Add lesson about comments

src/data/courses/sql/chapters/sql-basics/lessons/comments.md

@@ -0,0 +1,56 @@
+---
+title: Comments
+description: Learn how to add comments to your SQL queries.
+order: 180
+type: lesson-challenge
+---
+
+Comments are a way to add notes to your SQL queries. They are not executed by the database and are only used to explain what the query is doing.
+
+## Single Line Comments
+
+Single line comments are created using `--` followed by the comment text. For example, in the following queries we have added some single line comments showing how you can use them.
+
+```sql
+-- This is a single line comment
+-- you can add as many comments as you want.
+-- They are ignored by the database.
+SELECT * FROM customers;
+
+-- You can use them to comment 
+-- out parts of your query.
+SELECT * FROM customers; -- WHERE id = 1;
+
+-- Add notes to your query for later reference.
+SELECT * 
+FROM customers 
+WHERE country = 'USA';  -- USA users only
+```
+
+## Multi Line Comments
+
+Multi line comments are created using `/*` followed by the comment text and `*/`. For example, in the following query, we have added some multi line comments showing how you can use them.
+
+```sql
+/*
+  This is a multi line comment
+  It can span multiple lines
+*/
+SELECT * FROM customers;
+
+/*
+    -- Other queries for our database
+    SELECT * FROM customers;
+    SELECT * FROM orders;
+    SELECT * FROM books;
+*/
+
+-- You can also use them to comment 
+-- out parts of your query. e.g. 
+-- here we are commenting out the
+-- country condition so we are only 
+-- getting orders that are shipped.
+SELECT * 
+FROM orders 
+WHERE /* country = 'USA' AND */ status = 'shipped';
+```