How do you know the change in ml for a transition?

[Jt00]

For example, consider a transition of an electron from the 3p to the 1s shell. You know Δl = 1 because of the change from a p shell to an s shell, but I was taught that m_l = -l, -l + 1, ..., l. However, to be an allowed transition, Δm_l must be equal to zero. If m_l is a set of numbers, how do you know it's equal to zero so a transition can be allowed?

 

[atyy]

http://farside.ph.utexas.edu/teaching/qmech/Quantum/node121.html (but the rule in Eq 1149-1150 looks different from what you mention)