Is it a good practice to answer old unanswered questions? I've seen some old questions with no answers and am assuming it is fine to answer them. Also, is it bad to do so a lot?

"I have them all memorized" I thought have is used when something is done someone and the subject tell them to do so. Like : "I had my computer fixed" "I had my hair cut" So what's the meaning of "have" in "I have them all memorized"?