Where a database column becomes a typed Go value. Col[T] returns an Expr[T], and every comparison, function and aggregate over it carries that type forward.
A column name is the one value from outside that is rendered into SQL text rather than bound. No engine accepts a placeholder where an identifier goes, so dynamic selection depends entirely on the dialect doubling an embedded quote, which keeps a hostile string one identifier. TestCol_NameFromOutsideStaysOneIdentifier pins it here, where the name enters.
The page for this package is docs/expr/README.md, with runnable snippets — every one of them copied from a test whose expected output go test checks. The documentation index is ../docs/README.md and the repository README is ../README.md.