In MySQL, comments are used to explain sections of SQL code, making the code easier to understand, or to temporarily disable part of the code (prevent execution). MySQL supports both single-line and multi-line comments.
Single Line Comments
- Syntax:
-- comment - Any text following
--on the same line will be treated as a comment and ignored by the MySQL engine.
Example 1: Basic comment
-- Select all records from the Customers table:
SELECT * FROM Customers;
Example 2: Comment part of the statement
SELECT * FROM Customers -- WHERE City='Berlin';
Example 3: Comment out a full statement
-- SELECT * FROM Customers;
SELECT * FROM Products;
Multi-line Comments
- Syntax:
/* comment */ - Anything between
/*and*/is treated as a comment, even if it spans multiple lines.
Example 1: Multi-line comment as explanation
/* Select all the columns
of all the records
in the Customers table: */
SELECT * FROM Customers;
Example 2: Comment multiple statements
/*SELECT * FROM Customers;
SELECT * FROM Products;
SELECT * FROM Orders;
SELECT * FROM Categories;*/
SELECT * FROM Suppliers;
Example 3: Comment part of a statement
SELECT CustomerName, /*City,*/ Country FROM Customers;
Example 4: Complex comment within a statement
SELECT * FROM Customers WHERE (CustomerName LIKE 'L%'
OR CustomerName LIKE 'R%' /*OR CustomerName LIKE 'S%'
OR CustomerName LIKE 'T%'*/ OR CustomerName LIKE 'W%')
AND Country='USA'
ORDER BY CustomerName;
Summary
- Single-line comments are useful for brief notes or disabling a single line of code.
- Multi-line comments are ideal for longer explanations or temporarily disabling large sections of code.