Sometimes, we need to join same table twice in a query. The examples about this issue is at below.
Example1 : List the students who have the same date of birth.
from students as s1,students as s2
where s1.birthdate = s2.birthdate
and s1.studentId != s2.studentId
and s1.studentId<s2.studentId --for not list twice
order by s1.birthdate
Example2 : List the books what have the same page counts.
from books as b1,books as b2
where b1.pagecount = b2.pagecount
and b1.bookId != b2.bookId
and b1.bookId < b2.bookId
order by b1.pagecount