[jacques_chester]

They're good questions because they go past the two simplest query types:

    select from ... where ...;

and

    select from ... join ... where;

Those are the types 99% of programmers who use SQL for simple CRUD apps know. But they come up short for asking more useful business questions.

Eyeballing the list, it tests subqueries, GROUP BY, HAVING, OUTER JOIN, IN/NOT IN and SUM. Fairly useful primitives for general query writing.

I'd try to add a question that relies on UNION, INTERSECT or EXCEPT.

[j-kidd]

For interview question, the keyword I like best would be EXISTS. Sadly, many developers write SQL for years without knowing its existence. If I am not mistaken, the propel ORM of symfony doesn't even have native support for it.

As shown by codegeek, the 6 questions here can be answered without needing sub-query. Maybe we can add something like "List employees who are not working alone in their department"