Two phone screens followed, after which I was called for an on-site interview. The two phone screens were also fairly tricky, and I felt lucky to have come up with the right answer at the right time.

1. Imagine a table keeping track of all the rides a driver undertakes in a given day. How do you query that table to find out the times when he did not have any rides? The table may have a record when he picks up his first ride, and then he picks another, and then drops both of them. After a while, he picks up a few more, which again is recorded in the table. So, with all that, how do you find the times when he does not have any rides?