– 查询出只有两门课程的全部学生的学号和姓名
SELECT
st.s_id,
st.s_name
FROM student st
LEFT JOIN score sc ON sc.s_id=st.s_id
INNER JOIN course c ON c.c_id=sc.c_id
GROUP BY st.s_id HAVING COUNT(1)=2
– 查询出只有两门课程的全部学生的学号和姓名
SELECT
st.s_id,
st.s_name
FROM student st
LEFT JOIN score sc ON sc.s_id=st.s_id
INNER JOIN course c ON c.c_id=sc.c_id
GROUP BY st.s_id HAVING COUNT(1)=2