I want to do something like this inside a stored procedure:

WHERE
  transaction_timestamp::DATE IN 
  (COALESCE(
    (SELECT DISTINCT trx_date FROM query_1), 
    (SELECT DISTINCT CASE WHEN date_1::DATE IS NOT NULL THEN date_1::DATE ELSE date_2::DATE END AS trx_date FROM query_2)
    )
  )

It’s giving me:

Single-row subquery returns more than one row.