Using a condition within "field"

[haymanezzeldin]

Hello,
I would to have the field based on the output of a condition.
For example:
if A > B, then "field":"A",
if B > A, then "field":"B",
else "field":"C"

Here is my code:

{
  "mark": {
    "type": "text",
    "fontSize": 16
  },
  "encoding": {
    "x": {
      "aggregate": "max",
      "field": "Target"
    },
    "text": {
      "field": "string_TOP"
    }
  }
}

Your help is so much appreciated.