偏移量函数,可以实现将当前记录的 (前、后)记录的字段值,当成当前记录的字段展示。
一、资源准备:
1:建表
create table TEST_1 ( c1 VARCHAR2(10), c2 VARCHAR2(10), c3 VARCHAR2(10) );
2:数据准备
insert into TEST_1 (C1, C2, C3) values ('A', '2012-01', '201201'); insert into TEST_1 (C1, C2, C3) values ('B', '2012-01', '201201'); insert into TEST_1 (C1, C2, C3) values ('C', '2012-01', '201201'); insert into TEST_1 (C1, C2, C3) values ('C', '2012-02', '201202'); insert into TEST_1 (C1, C2, C3) values ('C', '2012-03', '201203'); insert into TEST_1 (C1, C2, C3) values ('A', '2012-02', '201202'); insert into TEST_1 (C1, C2, C3) values ('D', '2012-01', '201201'); insert into TEST_1 (C1, C2, C3) values ('C', '2012-06', '201206');
二、执行
SELECT T.C1, T.C2, (T.C3) CURRENT_MONTH, decode(lag(c2) over(PARTITION BY T.C1 order by c2), to_char(add_months(to_date(c2, 'yyyy-mm'), -1), 'yyyy-mm'), LAG(C3) OVER(PARTITION BY T.C1 ORDER BY T.C2), 0) PREV_MONTH, LEAD(C3,1,0) OVER(PARTITION BY T.C1 ORDER BY T.C2) NEXT_MONTH FROM TEST_1 T GROUP BY T.C1, T.C2, T.C3 ORDER BY T.C1, T.C2;
三、结果
四、解读
LAG使用的DECODE,如果不是前一个月的数据,则用0表示,
LEAD则没有这样判断,只要是同一个PARTITION的数据,则按ORDER BY 的顺序列出。