What is Instr in MySQL?

A function to return the position of occurrence of one string within another

So

  1. SELECT INSTR(‘This is a test string’,‘test’)

will return 11 which is the starting position of the string test within the string This is a test string
This can also be used to check if string exists within another string (if it returns value of 0 then it means the searched string doesnt exist within the main string )