Add single quotes to results in a column from a SQL query

So I currently have the following SQL:

SELECT opportunities.ref FROM opportunities GROUP BY opportunities.ref

which returns the following results:

OP10
OP252
OP52905
OP42

I am trying to achieve the following:

'OP10'
'OP252'
'OP52905'
'OP42'

I have tried the following:

SELECT opportunities.ref FROM opportunities
CASE WHEN opportunities.ref IS NOT NULL THEN "'"+opportunitiesref+"'"
GROUP BY opportunities.ref

but this does not work.

I do not want to update the column, or use any @declare functions as this. I am currently using SQL server 2008. Im not sure how to go about this. Help?

Answer

If you are using SQL SERVER then modify your query according to below :

SELECT opportunities.ref FROM opportunities
CASE WHEN opportunities.ref IS NOT NULL THEN ''''+opportunitiesref+''''
GROUP BY opportunities.ref