SELECT *
FROM
(SELECT a.a AS a, a.b AS b, d.c AS c FROM table_a a JOIN table_d d on d.a = a.a),
(SELECT c.a AS a, c.b AS b, d.c AS c FROM table_c c JOIN table_d d on d.a = ca.a),
(SELECT b.a AS a, b.b AS b, d.c AS c FROM table_b b JOIN table_d d on d.a = b.a)

Union results in ambiguous schema [d.c] is ambiguous and is aliasing multiple fields

select *
from
(select a, b
from table_a, table_b, table_c) as a
join
(select a, c
from table_d) as d
on a.a = d.a