A summary calculation for a calculated item consists of a single computation performed on all records of a single item in a single data block.
When working with summary calculations, draw a distinction between two similar terms:
Null Value. If the summarized item's block is empty—or if each instance of the item has a null value—then the item's value is null, unless its Summary Function property is set to Count, in which case the item's value is zero. For purposes of computing summary items, a block is treated as empty while it is in Enter-Query mode, and while it is being selected into (up to but not including the firing of an ON_CLOSE trigger), unless the block's Precompute Summaries property is set to Yes.
Valid Datatypes. You must set the Data Type property of a summary item to Number, unless the item's Summary Type property is Max or Min, in which case the datatype must mirror that of its associated summarized item. For example, a calculated item that displays the most recent (i.e., maximum) Date in the HIRE_DATE column must have a datatype of Date.