How to deal with concurrent updates in databases?

Question

What\'s the common way to deal with concurrent updates in an SQL database ?

Consider a simple SQL schema(constraints and defaults not shown..) like

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Answer 1

For the first scenario you could add another condition in the where-clause to make sure you won't overwrite changes made by a concurrent user. E.g.

update credits set creds= 150 where userid = 1 AND creds = 0;