Adds a filter clause with a nested where clause after the function, where both sides of the operator are references to columns.

Similar to WhereInterface's whereRef method.

Examples

Count people with same first and last names versus general public:

const result = await db
  .selectFrom('person')
  .select((eb) => [
    eb.fn
      .count<number>('id')
      .filterWhereRef('first_name', '=', 'last_name')
      .as('repeat_name_count'),
    eb.fn.count<number>('id').as('total_count'),
  ])
  .executeTakeFirstOrThrow()

The generated SQL (PostgreSQL):

select
  count("id") filter(where "first_name" = "last_name") as "repeat_name_count",
  count("id") as "total_count"
from "person"