# Does a trade always change the mid-price?

Let assume that the mid-price now is m0, then a trade happens and the new mid-price is m1. Is it possible to have m1 = m0 or it should be always different?

This also implies that no trade is necessary to change the price. If the best bid or ask changes, the result of $$\frac{\mathrm{best\,bid} + \mathrm{best\,ask}}{2}$$ will also change.