问题
1.是把 score表分成了两个表吗?是一个值不能同时等于两个值吗?
SELECT
a.*
FROM
students a,
score b,
score c
WHERE
a.id = b.stu_id AND
b.c_name = '计算机' AND
a.id = c.stu_id and
c.c_name = '英语';
2. id分组后就相当于去掉重复的了,为什么还能计算值呢?
SELECT
b.stu_id
FROM
score b
GROUP BY
b.stu_id
HAVING
count(b.stu_id) < 2;