PostgreSQL conditional anonymous code block

gistfile1.sql

DO $$
  DECLARE updated_rows INT;
  BEGIN

    UPDATE some_table
    SET some_column = 'new'
    WHERE some_column = 'old';

    GET DIAGNOSTICS updated_rows = ROW_COUNT;

    -- Replace 1 with expected row count
    IF updated_rows <> 1 THEN                 
      RAISE EXCEPTION 'Unexpected number of rows would be modified = %', updated_rows;
    ELSE
      RAISE NOTICE 'Success. Updated % row(s)', updated_rows;
    END IF;
  END;
$$;