Indeed, I would be surprised if there exists any RDBMS where the content of the select list in an EXISTS subquery matters. Postgres's SQL dialect even lets you use an empty select list (`... WHERE EXISTS (SELECT FROM related_table WHERE related_id = id_from_outer_query)`). In T-SQL, however, a non-empty select list is required, and in my experience, developers writing T-SQL tend to prefer a constant value like `1` or `NULL` over `*` -- I suspect there's some superstition there related to both the common wisdom that `SELECT *` queries are to be avoided (which is true -- you really should only ever select the columns of interest) and a lack of truly understanding that EXISTS only reads enough of the table to ascertain whether rows exist that satisfy the subquery's predicate, and returns a boolean value, not a result set.

You could. But `SELECT 1` is a very common pattern in SQL Server, intended exactly to declare intent to the reader that "this subquery doesn't return any real data" - in the event the `EXISTS` that surrounds it isn't enough of a clue. You could use `SELECT NULL` or `SELECT 0` or `SELECT *` or `SELECT 12/0` and the behavior would be identical, but most of those would likely raise even more questions to an inexperienced reader than `SELECT 1`.