外部 SELECT 获取嵌套 SELECT 的第一个结果

Question

我正在尝试获取课程中员工的最后一个操作（状态）为“已注册”的课程

这是所有课程中的table

CREATE TABLE course (
    id integer primary key,
    name text
);

这是所有员工的table， nid = 国民身份证

CREATE TABLE employee (
    id integer primary key,
    nid integer,
    name text
);

这是一个table，我在其中记录所有课程的员工的所有行为

CREATE TABLE tran (
    id integer primary key,
    nid integer,
    cid integer,
    state text
);

这是我的数据

INSERT INTO course VALUES (1, "c1"); --> required result
INSERT INTO course VALUES (2, "c2");
INSERT INTO course VALUES (3, "c3"); --> required result

INSERT INTO employee VALUES (1, 111, "e1");
INSERT INTO employee VALUES (2, 222, "e2");
INSERT INTO employee VALUES (3, 333, "e3");

INSERT INTO tran VALUES (1, 111, 1, "enrolled"); --> latest action
INSERT INTO tran VALUES (3, 222, 3, "enrolled");
INSERT INTO tran VALUES (4, 222, 3, "rejected");
INSERT INTO tran VALUES (5, 111, 2, "enrolled");
INSERT INTO tran VALUES (6, 111, 2, "withdrawn");
INSERT INTO tran VALUES (7, 111, 3, "enrolled"); --> latest action

我应该得到上面标记的行

我可以使用这个获取每门课程的最新操作

SELECT cid FROM tran AS t1
WHERE t1.id = (
    SELECT MAX(t2.id) FROM tran AS t2
    WHERE t1.nid = 111
    AND t1.cid = t2.cid
)
AND t1.state = "enrolled";

Output: 
1
3

但是当我尝试用另一个 SELECT 语句将其包装起来以获取课程名称时，我只得到第一个

SELECT id, name FROM course AS c
WHERE c.id = (
    SELECT cid FROM tran AS t1
    WHERE t1.id = (
        SELECT MAX(t2.id) FROM tran AS t2
        WHERE t1.nid = 111
        AND t1.cid = t2.cid
    )
    AND t1.state = "enrolled"
);

Output: 
1|c1

Answer 1

乍一看，您可能会尝试使用 IN 运算符。

SELECT id, name FROM course AS c
WHERE c.id IN (
    SELECT cid FROM tran AS t1
    WHERE t1.id = (
        SELECT MAX(t2.id) FROM tran AS t2
        WHERE t1.nid = 111
        AND t1.cid = t2.cid
    )
    AND t1.state = "enrolled"
);

不过我自己还没有测试过。

Answer 2

一种方法使用聚合：

select t.nid, t.cid
from tran t
group by t.nid, t.cid
having max(t.id) = max(case when t.state = 'enrolled' then id end);

此 returns employee/course 组合，其中 tran 中的最后一个 ID 是 'enrolled'。

如果您需要更多信息，可以在其他表格中join。

外部 SELECT 获取嵌套 SELECT 的第一个结果

Outer SELECT takes the first result of the nested SELECT

sql

nested-queries

nested-query