It's essential to ask permission from the first customer to greet a new one that entered the store. Still, it's necessary to wait until you get an answer before you leave! Please, do not leave them or yell at the new customer! It's even better if you keep them busy with something, for example, looking at or reading about a product. Also, it's essential to come back to this customer quickly and continue with the sale.
We found an interesting video about how to greet customers; take a look!