To write Custom Reports an implementer should be good with SQL statements. Most reports require you to pull database from MySQL / Postgres DB, and having a good grasp on SQL will help.

Given below a set of common SQL clauses an implementer should learn to master custom reports.

• SELECT
• WHERE clause
• HAVING clause
• INNER JOIN
• LEFT OUTER JOIN
• RIGHT OUTER JOIN
• GROUP BY
• ORDER BY
• SUM
• IF
• DATEDIFF
• DATEADD
• CREATE VIEW
• UNION
• INTERESECT
• Sql Functions
  
  

Some resources are given below:

• W3 Schools 
• Learn SQL the hard way 
• Khan Academy
• SQL Course
• SQL Course 2