In a structure with Programs, many Events, and many Registrants, I'm wondering if there is a way to count the number of unique registrants for a given Program. I think two sub queries are needed but my mind blanks over. Do you have a lesson on sub queries?

Here's the structure in essence:
ProgT
    ProgID       PK
    PrName      Text

EventT
    EventID     PK
    ProgID       FK
    DtStart      Date

RegT
    RegID        PK
    EventID      FK
    EID            FK {registrant}

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Reply from Richard Rost:

You could do it with a single aggregate query. Use criteria to limit the ProgID and then group by EventID, sum the RegID.