[cbreezyyall]

This feels like an interesting interview question. I think you could simulate this with a full outer join on the entire select list and coalesce? For a UNION ALL you could put some literal column in the selects from both tables that you set to different values and include that in the join so you'd get a result set that will have all nulls in the right table columns for the rows in the left table and vice versa. Something like

  WITH top_t AS (
   SELECT
     a
    ,b
    ,c
    ,'top' as nonexistent_col
   FROM table_1
  ), bottom_t AS (
   SELECT
     a
    ,b
    ,c
    ,'bottom' as nonexistent_col
   FROM table_2
  )
  SELECT
    COALESCE(top_t.a, bottom_t.a) AS a
   ,COALESCE(top_t.b, bottom_t.b) AS b
   ,COALESCE(top_t.c, bottom_t.c) AS c
  FROM top_t FULL OUTER JOIN bottom_t 
    ON top_t.a = bottom_t.a
    AND top_t.b = bottom_t.b
    AND top_t.c = bottom_t.c
    AND top_t.nonexistent_col = bottom_t.nonexistent_col -- remove this for a normal UNION

[tzot]

This is a nice trick using full outer join! My question to wood_spirit still stands, though.

[wood_spirit]

A fun challenge :)

Assuming a null we can use as sentinel:

      SELECT COALESCE(a.col, b.col) AS col
      FROM a
      LEFT JOIN b ON (TRUE)
      WHERE (a.col IS NULL) IS DISTINCT FROM (b.col IS NULL)

(Getting that sentinel might take effort, depending on eg whether there are useful window functions. Here is a way to do it with only left joins and the assumption the column has no duplicate values:

  WITH crossed AS (
     SELECT * FROM UNNEST([1, 2]) AS sentinal
  )
  SELECT IF(sentinal = 1, col, NULL) AS col
  FROM a
  LEFT JOIN crossed ON TRUE
  WHERE sentinal = 1 OR col = (SELECT * FROM a LIMIT 1)

[wood_spirit]

Bravo!!