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      MySQL Tutorial

      This SQL tutorial is structured for beginners to guide them from the foundational concepts to advanced data manipulation and querying techniques in SQL. By the end of this tutorial, you will have developed a robust understanding of SQL, equipping you with the knowledge to efficiently manage and analyze data across various database systems. This guide sets the stage for your growth into a skilled data professional, ready to tackle complex data challenges and contribute to the field of data analysis and database management.

      Comments

      The MySQL Comments

      The MySQL Comment is a textual explanation added to a piece of code to provide additional information about the code. Comments are not meant to be executed as part of the code. It serve as notes for readers, including developers, to understand the purpose of the code, functionality, or any other relevant details.
      There are two types of comments in MySQL: Single-line comments and Multi-line comments

      The MySQL Single Line Comments

      Single-line comments are used for brief explanations on a single line. To create a single-line comment in MySQL, use two hyphens (--) followed by your comment text.

      Example

      In the following query, we are using a single line comment to write a text.
      SELECT * FROM customers; -- This is a comment

      The MySQL Multi-line Comments

      Multi-line comments in MySQL are used for longer explanations or to comment out multiple lines of code. These comments start with /* and end with */. Everything between them is considered a comment.

      Example

      The following example uses multi-line comment as an explanation of the query 
      /*
      This is a multi-line comment.
      You can use it to explain complex queries or comment out multiple lines of code.
      
      SELECT *
      FROM products
      WHERE price > 50;
      */

      Where to Place Comments

      You can place comments almost anywhere in your SQL code. Common places include
      • Before or after a SQL statement.
      • Within a SQL statement to explain a specific part of it.
      • At the beginning of a script or stored procedure to describe its purpose.
      -- This is a comment before a query
      SELECT * FROM orders;
      
      SELECT /* This is an inline comment */ customer_name
      FROM customers;
      
      /* This is a comment block at the beginning of a script */
      DELIMITER //
      CREATE PROCEDURE CalculateDiscount(IN product_id INT)
      BEGIN
          -- Calculate discount logic here
      END //
      DELIMITER ;

      Comments Using a Client Program

      We can also comment any value using the client program.

      Syntax

      PHPNodeJSJavaPython
      To comment any value or query through a PHP program, we need to execute the following comment methods using the mysqli function query() as follows 
      single line comment
      --
      multiline comment
      /**/
      
      (using Query)
      
      $sql = "SELECT ID /*NAME, ADDRESS*/ FROM CUSTOMERS WHERE ADDRESS = 'Mumbai'";
      $mysqli->query($sql);

      Example

      Following are the programs
      PHPNodeJSJavaPython
      $dbhost = 'localhost';
      $dbuser = 'root';
      $dbpass = 'password';
      $db = 'TUTORIALS';
      $mysqli = new mysqli($dbhost, $dbuser, $dbpass, $db);
      if ($mysqli->connect_errno) {
          printf("Connect failed: %s", $mysqli->connect_error);
          exit();
      }
      //printf('Connected successfully.');
      $sql = "SELECT ID /*NAME, ADDRESS*/ FROM CUSTOMERS WHERE ADDRESS = 'Mumbai'";
      if($mysqli->query($sql)){
          printf("Select query executed successfully...!\n");
      }
      printf("Table records: \n");
      if($result = $mysqli->query($sql)){
          while($row = mysqli_fetch_array($result)){
              printf("Id: %d",
              $row['ID']);
              printf("\n");
          }
      }
      if($mysqli->error){
          printf("Error message: ", $mysqli->error);
      }
      $mysqli->close();

      Output

      The output obtained is as shown below 
      Select query executed successfully...!
      Table records:
      Id: 4