select 1 from table will return the constant 1 for every row of the table. It's useful when you want to cheaply determine if record matches your where clause and/or join.
Here is what the above code is Doing:
1. We’re selecting the constant 1 from the table.
2. We’re using the where clause to filter out the rows that don’t match the condition.
3. We’re using the exists clause to check if the subquery returns any rows.