Data and selection expression
Explain data expression and selection expression?
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Sorry, what's the context? SQL ? COS ?
COS
Please give us more context. Where is the question arising from? It looks a bit like a homework assignment after a class covering a specific area of the COS language.
online docs don't give a direct hint.
so my shot from the hip:
selection expression - $select(), $case() where you get a value based on a condition
data expression - anything that sets, modifies, composes data.
this is a quite academic view since data expressions may contain selection expressions
and selection expressions may contain data expressions.
both variants and any mix of it are quite common in COS
HTH