第94页 增长、减少、维持现状

--求与上一年营业额相同的年份：使用关联子查询
SELECT
	* 
FROM
	Sales s1 
WHERE
	s1.sale = ( SELECT s2.sale FROM Sales s2 WHERE s2.year = s1.year - 1 ) 
ORDER BY
	s1.year
--求与上一年营业额相同的年份：使用自连接
SELECT
	s1.* 
FROM
	Sales s1,
	Sales s2 
WHERE
	s1.sale = s2.sale 
	AND s2.year = s1.year - 1 
ORDER BY
	s1.year