SQL Comments

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SQL comments are used to add explanatory notes or documentation within the SQL code. Comments are not executed by the database and do not affect the query or stored procedure logic.

There are two commonly used types of comments in SQL:

1.     Single-line comments: These are used to add comments that span a single line. In most SQL databases, single-line comments start with two hyphens (--) and continue until the end of the line. For example:

-- This is a single-line comment in SQL

SELECT * FROM users; -- This is another comment

2.     Multi-line comments: These are used to add comments that span multiple lines. In most SQL databases, multi-line comments start with /* and end with */. For example:

/*

This is a multi-line comment

in SQL. It can span

multiple lines.

*/

SELECT * FROM orders;

It's important to note that comments are purely for human readability and are ignored by the database engine when executing the SQL code. They can be used to provide explanations, documentations, or temporary code removal without affecting the functionality of the SQL code.

Example 

--Select all:

SELECT * FROM Customers; 

The following example uses a single-line comment to ignore the end of a line: 

Example 

SELECT * FROM Customers

-- WHERE City='Berlin'; 

The following example uses a single-line comment to ignore a statement: 

Example 

--SELECT * FROM Customers;

SELECT * FROM Products;

Example

/*Select all the columns

of all the records

in the Customers table:*/ 

SELECT * FROM Customers; 

The following example uses a multi-line comment to ignore many statements: 

Example 

/*SELECT * FROM Customers;

SELECT * FROM Products;

SELECT * FROM Orders;

SELECT * FROM Categories;*/

SELECT * FROM Suppliers; 

To ignore just a part of a statement, also use the /* */ comment.

 The following example uses a comment to ignore part of a line:

 Example 

SELECT CustomerName, /*City,*/ Country

FROM Customers;

 

The following example uses a comment to ignore part of a statement: 

Example 

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;

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