Oracle SQL 连接和计数案例

Question

我有一门课程，学生按年份注册，他们有出勤记录，我需要从每年的总注册人数中知道有多少人实际参加了该课程，实现该目标所需的 SQL 查询是什么？

预期输出：

YEAR COURSE ENROLED ATTENDED
---- ------ ------- --------
2017 IT           3        2
2018 IT           2        0 

Oracle 脚本示例：

--
DROP TABLE ATTENDANCE;
DROP TABLE ENROLMENT;
DROP TABLE STUDENT;
DROP TABLE COURSE;
--
CREATE TABLE COURSE (ID VARCHAR2(1), NAME VARCHAR2(20), PRIMARY KEY(ID));
CREATE TABLE STUDENT (ID VARCHAR2(1), NAME VARCHAR2(20), PRIMARY KEY(ID));
CREATE TABLE ENROLMENT (ID VARCHAR2(1), COURSE_ID VARCHAR2(1), STUDENT_ID VARCHAR2(1), YEAR VARCHAR2(4), PRIMARY KEY(COURSE_ID, STUDENT_ID, YEAR));
CREATE TABLE ATTENDANCE (ENROLMENT_ID VARCHAR2(1), PRIMARY KEY(ENROLMENT_ID));
--
INSERT INTO COURSE VALUES('1', 'IT');
INSERT INTO STUDENT VALUES('1', 'John');
INSERT INTO STUDENT VALUES('2', 'Mary');
INSERT INTO STUDENT VALUES('3', 'Peter');
INSERT INTO ENROLMENT VALUES('1', '1', '1', '2017');
INSERT INTO ENROLMENT VALUES('2', '1', '2', '2017');
INSERT INTO ENROLMENT VALUES('3', '1', '3', '2017');
INSERT INTO ENROLMENT VALUES('4', '1', '1', '2018');
INSERT INTO ENROLMENT VALUES('5', '1', '2', '2018');
INSERT INTO ATTENDANCE VALUES('1');
INSERT INTO ATTENDANCE VALUES('2');
COMMIT;
--

Answer 1

怎么样：

select e.year, c.name, 
    count(distinct s.id) as enroled, 
    count(distinct a.enrolment_id) as attended
  from enrolment e
  join course c on e.course_id = c.id
  left join student s on s.id = e.student_id
  left join attendance a on a.enrolment_id = e.id
  group by e.year, c.name

结果：

year  name  enroled  attended
----  ----  -------  -----
2017  IT    3        2
2018  IT    2        0