Hilary Forbes <[EMAIL PROTECTED]> writes:
> I need to set the value of a field in table A to a value in table B depending 
> on the existence of the record in table B.  So what I have done is

> UPDATE tableA set a.val1=b.somefield FROM tableA a, tableB b WHERE 
> a.key1=b.key1;

You've written an unconstrained join to a second copy of tableA.

                        regards, tom lane

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