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Groups > comp.databases.ms-sqlserver > #2092

UPDATE FROM

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Hello, all.  The MSSQL documentation for `UPDATE FROM' is
not very clear on how the update source should be specified
and how its rows are matched against those of the table
being updated. I think the following methods are equivalent
and correct:

   -- 1.
   UPDATE my_alias
   SET col = data_tab.col
   FROM upd_tab my_alias
   JOIN data_tab ON data_tab.code = my_alias.code

   -- 2.
   UPDATE upd_tab
   SET col = data_tab.col
   FROM upd_tab my_alias
   JOIN data_tab ON data_tab.code = my_alias.code

But in some old code that seems to have been working for
about ten years I have found an `UPDATE' with the following
structure:

   -- 3.
   UPDATE upd_tab
   SET col = data_tab.col
   FROM data_tab
   WHERE data_tab.code = my_alias.code

which, unlike the previous two commands, works non-
deterministically, although there are no more than one row
in data_tab from each row in upd_tab. Is it because the FROM
clause does not mention upd_tab, whereas it must?  If so,
how is the WHERE predicate above interpreted and how does it
affect the result?

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