1. SELECT
What it is: Used to choose columns from a table
What it does: Returns specific columns of data
Query: Fetch name and salary
SELECT name, salary
FROM employees;
2. FROM
What it is: Specifies the table
What it does: Tells SQL where to get data from
Query: Fetch all data from employees
SELECT *
FROM employees;
3. WHERE
What it is: Filters rows based on condition
What it does: Returns only matching rows
Query: Employees with salary > 30000
SELECT *
FROM employees
WHERE salary > 30000;
4. ORDER BY
What it is: Sorts the data
What it does: Arranges rows in order
Query: Sort by salary (highest first)
SELECT *
FROM employees
ORDER BY salary DESC;
5. COUNT()
What it is: Counts rows
What it does: Returns total records
Query: Count employees
SELECT COUNT(*)
FROM employees;
6. AVG()
What it is: Calculates average
What it does: Returns mean value
Query: Average salary
SELECT AVG(salary)
FROM employees;
7. GROUP BY
What it is: Groups rows by column
What it does: Applies aggregation per group
Query: Avg salary per department
SELECT department, AVG(salary)
FROM employees
GROUP BY department;
8. HAVING
What it is: Filters grouped data
What it does: Returns filtered groups
Query: Departments with avg salary > 40000
SELECT department, AVG(salary)
FROM employees
GROUP BY department
HAVING AVG(salary) > 40000;
9. INNER JOIN
What it is: Combines matching rows from tables
What it does: Returns common data
Query: Employees with department names
SELECT e.name, d.department_name
FROM employees e
INNER JOIN departments d
ON e.dept_id = d.dept_id;
10. LEFT JOIN
What it is: Combines all left + matching right
What it does: Returns all left table data
Query: All employees with departments
SELECT e.name, d.department_name
FROM employees e
LEFT JOIN departments d
ON e.dept_id = d.dept_id;
11. CASE WHEN
What it is: Conditional logic
What it does: Creates values based on condition
Query: Categorize salary
SELECT name,
CASE
WHEN salary > 40000 THEN 'High'
ELSE 'Low'
END AS category
FROM employees;
12. SUBQUERY
What it is: Query inside another query
What it does: Uses result of inner query
Query: Salary above average
SELECT name, salary
FROM employees
WHERE salary > (
SELECT AVG(salary)
FROM employees
);
13. RANK()
What it is: Window function
What it does: Assigns rank without grouping
Query: Rank employees by salary
SELECT name, salary,
RANK() OVER (ORDER BY salary DESC) AS rank
FROM employees;
14. DISTINCT
What it is: Removes duplicates
What it does: Returns unique values
Query: Unique departments
SELECT DISTINCT department
FROM employees;
15. LIKE
What it is: Pattern matching
What it does: Filters text patterns
Query: Names starting with A
SELECT *
FROM employees
WHERE name LIKE 'A%';
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