Do bartenders have to memorize all the possible drink mixes and tails etc.?

i was just wondering...like if sum1 comes in and orders a screwdriver..that's simple, so you know it's just OJ and vodka..but what happens when sum1 comes in and orders a more difficult drink mix or a tail that not many ppl know about or order regularly. does a bartender have to know every single drink ? and im sure if he is working a busy night at a club or something he wouldn't be able to ask each person the description of their drink...if u kno what i mean..( i probably sound so confusing)