SQL Basics — SELECT, FROM, WHERE

SQL (Structured Query Language) is used to query and manipulate data stored in relational databases.

Data analysts use SQL daily to extract insights from databases — filtering sales data, aggregating customer records, joining multiple tables to answer business questions.

SELECT * FROM employees;

The * wildcard selects all columns. In production, always name specific columns instead of * for better performance and clarity.

SELECT * FROM employees
WHERE salary > 50000;

WHERE filters rows before they are returned. Common operators: >, <, >=, <=, =, != (or <>).

SELECT * FROM employees
WHERE department = 'Sales';

String values in WHERE must use single quotes. SQL is case-insensitive for keywords but case-sensitive for string values depending on the database.

SELECT * FROM employees
WHERE salary > 40000
  AND department = 'HR';

AND requires both conditions to be true. OR requires at least one. Use parentheses when combining AND/OR to control precedence.

-- WHERE filters individual rows BEFORE grouping:
SELECT department, COUNT(*)
FROM employees
WHERE salary > 30000
GROUP BY department;

-- HAVING filters groups AFTER grouping:
SELECT department, COUNT(*)
FROM employees
GROUP BY department
HAVING COUNT(*) > 5;

WHERE cannot reference aggregate functions (COUNT, SUM, etc.) because it runs before GROUP BY. HAVING runs after GROUP BY and can filter on aggregates.