In tMap, you can use the column expression like this: Var.a<Var.b?myValue1:myValue2
Use a routine and code your while into, then call this routine from tMap.
Here are online documentations about TalendHelpCenter:tMap scenarios
What's your job requirement about while in tMap? Does the If / else statement work for you?
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Part 2 of a series on Context Variables
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Find out how to migrate from one database to another using the Dynamic schema