Note that SQLAlchemy’s clause constructs take operator precedence into account - so parenthesis might not be needed, for example, in an expression like will produce a new construct, each one with the parameter list modified to include the new parameters sent.
In the typical case of a single dictionary of parameters, the newly passed keys will replace the same keys in the previous construct.
For DELETE, the values are those of the rows which were deleted.
Upon execution, the values of the columns to be returned are made available via the result set and can be iterated using and similar.
SQLAlchemy detects Add one or more expressions following the statement keyword, i.e. E.g.: Upon compilation, a RETURNING clause, or database equivalent, will be rendered within the statement.
For INSERT and UPDATE, the values are the newly inserted/updated values.
However, the basic rule of thumb I follow in data-modification cases is similar to a rule of thumb you should follow when eating a nice juicy steak: You might choke and die if you try to swallow the entire steak in one bite, so don't do it.Eating the steak in multiple, smaller bites is much easier, safer, and tastier.The same advice holds true when modifying massive amounts of data. Conceptually, you can update the row through a single UPDATE statement, or you can do it in multiple UPDATE statements, updating smaller "bites" of data with each statement.When the subquery fails to find a matching row in tbl B, the subquery returns NULL.But since the UPDATE has no WHERE clause, it will update all rows, even those where the subquery returns NULL.Special semantics regarding UNION can also be employed to allow “recursive” queries, where a SELECT statement can draw upon the set of rows that have previously been selected.