"They compare this number with our database."

Translation: Sie vergleichen diese Nummer mit unserer Datenbank.

[retsaylor]

Can someone help me with when to use "Nummer" and when to use "Zahl" please?

[amoussa]

Zahl = any number

Nummer = a number that forms part of a set of numbers e.g. the Hausnummer (house number), Telefonnummer (telephone number)

Ziffer = digit i.e. 0,1,2,3,4,5,6,7,8,9