GROUP_CONCAT will always return a string value . It can be easily explain by the example.

Example:

  bookid    | bookname    | isbnno   | cateid    | autid     |  pubid 

    | BK001   | Introduction to Electrodynamics     | 0000979001  | CA001   | AUT001 | P003   
    | BK001  | Introduction to Electrodynamics    |  0000979002  | CA002   | AUT002 | P002
    | BK003   | Guide to Networking                 | 0000979003  | CA003   | AUT003 | P002
    | BK004   | Transfer  of Heat and Mass          | 0000979004  | CA002   | AUT004 | P004
    | BK005   | Conceptual Physics                  | 0000979005  | CA001   | AUT005 | P006
    | BK006   | Fundamentals of Heat                | 0000979006  | CA001   | AUT006 | P005  
    | BK007   | Advanced 3d Graphics                | 0000979007  | CA003   | AUT007 | P002
    | BK008   | Human Anatomy                       | 0000979008  | CA005   | AUT008 | P006 
    | BK009   | Mental Health Nursing               | 0000979009  | CA005   | AUT009 | P007
    | BK010   | Fundamentals of Thermodynamics      | 0000979010  | CA002   | AUT010 | P007 

This is book table.

mysql_ SELECT pubid,GROUP_CONCAT(CATEID)
    -> FROM book
    -> GROUP BY pubid;
+--------+-----------------------+
| pubid | GROUP_CONCAT(CATEID) |
+--------+-----------------------+
| P001   | CA002,CA004           | 
| P002   | CA003,CA003           | 
| P003   | CA001,CA003           | 
| P004   | CA005,CA002           | 
| P005   | CA001,CA004           | 
| P006   | CA005,CA001           | 
| P007   | CA005,CA002           | 
| P008   | CA005,CA004           | 
+--------+-----------------------+

This will explain the use of GROUP_CONCAT.