lag的用法：
lag (expression) over (order-by-clause)
lag (expression, offset) over (order-by-clause)
lag (expression, offset, default-value) over (order-by-clause)
lag (expression) over (query-partition-clause, order-by-clause)
lag (expression, offset) over (order-by-clause)
lag (expression, offset, default-value) over (order-by-clause)
lag (expression) over (query-partition-clause, order-by-clause)
lag分析函数用来查找当前行的下N行的记录，如果要查找当前行的上N行，使用lead
例子：
select t.field1,lag(field1,1) over (order by rownum) from table1 t