how to apply case when on two tables

Question

I have a question

I have two tables table A and table B

Now I want to update a flag as 1 when records are common in both the tables and update flag as 0 when no common records are there . Adding a snippet from my code

dataframe = spark.sql("""select b.* 
                         from ( Select * 
                                from db.A 
                                where source = 'fileFormat.xlsx'
                                ) a
                         left join table B b on a.column1 = b.column1
                         and a.column2 = b.column2
                         and a.column3 = b.column3
                         where b.column2 is not null
                       """) 

In layman's language how do i find the common records and update the flags.