问题：

[单选]

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.