Key Points: Comments in SQL

Importance of Comments:

• Comments provide valuable feedback to video creators. Similarly, in programming, comments serve as messages to help others understand the code.
• Good documentation through comments helps avoid errors and improves productivity, especially in cross-functional team environments.

Purpose of Comments:

• Describe or explain sections of SQL statements.
• Facilitate good documentation for knowledge preservation, especially with new employees or multidisciplinary teams.

Comments in SQL:

• Comments in SQL are ignored during execution and are used solely for documentation and explanation.
• Each database software has its own style for implementing comments and not all may support them (e.g., Microsoft Access).

Types of Comments:

1. Single-Line Comments:

   • Use double dashes (--).
   • Example: -- This is a single-line comment
   • Ignored by SQL engine, facilitating clear documentation alongside code.

2. Multi-Line Comments:

   • Start with /* and end with */.
   • Example:

     /*
     This is a
     multi-line comment
     */
     

   • Useful for describing paragraphs or multiple lines of comments.

Examples:

• Example 1: Single-Line Comment on Separate Line

  • -- Select all columns from teachers
    SELECT * FROM teachers;
    

• Example 2: Single-Line Comment on Same Line

  • SELECT * FROM teachers; -- where city is 'Kolkata'
    

• Example 3: Single-Line Comment with Ignored SQL

  • -- SELECT * FROM customers;
    SELECT * FROM teachers;
    

• Example 4: Multi-Line Comment

  • /*
    Multi-line comment
    explaining the query
    */
    SELECT * FROM customers;
    

• Example 5: Multi-Line Comment Ignoring Multiple Queries
  • All queries inside the comment are ignored, executing only the query outside.
• Example 6: Using Multi-Line Comment for a Word
  • Useful to ignore specific parts of a line rather than the entire line.

Homework Question:

• A logical exercise is provided where the viewers are asked to determine which part of the SQL query is executed by the SQL engine by identifying and ignoring comments.

This structured exercise involves real-life SQL queries, demonstrating the effective use of comments for maintaining clarity and functionality within codes.