SQL (Structured Query Language) has a set of rules and conventions that must be followed to ensure correct queries are executed. Here’s an overview of SQL syntax and key conventions:
1. Basic SQL Syntax
SQL syntax is simple and structured, typically consisting of statements or commands. A basic SQL command follows this general structure:
- Action: The operation you wish to perform (e.g.,
SELECT,INSERT,UPDATE,DELETE). - Object: The table or view you are interacting with (e.g.,
FROM table_name). - Optional Clauses: Additional conditions like
WHERE,ORDER BY,GROUP BY, etc.
2. SQL Keywords
SQL uses keywords (reserved words) to define operations. These keywords are the core of SQL commands like SELECT, INSERT, UPDATE, and DELETE.
- Examples of SQL keywords:
SELECT,FROM,WHERE,JOIN,ORDER BY,GROUP BY,HAVING,INSERT,UPDATE,DELETE,CREATE,ALTER,DROP,TRUNCATE,GRANT,REVOKE,COMMIT, etc.
3. SQL Syntax Conventions
There are certain conventions that make your SQL queries readable, efficient, and maintainable:
Statements End with a Semicolon (
;): SQL statements usually end with a semicolon (;), although many database systems allow you to omit it for single statements.
SELECT * FROM employees;
SELECT first_name, last_name
FROM employees
WHERE department = 'HR';
Comments: You can add comments to explain the SQL code. Comments are ignored by the SQL engine.
- Single-line comments start with —
-- This is a single-line comment
SELECT * FROM employees;
Multi-line comments are enclosed in /* */
/* This is a
multi-line comment */
SELECT * FROM employees;
Case Sensitivity in SQL
SQL has specific rules regarding case sensitivity, which vary depending on the database system and the context. Here’s an overview:
1. SQL Keywords Case Sensitivity
SQL keywords (e.g., SELECT, FROM, WHERE, etc.) are not case-sensitive in most database systems, meaning they can be written in lowercase, uppercase, or a mix of both. The following queries are equivalent:
SELECT * FROM employees;
select * from employees;
SeLeCt * fRoM employees;
-- All commands will work fine ins sql
