Is it possible to have a conditional value within groq?

Yes, absolutely! You can use the select() function in GROQ to handle conditional values. This is exactly what you need for your use case.

The select() function works like a switch statement and evaluates conditions in order, returning the first matching value. Here's how you'd solve your specific case:

*[_type == "yourType"] {
  _id,
  "date": select(
    defined(customDate) => customDate,
    _updatedAt
  )
}

This will:

1. Check if customDate exists and has a value
2. Return customDate if it's defined
3. Fall back to _updatedAt if customDate is not set

The defined() function checks whether a field exists and has a non-null value, which is perfect for your scenario. You can also use multiple conditions if needed:

*[_type == "yourType"] {
  _id,
  "date": select(
    defined(customDate) && customDate != "" => customDate,
    defined(publishedDate) => publishedDate,
    _updatedAt
  )
}

This approach ensures your date property always has a value directly in the query, exactly as you wanted. The select() function stops evaluating once it finds the first truthy condition, so it's efficient too.