取前一条记录：LAG(Field, 1, DefaultValue)
LAG (scalar_expression [,offset] [,default]) OVER ( [ partition_by_clause ] order_by_clause )
取后一条记录：LEAD(Field, 1, DefaultValue)：
LEAD ( scalar_expression [ ,offset ] , [ default ] ) OVER ( [ partition_by_clause ] order_by_clause )
详细见：https://docs.microsoft.com/zh-cn/sql/t-sql/functions/lag-transact-sql?view=sql-server-2017