1 the case of outer joins with the using clause is a

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1 The case of outer joins with the using clause is a little more complicated; with a right outer join it is possible that R 1 . A 1 is null, but R 2 . A 1 is not, and the output should contain the non-null value. The SQL coalesce function can be used, replacing S by coalesce ( R 1 . A 1 , R 2 . A 1) , coalesce ( R 1 . A 2 , R 2 . A 2) , . . . coalesce ( R 1 . An , R 2 . An ), followed by the other attributes of R 1 and R 2.
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