What is the difference between Single and Market Demand Curves?

The Market Demand Curve is represented by the sum of the quantity demanded of all individuals for that product’s price point.

So if Mary and Susan make up for the whole market: If Mary is willing to buy 12 units at $1.00, and Susan is willing to buy 11 at the same price, the Market Demand for that product at $1.00 is 23 units(12+11=23).

You can draw essentially 3 graphs: one for Mary, other for Susan and the last onf for the Market. All these will have quantity values for the same price.