Query performance

Hi

I have a stored procedure which queries in 5 secs.

Recently i have added a UDF in select

select col1,col2,col3,UDF_function(col1),col4 from table inner join table 1 ..........11 tables are joined here.

after adding udf_function() my query execution time is taking minimum 15 mins which is horrible. Kindly help me to how to improve the performance.

the function returns the values of a column as comma seperated string. it means i am looping through the records in the function.

The reason why i am using UDF is becos of the below table structure and requirement

Table1

p.k 	
col1    col2    col3
1       x       z
2       v       q
3       n       w
4       m       e

Table2

f.k 
1       a
1       b
1       c
2       a
2       d

Output

col1    col2
1       a,b,c
2       a,d
3
4

This is mainly to avoid duplicates

Thanks in advance Usha