How to create an enumeration or position of a child table record

I have seen this question in many user forums. Lets consider pubs database in MS-SQL SERVER. Now see how to generate the position number of each titles of an author in the titleauthor table.

SELECT * FROM
(
        SELECT
               ( SELECT COUNT(au_Id)
                  FROM titleauthor a
                  WHERE a.au_Id = b. au_Id 
                  AND a.title_ID <= b.title_ID
                  ) as pos ,
               au_Id,
              title_ID
         FROM titleauthor b
          GROUP BY au_Id ,title_ID
)  T

ORDER BY  au_Id 

This is the result we get

pos             au_Id         title_ID
--------------------------------------
1         172-32-1176         PS3333
1         213-46-8915         BU1032
2         213-46-8915         BU2075
1         238-95-7766         PC1035
1         267-41-2394         BU1111
2         267-41-2394         TC7777
1         274-80-9391         BU7832
1         409-56-7008         BU1032
1         427-17-2319         PC8888
1         472-27-2349         TC7777
1         486-29-1786         PC9999
2         486-29-1786         PS7777
1         648-92-1872         TC4203
1         672-71-3249         TC7777
1         712-45-1867         MC2222
1         722-51-5454         MC3021
1         724-80-9391         BU1111
2         724-80-9391         PS1372
1         756-30-7391         PS1372
1         807-91-6654         TC3218
1         846-92-7186         PC8888
1         899-46-2035         MC3021
2         899-46-2035         PS2091
1         998-72-3567         PS2091
2         998-72-3567         PS2106

=====================================