SQL Server has two different ways to comment code.
1) Line comments
Line comments start with two dashes (–) and ends with carriage returns. Everything between two dashes(–) and carriage returns is considered as comment. This comment can be placed anywhere in the code. This comment can be placed after code in the same line and everything before two dashes (–) will execute like normal T-SQL and after two dashes(–) will not be executed and considered as comment.
SELECT * FROM Sales.Products --This table name WHERE ProductID > 10 --This is a filter condition
2) Block comments
Block comments start with a forward-slash and asterisk (/*) and ends with an asterisk and forward-slash (*/). Everything between forward-slash and asterisk (/*) and ends with an asterisk and forward-slash (*/) is considered as comments. This comment is usually placed before or after a big chunk of code, however it can be placed anywhere in the code. This comment can span more than one line.
/*This is an example of blog comment Following table is an example of same retrieve and filter condition*/ SELECT * FROM Sales.Products WHERE ProductID > 10
There is no limit for the length of the comment.
Comments can be nested.
GO Command is not allowed anywhere in commenting text.
Reference : Pinal Dave (http://blog.SQLAuthority.com)