SQL语句 求教！

Question

有一张表格 
姓名（SNAME） 出生日期（birthday） 入职日期（jointime)A            1981                         1990
B            1981                         1991
C            1980                         1990
D            1981                         1991
E            1980                         1991求得结果表为
姓名（SNAME1）姓名（SNAME2） 出生日期（birthday）  入职时间差（jointime1）
A           B                  1981                   1
A           D                  1981                   1
B           D                  1981                   0
C           E                  1980                   1SQL语句怎么写，求教sql

Answer 1

select t1.sname sname1,t2.sname sname2,t1.birthday,jointime1=t2.jointime-t1.jointime
from tb t1 join tb t2 on t1.birthday=t2.birthday and t1.sname<t2.sname

Answer 2

谢谢,但为什么要T1.SNAME<T2.SNAME不太理解！