Master SQL Comments

Comments and operators play a vital role in enhancing the readability and functionality of SQL code. Comments allow developers to include notes or explanations in their SQL scripts, while operators enable various operations on data. This section discusses the types of comments, operators used in SQL, and their syntax across different database systems.

1. SQL Comments

Comments in SQL are used to explain code, make it more readable, or temporarily disable code without deleting it. There are two types of comments in SQL:

Types of Comments

Comment Type	Syntax Example	Description
Single-line Comment	— This is a single-line comment	Used to comment a single line.
Multi-line Comment	/* This is a multi-line comment */	Used to comment multiple lines.

Examples:

-- This is a single-line comment
SELECT * FROM Employees;  -- Fetch all employees


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

Note: Comments are ignored by the SQL parser and do not affect query execution.

2. Operators

Operators are special symbols used to perform operations on operands (data values). SQL provides various types of operators that can be classified as follows:

a. Arithmetic Operators

Arithmetic operators are used to perform basic mathematical operations.

Operator	Description	Example
+	Addition	SELECT 2 + 3;
–	Subtraction	SELECT 5 – 2;
*	Multiplication	SELECT 4 * 3;
/	Division	SELECT 8 / 2;
%	Modulus (remainder)	SELECT 10 % 3;

Example:

SELECT FirstName, LastName, Salary, Salary * 0.10 AS Bonus
FROM Employees;

b. Comparison Operators

Comparison operators are used to compare two values and return a boolean result (TRUE, FALSE, or UNKNOWN).

Operator	Description	Example
=	Equal to	SELECT * FROM Employees WHERE Age = 30;
!= or <>	Not equal to	SELECT * FROM Employees WHERE Age != 30;
>	Greater than	SELECT * FROM Employees WHERE Salary > 50000;
<	Less than	SELECT * FROM Employees WHERE Salary < 30000;
>=	Greater than or equal to	SELECT * FROM Employees WHERE Salary >= 40000;
<=	Less than or equal to	SELECT * FROM Employees WHERE Salary <= 20000;
BETWEEN	Within a range	SELECT * FROM Employees WHERE Salary BETWEEN 30000 AND 60000;
LIKE	Pattern matching	SELECT * FROM Employees WHERE FirstName LIKE ‘A%’;

Example:

SELECT FirstName, LastName 
FROM Employees 
WHERE DepartmentID = 3 AND Salary > 60000;

c. Logical Operators

Logical operators are used to combine multiple conditions.

Operator	Description	Example
AND	Both conditions must be TRUE	SELECT * FROM Employees WHERE Age > 30 AND Salary > 50000;
OR	At least one condition must be TRUE	SELECT * FROM Employees WHERE Age < 30 OR Salary < 30000;
NOT	Reverses the result of a condition	SELECT * FROM Employees WHERE NOT (Age > 30);

Example:

SELECT FirstName, LastName 
FROM Employees 
WHERE (DepartmentID = 1 OR DepartmentID = 2) AND Salary < 40000;

Conclusion

Understanding comments and operators is essential for writing clean, efficient, and readable SQL code. Comments help clarify code functionality, while operators allow for diverse data manipulations, comparisons, and logical evaluations. Mastery of these elements enhances your ability to interact with databases effectively.

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