Avoiding people being in two places at the same time

[Cran29]

I have a problem where at the moment my data base will allow one person to be allocated to two places on the same date. I have an Employee Table with a list of employees. At the moment it will allow me to put the same employee on the same date on two or more sites. What would I need to do in order to ensure that if an employee has a [date from] and a [dateTo] to allocated to him it will not allow me to put him to another site on those dates. I suspect the solution may involve the dates.

[RuralGuy]

I suspect this link could help: http://allenbrowne.com/appevent.html