I have a Calendar List. How can i validate to prevent double booking of the event in my calendar using the Nintex Form rules.
You can use the Lookup function in the validation formula. Ensure to use Named Controls (NOT Item Properties) for the filter in the lookup. Í haven't tested it, so I can't assure you will be successful. You might need to add a calculated field (e.g. = <StartDateTime>-<EndDateTime>) to your Calendar list that is used for the Lookup filter......
The syntax for the lookup formula function is:
lookup("list title","column to filter on", value to filter on, "output column". [multiple values-boolean] , [value data type])
Which takes only 1 filter value. If that's sufficient e.g. Start Time can't be the same, and your start time control is Named "Start", then your lookup might look like:
lookup("Calendar", "Start Time", Start, "ID", true)
If it's EmptyOrNul, then no other entries are starting. Combine this with a lookup for End Time. Alternatively you could include a calculated Calendar column (e.g. EventWindow concatenating start and end time and use:
lookup("Calendar", "Event Window", Start-End, "ID", true)
Again, the above I've not yet tried myself, but you should be able to get this to work.