下面演示了使用UNION ALL生成原始帖子中描述的结果集的替代方法：

CREATE TABLE table_a (
  id integer PRIMARY KEY,
  tag text);

CREATE TABLE table_b (
  id integer PRIMARY KEY,
  a_id integer REFERENCES table_a(id),
  tag text);

CREATE TABLE table_c (
  id integer PRIMARY KEY,
  a_id integer REFERENCES table_a(id),
  tag text);

INSERT INTO table_a (id, tag)
VALUES (1, 'a1'), (2, 'a2');

INSERT INTO table_b (id, a_id, tag)
VALUES (1, 1, 'b1'), (2, 1, 'b2'), (3, 2, 'b3'), (4, 2, 'b4');

INSERT INTO table_c (id, a_id, tag)
VALUES (1, 1, 'c1'), (2, 1, 'c2'), (3, 2, 'c3'), (4, 2, 'c4');

SELECT a.tag AS table_a_cols, b.tag AS table_b_cols, c.tag AS table_c_cols
  FROM (VALUES ('b'), ('c')) s(selector)
  CROSS JOIN table_a a
  LEFT JOIN table_b b ON (s.selector = 'b' AND a.id = b.a_id)
  LEFT JOIN table_c c ON (s.selector = 'c' AND a.id = c.a_id)
ORDER BY b.tag, c.tag, a.tag;

这种方法比使用UNION ALL更简洁，并且可以很容易地扩展以包含其他表。