Thank you for your reply,It's a pleasure to discuss it with you.We are not only discussing the query side of SQL (DML), it is the essence of SQL, relational alg

Thank you for your reply,It's a pleasure to discuss it with you.We are not only discussing the query side of SQL (DML), it is the essence of SQL, relational algebra, and the examples in the post are just for a more intuitive representation of this. The efficiency of executing a complex SQL statement can usually only be achieved through the automatic optimization function of the database engine. As you said, SQL does not specify ordered vs hash grouping nor merge instance of hash That is an engine implementation detail and only touched upon in DDL (schema declaration). Even if more optimization is done in DML, the selection of the underlying algorithm cannot be changed, at most, it makes writing SQL more convenient for users.