救命！！！帮帮忙行吗！！还是SQL统计问题1(15分)

我用如下语句：
SELECT s.isbn, SUM(s.linqu_total) + SUM(t.linqu_total) + SUM(l.buy_total) AS totalsum
FROM ystudentlinshu AS s, yteacherlinshu AS t, ylinshoutab AS l
WHERE (s.isbn = t.isbn) AND (s.isbn = l.isbn)
GROUP BY s.isbn
ORDER BY s.isbn
只统计出三表中具有相同字段值的记录，可是我还想让三张表中不相同的记录也显示出来，即不相同的既然不统计，但也显示它，应该如何实现呢？？谢谢！
如表别名s.t.l中有
s t l
isbn total isbn total isbn total
1-1 10 1-1 10 1-1 10
2-2 10 2-1 10 2-2 10
3-3 10 3-1 10 1-1 15
想得到如下结果
isbn totalsum
1-1 45
2-1 10
2-2 20
3-1 10
3-3 10

SUM 是统计 函数，这样做行不通的，
建议作个 汇总统计的 表吧

我用如下语句：
SELECT s.isbn, SUM(s.linqu_total) + SUM(t.linqu_total) + SUM(l.buy_total) AS totalsum
FROM ystudentlinshu AS s, yteacherlinshu AS t, ylinshoutab AS l
WHERE (s.isbn = t.isbn) AND (s.isbn = l.isbn)
GROUP BY s.isbn
UNION
select isbn, linqu_total, 0, 0 from ystudentlinshu where isbn not in (select isbn from yteacherlinshu union select isbn from ylinshoutab)
UNION
select isbn, 0, linqu_total, 0 from yteacherlinshu where isbn not in (select isbn from ystudentlinshu union select isbn from ylinshutab)
UNION
select isbn, 0, 0, buy_total from ylinshutab where isbn not in (select isbn from ystudentlinshu union select isbn from yteacherlinshu)

Pearl的语句运行时有错误！

嘻嘻， 写错了一点点嘛
SELECT s.isbn, SUM(s.linqu_total) + SUM(t.linqu_total) + SUM(l.buy_total) AS totalsum
FROM ystudentlinshu AS s, yteacherlinshu AS t, ylinshoutab AS l
WHERE (s.isbn = t.isbn) AND (s.isbn = l.isbn)
GROUP BY s.isbn
UNION
select isbn, linqu_total from ystudentlinshu where isbn not in (select isbn from yteacherlinshu union select isbn from ylinshoutab)
UNION
select isbn, linqu_total from yteacherlinshu where isbn not in (select isbn from ystudentlinshu union select isbn from ylinshutab)
UNION
select isbn, buy_total from ylinshutab where isbn not in (select isbn from ystudentlinshu union select isbn from yteacherlinshu)