LAG is an analytical function that can be used to
get the value of a column in a previous row. It provides access to a row at a
given offset prior to the current position.
LAG
(value_expression [,offset] [,default]) OVER ([query_partition_clause] order_by_clause)
value_expression - Can be a column or a built-in
function, except for other analytic functions.
offset - The number of rows preceding/following
the current row, from which the data is to be retrieved. The default value is
1.
default - The value returned if the offset is
outside the scope of the window. The default value is NULL.
query_partition_clause - Optional. It is used to
partition the results into groups based on one or more expressions.
order_by_clause - Optional. It is used to order
the data within each partition.
