初学，两个小问题

[住楼上的老丁]

1、sql语句问题，
三个表分别为member,degree,major,member中的degree1、major1和degree2、major2字段保存着degree和major的外键，degree和major中存的是名称。
想用sql 语句直接将member中的degree1、major1和degree2、major2的名称显示出来，于是写一sql语句：
select member.id,degree.name as degree1,major.name as major1,degree.name as degree2,major.name as major2 from member,degree,major where degree.id=member.degree1 and major.id=member.major1
还要把degree2和major2的名称显示出来我就不会往上加了。
2、sql表中某一字段什么也不存有时里头什么也没有，有时是<null>是怎么回事？我想都什么都没有，应该怎么做？

[psxfghost]

用外关联若关联不上,对应的关联数据就会为NULL

[psxfghost]

select a.id,
(select b.name from degree b where a.id=b.id) as degree1,
(select c.name from major c where a.id=c.id) as major1
(select b.name from degree b where a.id=b.id) as degree2,
(select c.name from major c where a.id=c.id) as major2
from member a

不过我不明白你的
select member.id,degree.name as degree1,major.name as major1,degree.name as degree2,major.name as major2 from member,degree,major where degree.id=member.degree1 and major.id=member.major1
中degree.name as degree1和degree.name as degree2显示的数据有什么不同

[psxfghost]

这样对吗??
select member.id,
(select b.name from degree b where a.degree1=b.id) as dname1,
(select c.name from major c where a.major1=c.id) as mname1
(select b.name from degree b where a.degree2=b.id) as dname2,
(select c.name from major c where a.major2=c.id) as mname2
from member a

[angelwa]

up!

[psxfghost]

select a.id,
(select b.name from degree b where a.degree1=b.id) as dname1,
(select c.name from major c where a.major1=c.id) as mname1
(select d.name from degree d where a.degree2=d.id) as dname2,
(select e.name from major e where a.major2=e.id) as mname2
from member a

[psxfghost]

select a.id,
(select b.name from degree b where a.id=b.id) as degree1,
(select c.name from major c where a.id=c.id) as major1,
(select b.name from degree b where a.id=b.id) as degree2,
(select c.name from major c where a.id=c.id) as major2
from member a

呵呵,漏了一个逗号!

[住楼上的老丁]

还是不对，说是子查询返回的值多于一个，当子查询跟随在=、！=、>=、<=,<,>,或子查询用作表达式时这种情况是不允许的，该怎么办？

[住楼上的老丁]

不行阿,就是member,degree,major 三个表 用你这个
select a.id,
(select b.name from degree b where a.id=b.id) as degree1,
(select c.name from major c where a.id=c.id) as major1
(select b.name from degree b where a.id=b.id) as degree2,
(select c.name from major c where a.id=c.id) as major2
from member a
分析器说from附近有错误.


不过我不明白你的
中degree.name as degree1和degree.name as degree2显示的数据有什么不同
数据类型是一样的,member表中考虑一个人可能有两个学历或专业,

[boat2002w]

先查一个degree,major 取出来做一个变量，再查另一个，然后在写入最后结果中去