Re: Order of execution

On 2013-05-06, JORGE MALDONADO <jorgemal1960(at)gmail(dot)com> wrote:
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> I have an UPDATE query which performs several opertions in one table.
>
> UPDATE table01 SET
> field1 = (query1 may contain any field),
> field2 = (query1 may contain any field),
> field3 = (query1 may contain any field)
> WHERE (condition)
>
> query1, query2 and query3 perform a logic to determine which records will
> be updated. Does field1 updates first, then field2 and, lastly, field3?

Updates are one row at a time in an unpredictble order.
this will be wrapped in a transaction so either all will
updated or none.

testing suggests that in simple cases the expressions are evaluated in
the order the columns appear in the table definition, but I wouldn't
rely on that.

> or What is the order in which updates are executed? If I need fields to be
> updated in a certain order, should I use 3 UPDATE commands instead?

yeah, if "simultaneously" isn't acceptable you'll need to do separate
updates.

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