Joins are used to combine rows from two or more tables based on a related column between them. Mastering joins is essential for almost any data-related interview.

• INNER JOIN: Returns records that have matching values in both tables.
• LEFT (OUTER) JOIN: Returns all records from the left table, and the matched records from the right table. (Unmatched right records result in NULL).
• RIGHT (OUTER) JOIN: Returns all records from the right table, and the matched records from the left table.
• FULL (OUTER) JOIN: Returns all records when there is a match in either left or right table.
• SELF JOIN: A regular join, but the table is joined with itself.
• CROSS JOIN: Returns the Cartesian product of the two tables.

• INNER JOIN: Only returns users who HAVE made an order.
• LEFT JOIN: Returns ALL users, including those who HAVEN'T made an order (order details will be NULL).

sql Standard
SELECT 
    Users.Name, 
    Orders.OrderDate
FROM Users
LEFT JOIN Orders ON Users.ID = Orders.UserID;

• Ambiguous Column Name: Forgetting to prefix columns with table names (e.g., Users.ID vs Orders.ID) when names overlap.
• Join Condition: Forgetting the ON clause or using the wrong column for joining.
• Performance: Joining huge tables without indexes on the join columns.