First, the term "double action" comes from the trigger having two functions relative to the hammer - (1) cock the hammer and (2) release it to fire the gun. In a "single action", the trigger has one function - release the hammer which has been cocked by some means other than the trigger.
Revolvers are either single action (the "cowboy" guns) or double action (which usually means both DA and SA). Some revolvers are double action only (DAO), usually because the hammer is concealed and cannot be cocked other than by the trigger.
Semi-auto pistols are more varied. Some are DAO; the hammer or striker is either hidden or can't be cocked. Some are "conventional DA/SA" meaning that for the first shot the hammer can be cocked and released with a pull of the trigger, but for subsequent shots the hammer is cocked by the operation of the pistol and the trigger is single action. Some "autos" (actually semi-autos) are single action only. With those, the hammer has to be cocked manually for the first shot, either by the action of loading the chamber or by pulling back the hammer.
Jim