SQL Basics¶

What is SQL?¶

SQL (Structured Query Language) is used to:
Store data
Retrieve data
Update data
Delete data from a database.
SQL works with relational databases like:
MySQL
PostgreSQL
SQL Server
Oracle
SQLite

What is a Database?¶

A database is a place where data is stored.

What is a Table?¶

A table stores data in rows and columns.
Example: employees table:

id	name	age	department	salary
1	Rahul	25	IT	50000
2	Anita	28	HR	45000

Row → one record
Column → one field

Basic SQL Command¶

SELECT Statement * Used to read data from a table.

SELECT * FROM employees;

* means all columns

Select Specific Columns

SELECT name, salary FROM employees;

WHERE Clause (Filtering Data)

SELECT * FROM employees
WHERE department = 'IT';

Using Conditions

SELECT * FROM employees
WHERE salary > 45000;

AND / OR

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

ORDER BY (Sorting Data) * Used to sort data in ascending (ASC) or descending (DESC) order. * Example: Sort by age (youngest first)

sql
  SELECT * FROM students
  ORDER BY age;

* (Default is ASC)

Oldest first

SELECT * FROM students
ORDER BY age DESC;

Sort by name alphabetically

SELECT * FROM students
ORDER BY name ASC;

LIMIT (Restrict Number of Rows) * Used to limit the output rows. * Get first 5 students

SELECT * FROM students
LIMIT 5;

Top 3 oldest students

SELECT * FROM students
ORDER BY age DESC
LIMIT 3;

ORDER BY + WHERE + LIMIT (Very Important Combo) * Get top 2 highest paid IT employees

SELECT * FROM employees
WHERE department = 'IT'
ORDER BY salary DESC
LIMIT 2;

SQL Operators (IN, BETWEEN, LIKE) * IN Operator - Used when you want to match multiple values. * Instead of:

WHERE department = 'IT' OR department = 'HR'

* Use:

 SELECT * FROM employees
 WHERE department IN ('IT', 'HR');

* BETWEEN Operator - Used for a range of values. * Example:

SELECT * FROM employees
WHERE salary BETWEEN 40000 AND 60000;

* Includes both 40000 & 60000 * LIKE Operator (Pattern Matching) - Used with text.

| Pattern | Meaning                  |
| ------- | ------------------------ |
| `%`     | Any number of characters |
| `_`     | Exactly one character    |

Names starting with 'A'

  SELECT * FROM students
  WHERE name LIKE 'A%';

Names ending with 'n'

  SELECT * FROM students
  WHERE name LIKE '%n';

Names with 'a' in middle

  SELECT * FROM students
    WHERE name LIKE '%a%';