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單項(xiàng)選擇題

Which of the following SQL statements will return the year and average salary for all employees hired within a given year that have a salary greater than $30,000?()

A.SELECT * FROM t1 UNION SELECT * FROM t2
B.SELECT * FROM t1 UNION DISTINCT SELECT * FROM t2
C.SELECT * FROM t1 INTERSECT SELECT * FROM t2
D.SELECT * FROM t1 WHERE (c1,c2)= (SELECT c1,c2 FROM t2)

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