table query

[JOHN FARRUGIA]

I have a table(test) as follows:

id|tags|value|readtime
1|part|9090|2005-1-1 1:21:12
2|total|122|2005-1-1 1:21:12
3|part|9090|2005-1-1 1:21:40
4|total|122|2005-1-1 1:21:40

I am looking to search the table and retrieve "readtime" of any row that has a tag value of "part". Then I want to take all the "readtime" values I just extracted and find (in the same table) all rows that contain the same "readtime" value and have tags equal "total"

I.E. select * from test where tags='total' and readtime=(select readtime from test where tag='part')

I get an error with this query "Operand should contain 1 column"