Added exercise directory and beginner questions on PostgreSQL

Author: mathncode-sid

exercises/beginner-exercises.md

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+# PostgreSQL Beginner Interview Questions and Answers
+
+This document contains a set of 30 beginner-friendly PostgreSQL questions along with detailed answers. These questions are suitable for interview preparation.
+
+---
+
+## 1. What is PostgreSQL?
+**Answer:**
+PostgreSQL is an open-source, object-relational database management system (ORDBMS) that supports both SQL (relational) and JSON (non-relational) querying. It is highly extensible, reliable, and supports advanced data types.
+
+---
+
+## 2. How do you create a new database in PostgreSQL?
+**Answer:**
+```sql
+CREATE DATABASE my_database;
+```
+
+---
+
+## 3. How do you list all databases in PostgreSQL?
+**Answer:**
+```sql
+\l
+-- or using SQL
+SELECT datname FROM pg_database;
+```
+
+---
+
+## 4. How do you connect to a specific database?
+**Answer:**
+```sql
+\c my_database
+```
+
+---
+
+## 5. How do you create a table in PostgreSQL?
+**Answer:**
+```sql
+CREATE TABLE employees (
+    id SERIAL PRIMARY KEY,
+    name VARCHAR(50) NOT NULL,
+    age INT CHECK (age > 0),
+    department VARCHAR(50)
+);
+```
+- `SERIAL` automatically generates a unique number for `id`.
+- `CHECK` enforces constraints on data integrity.
+
+---
+
+## 6. How do you insert data into a table?
+**Answer:**
+```sql
+INSERT INTO employees (name, age, department)
+VALUES ('John Doe', 30, 'IT');
+```
+
+---
+
+## 7. How do you retrieve all data from a table?
+**Answer:**
+```sql
+SELECT * FROM employees;
+```
+
+---
+
+## 8. How do you retrieve specific columns from a table?
+**Answer:**
+```sql
+SELECT name, department FROM employees;
+```
+
+---
+
+## 9. How do you update data in a table?
+**Answer:**
+```sql
+UPDATE employees
+SET department = 'HR'
+WHERE name = 'John Doe';
+```
+- `WHERE` clause ensures only the intended row(s) are updated.
+
+---
+
+## 10. How do you delete data from a table?
+**Answer:**
+```sql
+DELETE FROM employees
+WHERE name = 'John Doe';
+```
+
+---
+
+## 11. How do you add a new column to an existing table?
+**Answer:**
+```sql
+ALTER TABLE employees
+ADD COLUMN salary NUMERIC(10,2);
+```
+- `NUMERIC(10,2)` stores numbers with 10 digits total and 2 decimal places.
+
+---
+
+## 12. How do you remove a column from a table?
+**Answer:**
+```sql
+ALTER TABLE employees
+DROP COLUMN salary;
+```
+
+---
+
+## 13. How do you rename a column?
+**Answer:**
+```sql
+ALTER TABLE employees
+RENAME COLUMN department TO dept;
+```
+
+---
+
+## 14. How do you rename a table?
+**Answer:**
+```sql
+ALTER TABLE employees
+RENAME TO staff;
+```
+
+---
+
+## 15. How do you create an index on a column?
+**Answer:**
+```sql
+CREATE INDEX idx_name ON employees(name);
+```
+- Indexes improve query performance on frequently searched columns.
+
+---
+
+## 16. What is the difference between `CHAR`, `VARCHAR`, and `TEXT`?
+**Answer:**
+- `CHAR(n)`: Fixed-length string, pads with spaces if less than n characters.  
+- `VARCHAR(n)`: Variable-length string with a maximum limit of n characters.  
+- `TEXT`: Variable-length string with no maximum limit.
+
+---
+
+## 17. How do you find all employees older than 25?
+**Answer:**
+```sql
+SELECT * FROM employees
+WHERE age > 25;
+```
+
+---
+
+## 18. How do you sort query results by a column in ascending order?
+**Answer:**
+```sql
+SELECT * FROM employees
+ORDER BY age ASC;
+```
+
+---
+
+## 19. How do you sort query results in descending order?
+**Answer:**
+```sql
+SELECT * FROM employees
+ORDER BY age DESC;
+```
+
+---
+
+## 20. How do you count the number of rows in a table?
+**Answer:**
+```sql
+SELECT COUNT(*) FROM employees;
+```
+
+---
+
+## 21. How do you find the maximum and minimum age?
+**Answer:**
+```sql
+SELECT MAX(age) AS max_age, MIN(age) AS min_age
+FROM employees;
+```
+
+---
+
+## 22. How do you calculate the average age?
+**Answer:**
+```sql
+SELECT AVG(age) AS average_age FROM employees;
+```
+
+---
+
+## 23. How do you find unique departments?
+**Answer:**
+```sql
+SELECT DISTINCT department FROM employees;
+```
+
+---
+
+## 24. How do you filter records using multiple conditions?
+**Answer:**
+```sql
+SELECT * FROM employees
+WHERE age > 25 AND department = 'IT';
+```
+
+---
+
+## 25. How do you use the `LIKE` operator?
+**Answer:**
+```sql
+SELECT * FROM employees
+WHERE name LIKE 'J%'; -- Names starting with J
+```
+
+---
+
+## 26. How do you use `IN` to filter specific values?
+**Answer:**
+```sql
+SELECT * FROM employees
+WHERE department IN ('IT', 'HR');
+```
+
+---
+
+## 27. How do you use `BETWEEN` to filter a range?
+**Answer:**
+```sql
+SELECT * FROM employees
+WHERE age BETWEEN 25 AND 35;
+```
+
+---
+
+## 28. How do you join two tables?
+**Answer:**
+```sql
+SELECT e.name, d.name AS dept_name
+FROM employees e
+JOIN departments d ON e.department_id = d.id;
+```
+- `JOIN` combines rows from two tables based on a related column.
+
+---
+
+## 29. How do you group data?
+**Answer:**
+```sql
+SELECT department, COUNT(*) AS total_employees
+FROM employees
+GROUP BY department;
+```
+- `GROUP BY` aggregates rows with the same column value.
+
+---
+
+## 30. How do you delete a table?
+**Answer:**
+```sql
+DROP TABLE employees;
+```
+
+---
+