what need to learn in mysql?

You can start by learning the very basics. Go step by step and learn one topic at a time. There are plenty of free online resources from where you can start learning. The topics you should master are - 

SQL Syntax

SQL Select

SQL Select Distinct

SQL Where

SQL And, Or, Not

SQL Order By

SQL Insert Into

SQL Null Values

SQL Update

SQL Delete

SQL Select Top

SQL Min and Max

SQL Count, Avg, Sum

SQL Like

SQL Wildcards

SQL In

SQL Between

SQL Aliases

SQL Joins

SQL Inner Join

SQL Left Join

SQL Right Join

SQL Full Join

SQL Self Join

SQL Union

SQL Group By

SQL Having

SQL Exists

SQL Any, All

SQL Select Into

SQL Insert Into Select

SQL Case

SQL Null Functions

SQL Stored Procedures

SQL Comments

SQL Operators

These are the basics you should get started with.