sql - 使用 IF..Then..Else 和选择查询？

Question

我有 3 个这样的表：

comment桌子 ：

commentId pid    sid       text      vid
1          1     null    comment 1    1
2         null    1     comment 2    1
3          2     null    comment 3    1

student桌子：

sid     firstname   lastname
1         john       adam
2         joan       adam

professor桌子 ：

pid    firstname   lastname
1       mark        abram
2       sean        hoak

我想写一个查询，结果返回如下：

firstname    lastname
mark          abram
john          adam
sean          hoak

米

if (select query ==null) 
   then (select query 1) 
   else select (query 2)

我尝试了以下方法：

if((select pid from comment==null)
then select student.firstname , student.lastname from student where sid in (select sid from comment where vid=1)

else
(select professor.firstname ,professor.lastname from professor where pid in (select pid from comment where vid=1)

但没有运气。

任何想法如何达到想要的结果？

Answer 1

如果我理解您的问题，您想查看使用 vid =1 发表评论的人的姓名，无论他们是学生还是教授。这将做到这一点。ALEFT JOIN带来左表中的所有内容，包括空值。只需LEFT JOIN使用适当的 id，您就会得到所需的东西。

SELECT ISNULL(p.firstname,s.firstname), ISNULL(p.lastname,s.lastname) --,comment or whatever other things you want to show
FROM Comment C
LEFT JOIN professor P ON P.pid = c.pid
LEFT JOIN student s ON s.sid = c.sid
WHERE vid = 1

Answer 2

如果我理解正确，这个查询应该：

SELECT A.commentId, ISNULL(B.firstname,C.firstname) firstname, ISNULL(B.lastname,C.lastname) lastname
FROM comment A
LEFT JOIN student B
ON A.sid = B.sid
LEFT JOIN professor C
ON A.pid = C.pid

Answer 3

尝试这个：

SELECT COALESCE(p.firstname, s.firstname), COALESCE(p.lastname, s.lastname)
FROM comments c
LEFT JOIN Professors p
ON c.pid = p.pid
LEFT JOIN Students s
ON c.sid = s.sid

有点讨厌，如果您创建一个合并学生和教授的视图，可能会得到改进。

Answer 4

或者

select student.firstname , student.lastname 
from student 
where sid in (select sid from comment where vid=1 and pid is null)
UNION ALL
select professor.firstname ,professor.lastname 
from professor 
where pid in (select pid from comment where vid=1 and PID is not null)