These are some questions I have used when interviewing mysql programmers. I’m sure they can be improved, but they worked for me :)
Q: Can you tell me of a scaling incident with mysql (when a scaling or performance problem occured) and how you fixed it?
Q: can you explain the cardinality of an index in mysql?
a: how much it distinguishes unique rows
Q: how can you increase the cardinality of an index?
a: make length longer
Q: how could you increase it for an index on a field that contains urls, for example?
a: reverse urls
Q: How do innodb vs myisam tables affect performance in large tables?
Q: How do you analyze the slow query log?
Q: how is the size of a table limited in mysql
a: by storage filesystem
Q: what are the things a master/slave setup doesn’t solve for scaling?
Q: Show them a query and the explain output: what’s going on here?