How can I add comments in MySQL?

There are two primary ways to add comments in MySQL: single-line comments and multi-line comments.

1. Single-line Comments

In MySQL, single-line comments can be added in two ways:

Using -- Symbol

The comment must be followed by a space character. For example:

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

Using # Symbol

This is another way to add single-line comments, and compared to double hyphens, using # is more concise. For example:

sql
# This is a single-line comment
SELECT * FROM users;

2. Multi-line Comments

Multi-line comments are used to comment out a block of code, which is very useful for explaining complex SQL statements or temporarily disabling parts of the code. Multi-line comments start with /* and end with */. For example:

sql
/*
This is a multi-line comment
It can span multiple lines
*/
SELECT * FROM users;

Application Example

Suppose we are writing a complex query involving joining multiple tables and using multiple conditions; we might use comments to explain the purpose of each part:

sql
SELECT
    users.name,
    orders.order_date,
    products.name
FROM
    users
-- Joining users and orders tables
INNER JOIN orders ON users.id = orders.user_id
-- Joining orders and products tables
INNER JOIN products ON orders.product_id = products.id
WHERE
    orders.order_date > '2021-01-01'  -- Selecting orders after 2021
ORDER BY
    orders.order_date;

The above covers the methods for adding comments in MySQL along with practical examples. Proper use of comments can significantly improve the readability and maintainability of SQL code.