Delete, Update with derived tables?

Question

I have just studied FROM clause and derived tables in mysql and most of the websites provided the examples using SELECT command

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Answer 1

Derived tables exist only for the duration of the parent query they're a member of. Assuming that this syntax and the operations were allowed by MySQL, consider what happens:

a) Your main query starts executing
b) the sub-query executes and returns its results as a temporary table
c) the parent update changes that temporary table
d) the parent query finishes
e) temporary tables are cleaned up and deleted

Essentially you'll have done nothing except waste a bunch of cpu cycles and disk bandwidth.

UPDATE queries DO allow you to join against other tables to use in the WHERE clause, e.g..

UPDATE maintable
LEFT JOIN othertable ON maintable.pk = othertable.fk
SET maintable.somefield='foo'
WHERE othertable.otherfield = 'bar'