Trying to learn SOQL from scratch, from where can I get exactly full brief and learning. please suggest

Hi @Hitesh Sharma

, I would suggest starting with Trailhead and then using the Salesforce Developer documentation as the reference. 

Please walktrough the trailhead modules mentioned below one by one, 

1.

2.

Recommended learning order is, 

1.  Basic SELECT / FROM
2. WHERE and operators 
3. ORDER BY / LIMIT
4. AND / OR / IN / LIKE
5.  Date and DateTime queries 
6.  Relationship queries — Parent → Child and Child → Parent 
7.  Aggregate functions — COUNT, SUM, AVG, MIN, MAX
8. GROUP BY
9.  SOQL in Apex + governor limits 
10.  Dynamic SOQL 
11.  SOQL vs SOSL 

you just walkthrough the trailhead and learn the topics one by one with handson excercise. This level of understanding is great to work in SOQL. 

Hope it works.