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.
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
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