你在找一个 UNION ALL ```
select project, null as milestone, null as change
from projects
union all
select project, milestone, null as change
from milestones
union all
select project, null as milestone, change
from changes

两者的区别 `union` 以及 `union all` 后者不会删除输出中的重复行。

根据你的结果，你似乎想要：

select p.project, null as milestone, null as change
from projects p
where not exists (select 1 from milestones m where m.project = p.project) and
      not exists (select 1 from changes c where c.project = p.project)
union all
select m.project, m.milestone, null as change 
from milestones
union all
select c.project, null as milestone, c.change
from changes;

这是因为您的结果没有行：

A     NULL     NULL

所以你似乎只想和我吵架 projects 当其他表中没有相应的行时。