SQL - Syntax Rules

Structured Query Language (SQL) is the standard programming language for managing and manipulating relational databases. To write efficient and error-free SQL queries, it is essential to follow proper syntax rules. This guide explains the fundamental SQL syntax rules in detail.

What is SQL Syntax?

SQL syntax refers to the set of rules that define how SQL statements must be structured. These rules ensure that database systems correctly interpret and execute commands. Violating syntax rules leads to errors, making it crucial to write queries in the correct format.

Key SQL Syntax Rules

1) SQL Statements Must End with a Semicolon (;)

In most database systems, each SQL statement should end with a semicolon (;). This helps separate multiple queries in a single script.

SELECT * FROM employees;  
UPDATE customers SET status = 'Active' WHERE id = 101;

Exception: Some database tools (like MySQL Workbench) may allow single statements without a semicolon, but it’s best practice to include it.

2) SQL is Case-Insensitive (But Follow Conventions)

SQL keywords (e.g., SELECT, FROM, WHERE) are case-insensitive. However, it is a common convention to write them in uppercase for better readability.

-- These are equivalent  
SELECT name FROM users;  
select name from users;

Note: Table and column names may be case-sensitive in some databases (e.g., PostgreSQL, Linux-based MySQL).

3) String Values Must Be Enclosed in Single Quotes

Text data must be wrapped in single quotes (' '). Double quotes (" ") are typically used for column or table names (in some databases).

SELECT * FROM employees WHERE department = 'HR';  
-- Incorrect: SELECT * FROM employees WHERE department = HR;

4) Use Comments for Clarity

Comments improve readability and help document SQL code.

• Single-line comments: Use --
• Multi-line comments: Use /* */

-- Fetch all active users  
SELECT * FROM users WHERE status = 'Active';  

/*  
This query retrieves  
all employees in the Sales department  
*/  
SELECT * FROM employees WHERE department = 'Sales';

5) SQL Clauses Must Follow a Specific Order

SQL statements must follow a logical sequence of clauses:

SELECT → FROM → WHERE → GROUP BY → HAVING → ORDER BY → LIMIT

Example:

SELECT department, COUNT(*) AS employee_count  
FROM employees  
WHERE salary > 50000  
GROUP BY department  
HAVING COUNT(*) > 5  
ORDER BY employee_count DESC  
LIMIT 10;

6. Avoid Reserved Keywords for Table/Column Names

SQL has reserved keywords (e.g., SELECT, TABLE, ORDER) that should not be used as table or column names. If necessary, use backticks (`), double quotes (" "), or square brackets ([]) depending on the database.

-- MySQL  
SELECT `user`, `order` FROM `table`;  

-- PostgreSQL/SQL Server  
SELECT "user", "order" FROM "table";

7. Proper Use of Quotes for Identifiers 

Different databases use different quotation styles:

• MySQL: Backticks (`column_name`)
• PostgreSQL/SQLite: Double quotes ("column_name")
• SQL Server: Square brackets ([column_name])

8. Wildcards and Operators Must Be Used Correctly

• Wildcards: % (any sequence), _ (single character)
• Comparison Operators: =, !=, >, <, LIKE, IN

SELECT * FROM products WHERE name LIKE 'App%';  
SELECT * FROM orders WHERE status IN ('Pending', 'Shipped');

Following SQL syntax rules is essential for writing efficient and error-free database queries. Key practices include proper use of semicolons, quotes, case conventions, clause ordering, and avoiding reserved keywords. By adhering to these rules, developers can ensure smooth database operations and maintainable SQL code.