Concatenate values into comma-delimited string

This is a common and quite reasonable request. The answer is pretty straightforward, too. Unless you are on a database system which has the functionality built in, like MySQL version 4.1 or Sybase ASE, you can't do this with SQL.

Let's say you have a simple query, like this:

select CompanyID
     , EmployeeID
     , RoleID
  from yourtable
order
    by CompanyID
     , EmployeeID
     , RoleID

And let's say it returns this result:

CompanyID EmployeeID RoleID
   ABC       1003      23
   ABC       1005       1
   ABC       1005       9
   ABC       1005      37
   DEF        442       9   
   DEF        442      45   
   DEF        448      11

In most situations, you would have to write application code to:

1. loop over the result set

2. initialize control break variables for CompanyID and EmployeeID

3. initialize a string for the roles

4. concatenate each row's RoleID to the string

5. detect control breaks and output the concatenated string

6. don't forget to output the last line after end of file!

In MySQL 4.1, though, using the GROUP_CONCAT() function, it's drop dead simple by comparison:

select CompanyID
     , EmployeeID
     , group_concat(RoleID
           order by RoleID) as Roles
  from yourtable
group
    by CompanyID
     , EmployeeID
order
    by CompanyID
     , EmployeeID

Here's what the above query returns:

CompanyID EmployeeID Roles
   ABC       1003      23
   ABC       1005      1,9,37
   DEF        442      9,45   
   DEF        448      11

Neat, eh?