In MySQL, comments are used to add explanatory notes or remarks within SQL statements. They provide additional information about the code and are ignored by the MySQL engine when executing the query. There are two types of comments in MySQL:

1. Single-line comments: Single-line comments start with "--" or "#" and continue until the end of the line. They are useful for adding comments on a single line.

sql
-- This is a single-line comment
SELECT * FROM table; -- Another single-line comment

2. Multi-line comments: Multi-line comments start with "/*" and end with "*/". They can span multiple lines and are useful for longer comments or comment blocks.

sql
/* This is a multi-line comment
spanning multiple lines */
SELECT * FROM table;

Comments can be placed anywhere within the SQL statement, except within string literals or other quoted expressions. They can be used to explain the purpose of the query, provide clarification about specific parts of the code, or temporarily disable parts of the code during development.

Here's an example that demonstrates the usage of comments:

sql
SELECT * FROM customers; -- Retrieve all customers

/* The following query calculates the average order value
for each customer */
SELECT customer_id, AVG(order_value) AS avg_order_value
FROM orders
GROUP BY customer_id;

In this example, single-line and multi-line comments are used to provide explanations and describe the purpose of each query.

Comments are a helpful practice in SQL programming to improve code readability, documentation, and collaboration among developers. They allow you to annotate your SQL statements and make it easier to understand and maintain the codebase.