MySQL - WHERE Clause

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MySQL WHERE Clause

We know that the SQL SELECT command is used to fetch records from a MySQL table. In addition to that, we can also use a conditional clause called the WHERE Clause in conjunction with the SELECT statement to filter out the results. Using this WHERE clause, we can specify a selection criteria to select the required records from a table.

The WHERE clause works like an if condition in any programming language. This clause is used to compare the given value with the field value available in a MySQL table. If the given value from outside is equal to the available field value in the MySQL table, then it returns that row.

Operators Used in WHERE Clause

Here is the list of comparison operators, which can be used with the WHERE clause.

  • =: Checks if the values of the two operands are equal or not, if yes, then the condition becomes true.

  • !=: Checks if the values of the two operands are equal or not, if the values are not equal then the condition becomes true.

  • >: Checks if the value of the left operand is greater than the value of the right operand, if yes, then the condition becomes true.

  • <: Checks if the value of the left operand is less than the value of the right operand, if yes then the condition becomes true.

  • >=: Checks if the value of the left operand is greater than or equal to the value of the right operand, if yes, then the condition becomes true.

  • <=: Checks if the value of the left operand is less than or equal to the value of the right operand, if yes, then the condition becomes true.

Along with these, the WHERE clause can also contain logical operators, like AND, OR and NOT.

  • AND: If an AND operator is used in WHERE Clause with two conditions, the query will return true only if both the conditions are satisfied.

  • OR: If an OR operator is used in WHERE Clause with two conditions, the query will return true only if either of the conditions are satisfied.

  • NOT: If a NOT operator is used in WHERE Clause with a condition, the query will return true only if the table records does not satisfy the condition.

Fetching Data Using Where Clause

The WHERE clause is very useful when you want to fetch the selected rows from a table, especially when you use the MySQL Join. Joins are discussed in another chapter.

If the given condition does not match any record in the table, then the query would not return any row.

Syntax

Following is the generic SQL syntax of the SELECT command with the WHERE clause to fetch data from the MySQL table −

SELECT field1, field2,...fieldN table_name1, table_name2...
[WHERE condition1 [AND [OR]] condition2.....

  • You can use one or more tables separated by a comma to include various conditions using a WHERE clause, but the WHERE clause is an optional part of the SELECT command.

  • You can specify any condition using the WHERE clause.

  • You can specify more than one condition using the AND or the OR operators.

  • A WHERE clause can be used along with DELETE or UPDATE SQL command also to specify a condition.

Example

Firstly, let us create a table named CUSTOMERS using the following query −

CREATE TABLE CUSTOMERS (
   ID INT NOT NULL,
   NAME VARCHAR (20) NOT NULL,
   AGE INT NOT NULL,
   ADDRESS CHAR (25),
   SALARY DECIMAL (18, 2),       
   PRIMARY KEY (ID)
);

The following query inserts 7 records into the above-created table −

INSERT INTO CUSTOMERS (ID,NAME,AGE,ADDRESS,SALARY) VALUES 
(1, 'Ramesh', 32, 'Ahmedabad', 2000.00 ),
(2, 'Khilan', 25, 'Delhi', 1500.00 ),
(3, 'Kaushik', 23, 'Kota', 2000.00 ),
(4, 'Chaitali', 25, 'Mumbai', 6500.00 ),
(5, 'Hardik', 27, 'Bhopal', 8500.00 ),
(6, 'Komal', 22, 'Hyderabad', 4500.00 ),
(7, 'Muffy', 24, 'Indore', 10000.00 );

Execute the below query to fetch all the records of CUSTOMERS table −

Select * From CUSTOMERS;

Following is the CUSTOMERS table −

ID NAME AGE ADDRESS SALARY
1 Ramesh 32 Ahmedabad 2000.00
2 Khilan 25 Delhi 1500.00
3 Kaushik 23 Kota 2000.00
4 Chaitali 25 Mumbai 6500.00
5 Hardik 27 Bhopal 8500.00
6 Komal 22 Hyderabad 4500.00
7 Muffy 24 Indore 10000.00

Now, let us fetch the CUSTOMERS whose AGE is greater than 23 using the MySQL WHERE clause in conjunction with SELECT statement −

Select * From CUSTOMERS Where AGE > 23;

Output

Following are the records −

ID NAME AGE ADDRESS SALARY
1 Ramesh 32 Ahmedabad 2000.00
2 Khilan 25 Delhi 1500.00
4 Chaitali 25 Mumbai 6500.00
5 Hardik 27 Bhopal 8500.00
7 Muffy 24 Indore 10000.00

WHERE Clause Using a Client Program

Besides using MySQL Where clause to fetch the selected rows from a table, we can also use client programs like PHP, Node.js, Java, and Python to achieve the same result.

Syntax

Following are the syntaxes of this operation in various programming languages −

PHP NodeJS Java Python

To fetch selective records from a table through PHP program, we need to execute the SELECT statement using the mysqli function query() as −

$sql = "SELECT COLUMN_NAME1, COLUMN_NAME2,.. FROM TABLE_NAME WHERE CONDITION";
$mysqli->query($sql,$resultmode)

To fetch selective records from a table through Node.js program, we need to execute the SELECT statement using the query() function of the mysql2 library as −

sql= "SELECT field1, field2,...fieldN table_name1, table_name2...
[WHERE condition1 [AND [OR]] condition2....." ;
Con.query(sql);

To fetch selective records from a table through Java program, we need to execute the SELECT statement using the JDBC function executeUpdate() as −

String sql = "SELECT field1, field2,...fieldN table_name1, table_name2...
[WHERE condition1 [AND [OR]] condition2....";
statement.executeQuery(sql);

To fetch selective records from a table through Python program, we need to execute the SELECT statement using the execute() function of the MySQL Connector/Python as −

where_clause_query = SELECT column1, column2, ... 
   FROM table_name WHERE condition;
cursorObj.execute(where_clause_query)

Example

Following are the programs −

PHP NodeJS Java Python 
$dbhost = 'localhost';
$dbuser = 'root';
$dbpass = 'password';
$dbname = 'TUTORIALS';
$mysqli = new mysqli($dbhost, $dbuser, $dbpass, $dbname);

if($mysqli->connect_errno ) {
   printf("Connect failed: %s
", $mysqli->connect_error);
   exit();
}
//printf('Connected successfully.
');
$sql = "SELECT * FROM tutorials_tbl where tutorial_author = 'Sanjay'";
$result = $mysqli->query($sql);
if ($result->num_rows > 0) {
   while($row = $result->fetch_assoc()) {
      printf("Id: %s, Title: %s, Author: %s, Date: %d 
", 
         $row["tutorial_id"], 
         $row["tutorial_title"], 
         $row["tutorial_author"],
         $row["submission_date"]);               
   }
} else {
   printf('No record found.
');
}
mysqli_free_result($result);
$mysqli->close();

Output

The output obtained is as follows −

Id: 3, Title: JAVA Tutorial, Author: Sanjay, Date: 2007

$dbhost = 'localhost';
$dbuser = 'root';
$dbpass = 'password';
$dbname = 'TUTORIALS';
$mysqli = new mysqli($dbhost, $dbuser, $dbpass, $dbname);

if($mysqli->connect_errno ) {
   printf("Connect failed: %s
", $mysqli->connect_error);
   exit();
}
//printf('Connected successfully.
');
$sql = "SELECT * FROM tutorials_tbl where tutorial_author = 'Sanjay'";
$result = $mysqli->query($sql);
if ($result->num_rows > 0) {
   while($row = $result->fetch_assoc()) {
      printf("Id: %s, Title: %s, Author: %s, Date: %d 
", 
         $row["tutorial_id"], 
         $row["tutorial_title"], 
         $row["tutorial_author"],
         $row["submission_date"]);               
   }
} else {
   printf('No record found.
');
}
mysqli_free_result($result);
$mysqli->close();

Output

The output produced is as follows −

Connected!
--------------------------
[
  {
    tutorial_id: 3,
    tutorial_title: 'JAVA Tutorial',
    tutorial_author: 'Sanjay',
    submission_date: 2007-05-20T18:30:00.000Z
  }
]

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
public class WhereClause {
	public static void main(String[] args) {
		String url = "jdbc:mysql://localhost:3306/TUTORIALS";
		String user = "root";
		String password = "password";
		ResultSet rs;
		try {
			Class.forName("com.mysql.cj.jdbc.Driver");
            Connection con = DriverManager.getConnection(url, user, password);
            Statement st = con.createStatement();
            //System.out.println("Database connected successfully...!");
            String sql = "SELECT * from tutorials_tbl WHERE tutorial_author = 'Sanjay'";
            rs = st.executeQuery(sql);
            System.out.println("Table records: ");
            while(rs.next()){
            	String Id = rs.getString("tutorial_id");
            	String Title = rs.getString("tutorial_title");
            	String Author = rs.getString("tutorial_author");
            	String Date = rs.getString("submission_date");
            	System.out.println("Id: " + Id + ", Title: " + Title + ", Author: " + Author + ", Submission-date: " + Date);
            }
		}catch(Exception e) {
			e.printStackTrace();
		}
	}
}

Output

The output obtained is as shown below −

Table records: 
Id: 3, Title: Learning Java, Author: Sanjay, Submission-date: 2007-05-06

import mysql.connector
#establishing the connection
connection = mysql.connector.connect(
    host='localhost',
    user='root',
    password='password',
    database='tut'
)
cursorObj = connection.cursor()
where_clause_query = """SELECT tutorial_id, tutorial_title, tutorial_author, submission_date
FROM tutorials_tbl
WHERE tutorial_author = 'John Paul'
"""
cursorObj.execute(where_clause_query)
# Fetch all the matching rows
matching_rows = cursorObj.fetchall()
# Printing the matching rows
for row in matching_rows:
    print(row)
cursorObj.close()
connection.close()

Output

Following is the output of the above code −

(1, 'Learn PHP', 'John Paul', datetime.date(2023, 3, 28))
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