feat: Made documentation for joins (#29)

* Add Docker installation instructions for PostgreSQL

* Add comprehensive documentation for PostgreSQL joins with examples and best practices

Author: aditya-nv-06

psql/joins.md

@@ -0,0 +1,207 @@
+# PostgreSQL Joins
+## Sample Tables
+
+Let's use these sample tables for illustration:
+
+```sql
+CREATE TABLE employees (
+    id SERIAL PRIMARY KEY,
+    name TEXT,
+    manager_id INTEGER
+);
+
+CREATE TABLE departments (
+    id SERIAL PRIMARY KEY,
+    dept_name TEXT,
+    manager_id INTEGER
+);
+```
+
+Sample data:
+
+```sql
+INSERT INTO employees (name, manager_id) VALUES
+('Alice', NULL),
+('Bob', 1),
+('Charlie', 1),
+('Diana', 2);
+
+INSERT INTO departments (dept_name, manager_id) VALUES
+('HR', 1),
+('Engineering', 2),
+('Marketing', 3);
+```
+
+## Examples
+
+**INNER JOIN:** List employees and their department if they are a manager.
+
+```sql
+SELECT e.name, d.dept_name
+FROM employees e
+INNER JOIN departments d ON e.id = d.manager_id;
+```
+
+**LEFT JOIN:** List all employees and their department (if any).
+
+```sql
+SELECT e.name, d.dept_name
+FROM employees e
+LEFT JOIN departments d ON e.id = d.manager_id;
+```
+
+**RIGHT JOIN:** List all departments and their manager (if any).
+
+```sql
+SELECT d.dept_name, e.name AS manager
+FROM departments d
+RIGHT JOIN employees e ON d.manager_id = e.id;
+```
+
+**FULL JOIN:** Show all employees and departments, matching managers where possible.
+
+```sql
+SELECT e.name, d.dept_name
+FROM employees e
+FULL JOIN departments d ON e.id = d.manager_id;
+```
+
+**CROSS JOIN:** Every employee with every department.
+
+```sql
+SELECT e.name, d.dept_name
+FROM employees e
+CROSS JOIN departments d;
+```
+
+**SELF JOIN:** List employees and their managers.
+
+```sql
+SELECT e.name AS employee, m.name AS manager
+FROM employees e
+INNER JOIN employees m ON e.manager_id = m.id;
+```
+Joins in PostgreSQL allow you to combine rows from two or more tables based on related columns. This is essential for querying relational data efficiently.
+
+## Types of Joins
+
+### 1. INNER JOIN
+
+Returns rows when there is a match in both tables.
+
+```sql
+SELECT a.*, b.*
+FROM table_a a
+INNER JOIN table_b b ON a.id = b.a_id;
+```
+
+### 2. LEFT JOIN (LEFT OUTER JOIN)
+
+Returns all rows from the left table, and matched rows from the right table. Unmatched rows from the right table will have NULLs.
+
+```sql
+SELECT a.*, b.*
+FROM table_a a
+LEFT JOIN table_b b ON a.id = b.a_id;
+```
+
+### 3. RIGHT JOIN (RIGHT OUTER JOIN)
+
+Returns all rows from the right table, and matched rows from the left table. Unmatched rows from the left table will have NULLs.
+
+```sql
+SELECT a.*, b.*
+FROM table_a a
+RIGHT JOIN table_b b ON a.id = b.a_id;
+```
+
+### 4. FULL JOIN (FULL OUTER JOIN)
+
+Returns all rows when there is a match in one of the tables. Unmatched rows will have NULLs for columns from the other table.
+
+```sql
+SELECT a.*, b.*
+FROM table_a a
+FULL JOIN table_b b ON a.id = b.a_id;
+```
+
+### 5. CROSS JOIN
+
+Returns the Cartesian product of both tables (every row of the first table with every row of the second table).
+
+```sql
+SELECT a.*, b.*
+FROM table_a a
+CROSS JOIN table_b b;
+```
+
+### 6. SELF JOIN
+
+A table joined with itself.
+
+```sql
+SELECT a.*, b.*
+FROM employees a
+INNER JOIN employees b ON a.manager_id = b.id;
+```
+
+## Join Conditions
+
+- Use `ON` to specify the join condition.
+- You can join on multiple columns:
+
+```sql
+SELECT *
+FROM orders o
+INNER JOIN customers c ON o.customer_id = c.id AND o.region = c.region;
+```
+
+## Practical Example
+
+Suppose you have two tables:
+
+```sql
+CREATE TABLE authors (
+    id SERIAL PRIMARY KEY,
+    name TEXT
+);
+
+CREATE TABLE books (
+    id SERIAL PRIMARY KEY,
+    title TEXT,
+    author_id INTEGER REFERENCES authors(id)
+);
+```
+
+**Get all books with their authors:**
+
+```sql
+SELECT books.title, authors.name
+FROM books
+INNER JOIN authors ON books.author_id = authors.id;
+```
+
+**Get all authors and their books (including authors with no books):**
+
+```sql
+SELECT authors.name, books.title
+FROM authors
+LEFT JOIN books ON authors.id = books.author_id;
+```
+
+## Tips
+
+- Use table aliases for readability.
+- Always specify join conditions to avoid Cartesian products (except for CROSS JOIN).
+- Use `USING(column)` for joins on columns with the same name.
+
+```sql
+SELECT *
+FROM table_a
+INNER JOIN table_b USING (id);
+```
+
+## References
+
+- [PostgreSQL Documentation: Joins](https://www.postgresql.org/docs/current/tutorial-inheritance.html)
+- [PostgreSQL SELECT](https://www.postgresql.org/docs/current/sql-select.html)