSWORD“.
Follow the steps given below to use WHERE clause and perform CURD operations on a table in Apache Derby
Step 1: Register the driver
To communicate with the database, first of all, you need to register the driver. The forName() method of the class Class accepts a String value representing a class name loads it in to the memory, which automatically registers it. Register the driver using this method
Step 2: Get the connection
In general, the first step we do to communicate to the database is to connect with it. The Connection class represents the physical connection with a database server. You can create a connection object by invoking the getConnection() method of the DriverManager class. Create a connection using this method.
Step 3: Create a statement object
You need to create a Statement or PreparedStatement or, CallableStatement objects to send SQL statements to the database. You can create these using the methods createStatement(), prepareStatement() and, prepareCall() respectively. Create either of these objects using the appropriate method.
Step 4: Execute the query
After creating a statement, you need to execute it. The Statement class provides various methods to execute a query like the execute() method to execute a statement that returns more than one result set. The executeUpdate() method executes queries like INSERT, UPDATE, DELETE. The executeQuery() method results that returns data. Use either of these methods and execute the statement created previously.
Example
Following JDBC example demonstrates how to use WHERE clause and perform CURD operations on a table in Apache Derby using JDBC program. Here, we are connecting to a database named sampleDB (will create if it does not exist) using the embedded driver.
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.Statement;
import java.sql.ResultSet;
public class WhereClauseExample {
public static void main(String args[]) throws Exception {
//Registering the driver
Class.forName("org.apache.derby.jdbc.EmbeddedDriver");
//Getting the Connection object
String URL = "jdbc:derby:sampleDB;create=true";
Connection conn = DriverManager.getConnection(URL);
//Creating the Statement object
Statement stmt = conn.createStatement();
//Creating a table and populating it
String query = "CREATE TABLE Employees("
+ "Id INT NOT NULL GENERATED ALWAYS AS IDENTITY, "
+ "Name VARCHAR(255), Salary INT NOT NULL, "
+ "Location VARCHAR(255), "
+ "PRIMARY KEY (Id))";
String query = "INSERT INTO Employees("
+ "Name, Salary, Location) VALUES "
+ "('Amit', 30000, 'Hyderabad'), "
+ "('Kalyan', 40000, 'Vishakhapatnam'), "
+ "('Renuka', 50000, 'Delhi'), "
+ "('Archana', 15000, 'Mumbai'), "
+ "('Trupthi', 45000, 'Kochin'), "
+ "('Suchatra', 33000, 'Pune'), "
+ "('Rahul', 39000, 'Lucknow'), "
+ "('Trupti', 45000, 'Kochin')";
//Executing the query
String query = "SELECT * FROM Employees WHERE Salary>35000";
ResultSet rs = stmt.executeQuery(query);
while(rs.next()) {
System.out.println("Id: "+rs.getString("Id"));
System.out.println("Name: "+rs.getString("Name"));
System.out.println("Salary: "+rs.getString("Salary"));
System.out.println("Location: "+rs.getString("Location"));
System.out.println(" ");
}
}
}
Output
On executing the above program, you will get the following output −
Id: 2 Name: Kalyan Salary: 43000 Location: Chennai Id: 3 Name: Renuka Salary: 50000 Location: Delhi Id: 5 Name: Trupthi Salary: 45000 Location: Kochin Id: 7 Name: Rahul Salary: 39000 Location: Lucknow