replacing a subquery with an outer join?

in an earlier thread, the following query was submitted:

SELECT COUNT(*) FROM q
> WHERE NOT EXISTS (SELECT 1
> FROM t AS t
> WHERE t.mid = q.mid);

and as part of his answer, David Johnson responded

3. Also, you are using a correlated sub-query instead of a LEFT OUTER JOIN

This interests me. How would you rewrite the correlated subquery above
using the outer join?

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