I have an array of table names that I want to iterate over and for each table, get the number of rows.  I was hoping I could do something like:

set tStatement = ##class(%SQL.Statement).%New()
set tSC = tStatement.%Prepare("SELECT COUNT(*) AS ObservedRowCount FROM ?")
$$$ThrowOnError(tSC)
set tTableName = ""
for
{
  set tTableName = $O(tCounterAry(tTableName))
  quit:tTableName=""
   set tResult = tStatement.%Execute(tTableName)
  // ...
}

But that doesn't work:

ERROR #5540: SQLCODE: -1 Message:  IDENTIFIER expected, ?

What are the differences between a process private global and a percent variable?  Basically I have some code running in an Ensemble operation that processes requests, and it needs access to pieces of data that are scattered throughout XData blocks in various classes.