Merging as a one-keyword feature for all flavors of SQL is almost impossible.
It gets hairy when you have columns with composite types. E.g. depending on database, records can be JSON objects, protobufs, structs, or other composite types, like tuples.
It is possible to define semantics to merge each of these, but they vary and the merge operation becomes verbose to allow to handle each weirdness of each of these underlying types.
Merging is also sensitive to NULL vs 0 vs a DEFAULT, plus in some databases, NULL is still allowed even when a column is marked NOT NULL.
You'd almost need a sub-language for the merging operation, specifying how to handle each corner case.
It gets hairy when you have columns with composite types. E.g. depending on database, records can be JSON objects, protobufs, structs, or other composite types, like tuples.
It is possible to define semantics to merge each of these, but they vary and the merge operation becomes verbose to allow to handle each weirdness of each of these underlying types.
Merging is also sensitive to NULL vs 0 vs a DEFAULT, plus in some databases, NULL is still allowed even when a column is marked NOT NULL.
You'd almost need a sub-language for the merging operation, specifying how to handle each corner case.