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Whenever you want data (columns) to be fetched from more than a single table then think of JOINS in SQL.

It could be 2 or more tables.

*Mostly you join using Primary key and Foreign Key

1) if you want only matching data (rows) from related tables use EQUI JOIN

2) if you want matching and non-matching data (rows) from related tables think of OUTER JOINS

    a) LEFT and RIGHT OUTER JOINS basically means from which table you want to fetch NON-MATCHING rows

    b) FULL OUTER fetches NON-MATCHING rows from both the TABLES

3) If you want to fetch 2 rows from the same table in a SINGLE select you go in for SELF JOIN

4) Any join between to tables without '=' symbol is called NON-EQUI JOIN.

yes, just cover below few topics.

1)  SQL datatypes 2) object creation and alter

 3) data population (insert/update/delete)

4) system defined functions 

5) JOINs

There are many ways to find a solution to this problem.

1.One simple way is to Find the difference with respect to the Maximum, average or what so ever your requirement is and place it in a new column using CASE and WHEN.

2.Other such solution is Self-Join. Use the Self-join and make it look a little more complex

There are many courses out in the market but try to find a course that teaches to entire SQL. Stick to that one course/learning path only. You might feel other videos interesting on SQL and trust me you will land nowhere if you shift without finishing. A proper path is mandated while learning.

Practice what so ever you have learnt online. Practising boosts your confidence levels and makes you market ready