Deckard is a German American surname. All (or virtually all) of the Indiana and Missouri Deckards are descended from Rev. Johann Jacob Deckard (Deckert) II, born in Metz, Moselle, Lorraine, France, which at the time was part of Germany. He travelled to America with his father Johannes Jacob "Squire" Deckert I.
All (or virtually all) of the Ohio Deckards are descended from Henry Deckard (Hendrick Dekker), born in Germany. His son, Richard Deckard Sr., was born in New York City.
The Missouri Deckards are descended from the Indiana Deckards. The Ohio Deckards have a very distant relationship to the Indiana (and Missouri) Deckards, but this is because of Rev. Deckard's wife, Elizabeth Deckard (Vance), not Rev. Deckard himself. Elizabeth Deckard is Henry Deckard's 20th cousin twice removed.
If Find-A-Grave memorials are any indication, the Indiana, Missouri, and Ohio Deckards comprise about 70% of the Deckards within the United States.