SQL GROUP BY and a condition on COUNT

Question

I have the following SQL expression:

select id, count(oID) from MyTable group by oID;

and I get something like

+-------+---------------+
| id    | count(oID)    |
+-------+---------------+
|   002 |             6 |
|   104 |             1 |
|   101 |             1 |
|   908 |             1 |
+-------+---------------+

And now I'd like to pull out the rows (select id) where count(oID) is 1. How can I do this in one SQL statement?

Answer 1

Use a HAVING clause to filter an aggregated column.

SELECT   id, count(oID) 
FROM     MyTable 
GROUP BY oID 
HAVING   count(oID) = 1

UPDATE 1

wrap the results in a subquery

SELECT a.*
FROM tableName a INNER JOIN
    (
        SELECT   id 
        FROM     MyTable 
        GROUP BY id  
        HAVING   count(oID) = 1
    ) b ON a.ID = b.ID