Product of an entire Collumn MySQL
As explained by How do I create a user-defined aggregate function? there is no such aggregate function (also see here).

However you can emulate your product by:

log(product(x) ) = sum(log(x))

SELECT EXP(SUM(LOG(a))) AS product
FROM t ;


Beware, there could be rounding errors. If this is all integers, do ROUND(...) around the EXP.