SELECTING with multiple WHERE conditions on same column

Consider using INTERSECT like this:

SELECT contactid WHERE flag = 'Volunteer' 
INTERSECT
SELECT contactid WHERE flag = 'Uploaded'

I think it it the most logistic solution.

Use:

  SELECT t.contactid
    FROM YOUR_TABLE t
   WHERE flag IN ('Volunteer', 'Uploaded')
GROUP BY t.contactid
  HAVING COUNT(DISTINCT t.flag) = 2

The key thing is that the counting of t.flag needs to equal the number of arguments in the IN clause.

The use of COUNT(DISTINCT t.flag) is in case there isn't a unique constraint on the combination of contactid and flag -- if there's no chance of duplicates you can omit the DISTINCT from the query:

  SELECT t.contactid
    FROM YOUR_TABLE t
   WHERE flag IN ('Volunteer', 'Uploaded')
GROUP BY t.contactid
  HAVING COUNT(t.flag) = 2

can't really see your table, but flag cannot be both 'Volunteer' and 'Uploaded'. If you have multiple values in a column, you can use

WHERE flag LIKE "%Volunteer%" AND flag LIKE "%UPLOADED%"

not really applicable seeing the formatted table.

You can either use GROUP BY and HAVING COUNT(*) = _:

SELECT contact_id
FROM your_table
WHERE flag IN ('Volunteer', 'Uploaded', ...)
GROUP BY contact_id
HAVING COUNT(*) = 2 -- // must match number in the WHERE flag IN (...) list

(assuming contact_id, flag is unique).

Or use joins:

SELECT T1.contact_id
FROM your_table T1
JOIN your_table T2 ON T1.contact_id = T2.contact_id AND T2.flag = 'Uploaded'
-- // more joins if necessary
WHERE T1.flag = 'Volunteer'

If the list of flags is very long and there are lots of matches the first is probably faster. If the list of flags is short and there are few matches, you will probably find that the second is faster. If performance is a concern try testing both on your data to see which works best.