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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>SQL Guide - Learning Hub</title>
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<link href="https://fonts.googleapis.com/css2?family=Inter:wght@400;600;700;800&family=Fira+Code&display=swap" rel="stylesheet">
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<body>
<div class="particles" id="particles"></div>
<nav class="navbar">
<div class="nav-container">
<a href="index.html" class="nav-brand">📚 Learning Hub</a>
<div class="nav-links">
<a href="index.html">Home</a>
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<h3>📖 Contents</h3>
<ul>
<li><a href="#intro" class="active">Introduction</a></li>
<li><a href="#basics">SQL Basics</a></li>
<li><a href="#select">SELECT Queries</a></li>
<li><a href="#where">WHERE Clause</a></li>
<li><a href="#sorting">Sorting & Limiting</a></li>
<li><a href="#aggregates">Aggregate Functions</a></li>
<li><a href="#groupby">GROUP BY</a></li>
<li><a href="#joins">JOINS</a></li>
<li><a href="#subqueries">Subqueries</a></li>
<li><a href="#insert">INSERT Data</a></li>
<li><a href="#update">UPDATE Data</a></li>
<li><a href="#delete">DELETE Data</a></li>
<li><a href="#tables">Table Operations</a></li>
<li><a href="#indexes">Indexes</a></li>
<li><a href="#advanced">Advanced Concepts</a></li>
</ul>
</aside>
<main class="content">
<div class="hero">
<h1>🗄️ SQL Essentials</h1>
<p>Master database querying with SELECT, JOIN, aggregations, and database design</p>
</div>
<section id="intro" class="card">
<h2>1. Introduction to SQL</h2>
<p>SQL (Structured Query Language) is the standard language for managing and manipulating relational databases. It's used to query, insert, update, and delete data, as well as create and modify database structures.</p>
<div class="highlight-box">
<p><strong>Why SQL?</strong> Universal database language, essential for data analysis, works with all major databases (MySQL, PostgreSQL, SQL Server, SQLite, Oracle)</p>
</div>
<p>SQL is essential for:</p>
<ul>
<li><strong>Data Retrieval</strong> - Query and extract specific data</li>
<li><strong>Data Manipulation</strong> - Insert, update, delete records</li>
<li><strong>Data Definition</strong> - Create and modify database structures</li>
<li><strong>Data Analysis</strong> - Aggregate, filter, and join data</li>
</ul>
</section>
<section id="basics" class="card">
<h2>2. SQL Basics</h2>
<h3>Database Structure</h3>
<p>A relational database consists of tables (relations) with rows (records) and columns (fields).</p>
<div class="code-container">
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<span>SQL</span>
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<pre><code class="language-sql">-- Example table structure
CREATE TABLE employees (
id INT PRIMARY KEY,
name VARCHAR(100),
department VARCHAR(50),
salary DECIMAL(10, 2),
hire_date DATE
);
-- View all databases
SHOW DATABASES;
-- Use a database
USE company_db;
-- View all tables
SHOW TABLES;
-- Describe table structure
DESCRIBE employees;</code></pre>
</div>
<h3>SQL Keywords</h3>
<p>SQL keywords are NOT case-sensitive, but it's conventional to write them in UPPERCASE for readability.</p>
</section>
<section id="select" class="card">
<h2>3. SELECT Queries</h2>
<p>The SELECT statement is used to retrieve data from a database.</p>
<h3>Basic SELECT</h3>
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<span>SQL</span>
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<pre><code class="language-sql">-- Select all columns
SELECT * FROM employees;
-- Select specific columns
SELECT name, department, salary FROM employees;
-- Select with alias
SELECT name AS employee_name, salary AS annual_salary
FROM employees;
-- Select distinct values
SELECT DISTINCT department FROM employees;
-- Count distinct
SELECT COUNT(DISTINCT department) FROM employees;</code></pre>
</div>
<h3>Column Calculations</h3>
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<pre><code class="language-sql">-- Calculated columns
SELECT name, salary, salary * 12 AS annual_salary
FROM employees;
-- String concatenation
SELECT CONCAT(first_name, ' ', last_name) AS full_name
FROM employees;
-- Conditional column (CASE)
SELECT name, salary,
CASE
WHEN salary > 100000 THEN 'High'
WHEN salary > 50000 THEN 'Medium'
ELSE 'Low'
END AS salary_category
FROM employees;</code></pre>
</div>
</section>
<section id="where" class="card">
<h2>4. WHERE Clause - Filtering Data</h2>
<p>The WHERE clause filters records based on specified conditions.</p>
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<pre><code class="language-sql">-- Basic conditions
SELECT * FROM employees WHERE department = 'Sales';
SELECT * FROM employees WHERE salary > 75000;
SELECT * FROM employees WHERE hire_date >= '2020-01-01';
-- Multiple conditions (AND, OR)
SELECT * FROM employees
WHERE department = 'Sales' AND salary > 60000;
SELECT * FROM employees
WHERE department = 'Sales' OR department = 'Marketing';
-- BETWEEN
SELECT * FROM employees
WHERE salary BETWEEN 50000 AND 100000;
-- IN operator
SELECT * FROM employees
WHERE department IN ('Sales', 'Marketing', 'IT');
-- NOT IN
SELECT * FROM employees
WHERE department NOT IN ('HR', 'Admin');
-- LIKE pattern matching
SELECT * FROM employees WHERE name LIKE 'John%'; -- Starts with John
SELECT * FROM employees WHERE name LIKE '%son'; -- Ends with son
SELECT * FROM employees WHERE name LIKE '%ann%'; -- Contains ann
-- IS NULL / IS NOT NULL
SELECT * FROM employees WHERE manager_id IS NULL;
SELECT * FROM employees WHERE email IS NOT NULL;</code></pre>
</div>
</section>
<section id="sorting" class="card">
<h2>5. Sorting and Limiting Results</h2>
<h3>ORDER BY</h3>
<div class="code-container">
<div class="code-header">
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<pre><code class="language-sql">-- Sort ascending (default)
SELECT * FROM employees ORDER BY salary;
SELECT * FROM employees ORDER BY salary ASC;
-- Sort descending
SELECT * FROM employees ORDER BY salary DESC;
-- Sort by multiple columns
SELECT * FROM employees
ORDER BY department ASC, salary DESC;
-- Sort by column position
SELECT name, salary FROM employees
ORDER BY 2 DESC; -- Sort by 2nd column (salary)</code></pre>
</div>
<h3>LIMIT / TOP</h3>
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<pre><code class="language-sql">-- MySQL, PostgreSQL, SQLite
SELECT * FROM employees LIMIT 10;
-- Top 10 highest salaries
SELECT * FROM employees
ORDER BY salary DESC
LIMIT 10;
-- Skip first 10, get next 10 (pagination)
SELECT * FROM employees
ORDER BY id
LIMIT 10 OFFSET 10;
-- SQL Server / MS Access
SELECT TOP 10 * FROM employees;</code></pre>
</div>
</section>
<section id="aggregates" class="card">
<h2>6. Aggregate Functions</h2>
<p>Aggregate functions perform calculations on multiple rows and return a single value.</p>
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<pre><code class="language-sql">-- COUNT - count rows
SELECT COUNT(*) FROM employees;
SELECT COUNT(manager_id) FROM employees; -- Excludes NULLs
SELECT COUNT(DISTINCT department) FROM employees;
-- SUM - total
SELECT SUM(salary) FROM employees;
-- AVG - average
SELECT AVG(salary) FROM employees;
SELECT ROUND(AVG(salary), 2) AS avg_salary FROM employees;
-- MIN and MAX
SELECT MIN(salary) AS lowest_salary FROM employees;
SELECT MAX(salary) AS highest_salary FROM employees;
-- Multiple aggregates
SELECT
COUNT(*) AS total_employees,
AVG(salary) AS avg_salary,
MIN(salary) AS min_salary,
MAX(salary) AS max_salary,
SUM(salary) AS total_payroll
FROM employees;</code></pre>
</div>
</section>
<section id="groupby" class="card">
<h2>7. GROUP BY and HAVING</h2>
<p>GROUP BY groups rows that have the same values and is often used with aggregate functions.</p>
<h3>GROUP BY</h3>
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<pre><code class="language-sql">-- Group by single column
SELECT department, COUNT(*) AS employee_count
FROM employees
GROUP BY department;
-- Group with multiple aggregates
SELECT department,
COUNT(*) AS num_employees,
AVG(salary) AS avg_salary,
MAX(salary) AS max_salary
FROM employees
GROUP BY department;
-- Group by multiple columns
SELECT department, YEAR(hire_date) AS hire_year, COUNT(*)
FROM employees
GROUP BY department, YEAR(hire_date);
-- Order grouped results
SELECT department, AVG(salary) AS avg_salary
FROM employees
GROUP BY department
ORDER BY avg_salary DESC;</code></pre>
</div>
<h3>HAVING</h3>
<p>HAVING filters groups (use it instead of WHERE with aggregate functions).</p>
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<pre><code class="language-sql">-- Filter groups
SELECT department, COUNT(*) AS num_employees
FROM employees
GROUP BY department
HAVING COUNT(*) > 10;
-- Having with multiple conditions
SELECT department, AVG(salary) AS avg_salary
FROM employees
GROUP BY department
HAVING AVG(salary) > 70000 AND COUNT(*) > 5;
-- WHERE vs HAVING
-- WHERE filters rows before grouping
-- HAVING filters groups after grouping
SELECT department, AVG(salary) AS avg_salary
FROM employees
WHERE salary > 50000 -- Filter individual rows first
GROUP BY department
HAVING AVG(salary) > 75000 -- Then filter groups
ORDER BY avg_salary DESC;</code></pre>
</div>
</section>
<section id="joins" class="card">
<h2>8. JOINS - Combining Tables</h2>
<p>JOINS are used to combine rows from two or more tables based on a related column.</p>
<h3>Sample Tables</h3>
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<pre><code class="language-sql">-- Employees table
-- id | name | dept_id
-- 1 | Alice | 1
-- 2 | Bob | 2
-- 3 | Charlie | NULL
-- Departments table
-- id | dept_name
-- 1 | Sales
-- 2 | IT
-- 3 | HR</code></pre>
</div>
<h3>INNER JOIN</h3>
<p>Returns only matching rows from both tables.</p>
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<pre><code class="language-sql">SELECT employees.name, departments.dept_name
FROM employees
INNER JOIN departments ON employees.dept_id = departments.id;
-- Result: Alice-Sales, Bob-IT (Charlie excluded, no dept match)
-- Using table aliases
SELECT e.name, d.dept_name, e.salary
FROM employees e
INNER JOIN departments d ON e.dept_id = d.id;</code></pre>
</div>
<h3>LEFT JOIN</h3>
<p>Returns all rows from left table, matching rows from right table.</p>
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<pre><code class="language-sql">SELECT e.name, d.dept_name
FROM employees e
LEFT JOIN departments d ON e.dept_id = d.id;
-- Result: Alice-Sales, Bob-IT, Charlie-NULL (all employees included)</code></pre>
</div>
<h3>RIGHT JOIN</h3>
<p>Returns all rows from right table, matching rows from left table.</p>
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<pre><code class="language-sql">SELECT e.name, d.dept_name
FROM employees e
RIGHT JOIN departments d ON e.dept_id = d.id;
-- Result: Alice-Sales, Bob-IT, NULL-HR (all departments included)</code></pre>
</div>
<h3>FULL OUTER JOIN</h3>
<p>Returns all rows from both tables.</p>
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<pre><code class="language-sql">SELECT e.name, d.dept_name
FROM employees e
FULL OUTER JOIN departments d ON e.dept_id = d.id;
-- Result: All employees and all departments (with NULLs where no match)</code></pre>
</div>
<h3>Multiple JOINS</h3>
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<pre><code class="language-sql">SELECT e.name, d.dept_name, p.project_name
FROM employees e
INNER JOIN departments d ON e.dept_id = d.id
INNER JOIN projects p ON e.id = p.employee_id;</code></pre>
</div>
<h3>SELF JOIN</h3>
<p>Join a table to itself.</p>
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<pre><code class="language-sql">-- Find employees and their managers
SELECT e.name AS employee, m.name AS manager
FROM employees e
LEFT JOIN employees m ON e.manager_id = m.id;</code></pre>
</div>
</section>
<section id="subqueries" class="card">
<h2>9. Subqueries</h2>
<p>A subquery is a query nested inside another query.</p>
<h3>Subquery in WHERE</h3>
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<pre><code class="language-sql">-- Employees with above-average salary
SELECT name, salary
FROM employees
WHERE salary > (SELECT AVG(salary) FROM employees);
-- Employees in Sales department
SELECT name
FROM employees
WHERE dept_id = (SELECT id FROM departments WHERE dept_name = 'Sales');
-- Using IN with subquery
SELECT name
FROM employees
WHERE dept_id IN (SELECT id FROM departments WHERE dept_name IN ('Sales', 'IT'));</code></pre>
</div>
<h3>Subquery in SELECT</h3>
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<pre><code class="language-sql">SELECT name, salary,
(SELECT AVG(salary) FROM employees) AS avg_salary,
salary - (SELECT AVG(salary) FROM employees) AS diff_from_avg
FROM employees;</code></pre>
</div>
<h3>Subquery in FROM</h3>
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<pre><code class="language-sql">-- Derived table
SELECT dept_name, avg_salary
FROM (
SELECT dept_id, AVG(salary) AS avg_salary
FROM employees
GROUP BY dept_id
) AS dept_averages
JOIN departments ON dept_averages.dept_id = departments.id
WHERE avg_salary > 70000;</code></pre>
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<h3>EXISTS</h3>
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<pre><code class="language-sql">-- Departments that have employees
SELECT dept_name
FROM departments d
WHERE EXISTS (
SELECT 1 FROM employees e WHERE e.dept_id = d.id
);
-- Departments with NO employees
SELECT dept_name
FROM departments d
WHERE NOT EXISTS (
SELECT 1 FROM employees e WHERE e.dept_id = d.id
);</code></pre>
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</section>
<section id="insert" class="card">
<h2>10. INSERT Data</h2>
<p>Add new rows to a table.</p>
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<span>SQL</span>
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<pre><code class="language-sql">-- Insert single row
INSERT INTO employees (name, department, salary, hire_date)
VALUES ('John Doe', 'Sales', 65000, '2023-01-15');
-- Insert multiple rows
INSERT INTO employees (name, department, salary)
VALUES
('Alice Smith', 'IT', 75000),
('Bob Jones', 'Marketing', 60000),
('Charlie Brown', 'HR', 55000);
-- Insert without specifying columns (must match ALL columns in order)
INSERT INTO employees
VALUES (101, 'Jane Wilson', 'Finance', 70000, '2023-02-01');
-- Insert from another table
INSERT INTO employees_backup
SELECT * FROM employees WHERE department = 'Sales';
-- Insert with default values
INSERT INTO employees (name, department)
VALUES ('Test User', 'IT'); -- Other columns will use default values or NULL</code></pre>
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</section>
<section id="update" class="card">
<h2>11. UPDATE Data</h2>
<p>Modify existing rows in a table.</p>
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<span>SQL</span>
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<pre><code class="language-sql">-- Update single column
UPDATE employees
SET salary = 70000
WHERE id = 5;
-- Update multiple columns
UPDATE employees
SET salary = 80000, department = 'IT'
WHERE name = 'John Doe';
-- Update with calculation
UPDATE employees
SET salary = salary * 1.10
WHERE department = 'Sales';
-- Update based on condition
UPDATE employees
SET status = 'Senior'
WHERE YEAR(hire_date) < 2015;
-- Update with JOIN (MySQL)
UPDATE employees e
JOIN departments d ON e.dept_id = d.id
SET e.salary = e.salary * 1.05
WHERE d.dept_name = 'IT';
-- ⚠️ WARNING: Update without WHERE updates ALL rows!
-- UPDATE employees SET salary = 50000; -- Don't do this!</code></pre>
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</section>
<section id="delete" class="card">
<h2>12. DELETE Data</h2>
<p>Remove rows from a table.</p>
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<span>SQL</span>
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<pre><code class="language-sql">-- Delete specific rows
DELETE FROM employees WHERE id = 10;
-- Delete with condition
DELETE FROM employees WHERE hire_date < '2010-01-01';
-- Delete with subquery
DELETE FROM employees
WHERE dept_id IN (SELECT id FROM departments WHERE dept_name = 'Temp');
-- ⚠️ WARNING: Delete without WHERE deletes ALL rows!
-- DELETE FROM employees; -- Don't do this!
-- TRUNCATE - faster way to delete all rows (can't be rolled back)
TRUNCATE TABLE employees;</code></pre>
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</section>
<section id="tables" class="card">
<h2>13. Table Operations</h2>
<h3>CREATE TABLE</h3>
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<pre><code class="language-sql">-- Basic table creation
CREATE TABLE employees (
id INT PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(100) NOT NULL,
email VARCHAR(100) UNIQUE,
department VARCHAR(50),
salary DECIMAL(10, 2) DEFAULT 50000,
hire_date DATE,
is_active BOOLEAN DEFAULT TRUE,
created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);
-- With foreign key
CREATE TABLE projects (
id INT PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(100) NOT NULL,
employee_id INT,
start_date DATE,
FOREIGN KEY (employee_id) REFERENCES employees(id)
);
-- Create table from another table
CREATE TABLE employees_backup AS
SELECT * FROM employees;</code></pre>
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<h3>ALTER TABLE</h3>
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<pre><code class="language-sql">-- Add column
ALTER TABLE employees ADD COLUMN phone VARCHAR(20);
-- Drop column
ALTER TABLE employees DROP COLUMN phone;
-- Modify column
ALTER TABLE employees MODIFY COLUMN name VARCHAR(150);
-- Rename column
ALTER TABLE employees RENAME COLUMN name TO full_name;
-- Add constraint
ALTER TABLE employees ADD CONSTRAINT unique_email UNIQUE (email);
-- Add foreign key
ALTER TABLE projects
ADD CONSTRAINT fk_employee
FOREIGN KEY (employee_id) REFERENCES employees(id);</code></pre>
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<h3>DROP TABLE</h3>
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<pre><code class="language-sql">-- Drop table (permanent!)
DROP TABLE employees;
-- Drop if exists (no error if doesn't exist)
DROP TABLE IF EXISTS employees;</code></pre>
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<h3>Data Types</h3>
<div class="highlight-box">
<p><strong>Common Data Types:</strong> INT, BIGINT, DECIMAL(p,s), FLOAT, DOUBLE, VARCHAR(n), TEXT, DATE, TIME, DATETIME, TIMESTAMP, BOOLEAN, BLOB</p>
</div>
</section>
<section id="indexes" class="card">
<h2>14. Indexes</h2>
<p>Indexes speed up data retrieval but slow down inserts/updates.</p>
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<pre><code class="language-sql">-- Create index
CREATE INDEX idx_department ON employees(department);
-- Create unique index
CREATE UNIQUE INDEX idx_email ON employees(email);
-- Composite index (multiple columns)
CREATE INDEX idx_dept_salary ON employees(department, salary);
-- View indexes
SHOW INDEX FROM employees;
-- Drop index
DROP INDEX idx_department ON employees;</code></pre>
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<div class="highlight-box">
<p><strong>When to use indexes:</strong> Columns used in WHERE, JOIN, ORDER BY. Don't over-index – it slows down writes!</p>
</div>
</section>
<section id="advanced" class="card">
<h2>15. Advanced Concepts</h2>
<h3>UNION</h3>
<p>Combine results from multiple SELECT statements.</p>
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<pre><code class="language-sql">-- UNION (removes duplicates)
SELECT name FROM employees WHERE department = 'Sales'
UNION
SELECT name FROM employees WHERE department = 'IT';
-- UNION ALL (keeps duplicates, faster)
SELECT name FROM employees WHERE salary > 70000
UNION ALL
SELECT name FROM managers WHERE salary > 70000;</code></pre>
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<h3>Window Functions</h3>
<p>Perform calculations across rows related to the current row.</p>
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<pre><code class="language-sql">-- ROW_NUMBER - assign unique row number
SELECT name, salary,
ROW_NUMBER() OVER (ORDER BY salary DESC) AS salary_rank
FROM employees;
-- RANK - rank with gaps for ties
SELECT name, department, salary,
RANK() OVER (PARTITION BY department ORDER BY salary DESC) AS dept_rank
FROM employees;
-- DENSE_RANK - rank without gaps
SELECT name, salary,
DENSE_RANK() OVER (ORDER BY salary DESC) AS dense_rank
FROM employees;
-- Running total
SELECT name, salary,
SUM(salary) OVER (ORDER BY hire_date) AS running_total
FROM employees;
-- Moving average
SELECT name, salary,
AVG(salary) OVER (ORDER BY hire_date ROWS BETWEEN 2 PRECEDING AND CURRENT ROW) AS moving_avg
FROM employees;</code></pre>
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<h3>Common Table Expressions (CTE)</h3>
<p>Temporary named result set, more readable than subqueries.</p>
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<pre><code class="language-sql">-- Basic CTE
WITH high_earners AS (
SELECT name, department, salary
FROM employees
WHERE salary > 80000
)
SELECT department, COUNT(*) AS num_high_earners
FROM high_earners
GROUP BY department;
-- Multiple CTEs
WITH
sales_dept AS (
SELECT * FROM employees WHERE department = 'Sales'
),
high_performers AS (
SELECT * FROM sales_dept WHERE salary > 70000
)
SELECT * FROM high_performers;
-- Recursive CTE (for hierarchical data)
WITH RECURSIVE employee_hierarchy AS (
-- Anchor: start with top-level employees
SELECT id, name, manager_id, 1 AS level
FROM employees
WHERE manager_id IS NULL
UNION ALL
-- Recursive: get subordinates
SELECT e.id, e.name, e.manager_id, eh.level + 1
FROM employees e
JOIN employee_hierarchy eh ON e.manager_id = eh.id
)
SELECT * FROM employee_hierarchy;</code></pre>
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<h3>Views</h3>
<p>Virtual tables based on queries.</p>
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<pre><code class="language-sql">-- Create view
CREATE VIEW high_earners AS
SELECT name, department, salary
FROM employees
WHERE salary > 80000;
-- Use view like a table
SELECT * FROM high_earners;
SELECT department, AVG(salary) FROM high_earners GROUP BY department;
-- Drop view
DROP VIEW high_earners;</code></pre>
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<h3>Transactions</h3>
<p>Group of SQL statements executed as a single unit.</p>
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<pre><code class="language-sql">-- Begin transaction
START TRANSACTION; -- or BEGIN;
-- Execute statements
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
-- Commit if successful
COMMIT;
-- Or rollback if error
ROLLBACK;
-- ACID properties: Atomicity, Consistency, Isolation, Durability</code></pre>
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<h3>String Functions</h3>
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<pre><code class="language-sql">-- Common string functions
SELECT UPPER(name) FROM employees; -- Uppercase
SELECT LOWER(name) FROM employees; -- Lowercase
SELECT LENGTH(name) FROM employees; -- String length
SELECT CONCAT(first_name, ' ', last_name) FROM employees; -- Concatenate
SELECT SUBSTRING(name, 1, 3) FROM employees; -- Extract substring
SELECT TRIM(name) FROM employees; -- Remove whitespace
SELECT REPLACE(name, 'John', 'Jane') FROM employees; -- Replace text</code></pre>
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<h3>Date Functions</h3>
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<pre><code class="language-sql">-- Current date/time
SELECT NOW(); -- Current datetime
SELECT CURDATE(); -- Current date
SELECT CURTIME(); -- Current time
-- Extract date parts
SELECT YEAR(hire_date) FROM employees;
SELECT MONTH(hire_date) FROM employees;
SELECT DAY(hire_date) FROM employees;
SELECT DAYNAME(hire_date) FROM employees;
-- Date arithmetic
SELECT DATE_ADD(hire_date, INTERVAL 1 YEAR) FROM employees;
SELECT DATE_SUB(hire_date, INTERVAL 6 MONTH) FROM employees;
SELECT DATEDIFF(NOW(), hire_date) AS days_employed FROM employees;
-- Format date
SELECT DATE_FORMAT(hire_date, '%Y-%m-%d') FROM employees;
SELECT DATE_FORMAT(hire_date, '%M %d, %Y') FROM employees;</code></pre>
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<h3>NULL Handling</h3>
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<pre><code class="language-sql">-- COALESCE - return first non-NULL value
SELECT name, COALESCE(phone, email, 'No contact') AS contact
FROM employees;
-- IFNULL / ISNULL (MySQL)
SELECT name, IFNULL(manager_id, 0) AS manager
FROM employees;
-- NULLIF - return NULL if values are equal
SELECT name, NULLIF(department, 'Unknown') AS dept
FROM employees;</code></pre>
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</section>
<section class="card">
<h2>SQL Best Practices</h2>
<ul>
<li><strong>Use meaningful names</strong> - Clear table and column names</li>
<li><strong>Always use WHERE with UPDATE/DELETE</strong> - Avoid accidental data loss</li>
<li><strong>Use indexes wisely</strong> - Speed up reads, but not too many</li>
<li><strong>Use transactions</strong> - For related operations that must succeed/fail together</li>
<li><strong>Avoid SELECT *</strong> - Specify needed columns for better performance</li>
<li><strong>Use JOINs over subqueries</strong> - Usually faster and more readable</li>
<li><strong>Normalize data</strong> - Reduce redundancy with proper table design</li>
<li><strong>Use prepared statements</strong> - Prevent SQL injection in applications</li>
<li><strong>Comment complex queries</strong> - Help future you understand the logic</li>
<li><strong>Test on small datasets first</strong> - Before running on production data</li>
</ul>
</section>
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