Why doesn't this code need c(" ") if it is needed at the end:

columns(alias_column<als>(&Employee::firstName) || " " || alias_column<als>(&Employee::lastName)

it is needed here:

 &Employee::firstName || c(" ") || &Employee::lastName),

Why doesn't this code need c(" ") if it is needed at the end:

columns(alias_column<als>(&Employee::firstName) || " " || alias_column<als>(&Employee::lastName)

it is needed here:

 &Employee::firstName || c(" ") || &Employee::lastName),

Because alias_column() returns alias_column_t<>, for which operator|| is overloaded.
Unfortunately we cannot overload C++ builtin types.