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Consider this syntax：   MERGE INTO t1  USING t2 ON （join predicate）…..   What does the MERGE syntax do？（）
A:It performs a merge join of the row from T2 only if it doesn’t exist in the T1 table. 
B:It creates a natural join of tables T1 and T2 for all columns that have the same name. 
C:It creates a Cartesian product of table T1 and table T2 for all columns that have the same name. 
D:For each row from T2, it updates the row if it exists within table T1, otherwise it inserts the row into T1.

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出自：Oracle认证

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