如何使用oracle的LAG和LEAD 等统计函数
Lag和Lead函数可以在一次查询中取出同一字段的前N行的数据和后N行的值。这种操作可以使用对相同表的表连接来实现，不过使用LAG和LEAD有更高的效率。以下是LAG和LEAD的例子：SQL> select year,region,profit ,lag (profit,1) over (order by year)
2 as last_year_exp from test;
YEAR REGION PROFIT LAST_YEAR_EXP
---- ------- ---------- -------------
2003 West 88
2003 West 88 88
2003 Central 101 88
2003 Central 100 101
2003 East 102 100
2004 West 77 102
2004 East 103 77
2004 West 89 103
SQL> select year,region,profit ,lead (profit,1) over (order by year)
2 as next_year_exp from test;
YEAR REGION PROFIT NEXT_YEAR_EXP
---- ------- ---------- -------------
2003 West 88 88
2003 West 88 101
2003 Central 101 100
2003 Central 100 102
2003 East 102 77
2004 West 77 103
2004 East 103 89
2004 West 89
Lag函数为Lag(exp,N,defval)，defval是当该函数无值可用的情况下返回的值。Lead函数的用法类似。
Lead和Lag函数也可以使用分组，以下是使用region分组的例子：
SQL> select year,region,profit ,
2 lag (profit,1,0) over (PARTITION BY region order by year)
3 as last_year_exp from test;
YEAR REGION PROFIT LAST_YEAR_EXP
---- ------- ---------- -------------
2003 Central 101 0
2003 Central 100 101
2003 East 102 0
2004 East 103 102
2003 West 88 0
2003 West 88 88
2004 West 77 88
2004 West 89 77