SQL – select based on value existence in another column

I want to create a SQL query to identify wells which do not have A3 events. I am using SQL server.

I have tried multiple ways like checking count, comparing with event A3 but still unable to get what i want.
From below example desired result would be W3 and W4

>Solution :

NOT EXISTS is a very performant approach

Example

Select * 
 From  YourTable A
 Where not exists ( select 1 
                     from  YourTable T 
                     where T.Well=A.Well and T.[Event]='A3' 
                  )

Results

Well    Event
W3      A1
W3      A4
W4      A2
W4      A4