MySQL:如何进行涉及连接和左连接的查询

Question

这只是一次大学练习，所以我并不着急。但我想知道答案了。考虑这些表和行:

create table course (
    numco   integer,
    nameco  varchar(20) not null,
    primary key (numco)
);
create table prereq (
    numco integer,
    numcoprereq integer,
    primary key (numco, numcoprereq),
    foreign key (numco) references course (numco),
    foreign key (numcoprereq) references course (numco)
);
insert into course values (1, 'course 1');
insert into course values (2, 'course 2');
insert into course values (3, 'course 3');
insert into course values (4, 'course 4');
insert into course values (1, 'course 5');
insert into course values (2, 'course 6');
insert into prereq values (4, 2);
insert into prereq values (2, 1);

这不是我做的。我只是翻译了它并删除了一些不相关的部分。我知道有两个 course 行具有相同的 PK，但我必须就此询问我的老师。

他要求我们编写查询以获取类(class)名称和类(class)先决条件的名称。如果它只是类(class)的编号，左连接就可以了。但我不能让它与名字一起工作。它应该输出:

course 1    (null)
course 2    course 1
course 3    (null)
course 4    course 2
course 5    (null)
course 6    course 1

搜了很多，写了两个尝试:

select C1.nameco, C2.nameco as namecoprereq
    from course C2 left join
    (course C1 join prereq P on C1.numco = P.numco)
    on C2.numco = P.numcoprereq;

select C1.nameco, C2.nameco as namecoprereq
    from (course C1 join prereq P on C1.numco = P.numco)
    left join course C2 on C2.numco = P.numcoprereq;

他们分别输出:

course 2    course 1
course 6    course 1
course 4    course 2
(null)      course 3
(null)      course 4
course 2    course 5
course 6    course 5
course 4    course 6

和

course 2    course 1
course 2    course 5
course 4    course 2
course 4    course 6
course 6    course 1
course 6    course 5

我知道(我想，我没试过)我可以用 union 和 minus 来做，但我想知道是否可以通过单次选择来做。关于如何做到这一点而又不费力的任何想法？

Answer 1

您几乎可以找到正确的查询答案，但我认为您的搜索有点过头了。只要用“英语”想一想:

1. 我有类(class)(FROM course)
2. 其中一些有先决条件(LEFT JOIN prereq)
3. 我需要先决条件的名称(LEFT JOIN course)

这应该有效:

SELECT
    c1.nameco,
    c2.nameco AS nameprereq
FROM course AS c1
LEFT JOIN prereq AS p ON p.numcoprereq = c1.numco
LEFT JOIN course AS c2 ON c2.numco = p.numco