You can't do this construct | timechart count as totalNumberOfPatches by X
| eval a=case(X=bla, 1...) because when you split a timechart by a field, the count AS totalNumberOfPatches does not resul...
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You can't do this construct | timechart count as totalNumberOfPatches by X
| eval a=case(X=bla, 1...) because when you split a timechart by a field, the count AS totalNumberOfPatches does not result in a field called totalNumberOfPatches, but the fields are the names of the values of the split by clause, in your case Computer_Name. I assume you have a stats rather than timechart, but as @richgalloway says, using dc() is the way to count distinct versions of a field.