LAG

LAG(COLUMN_EXPRESSION, [N], [FILL_VALUE])

Returns the row that precedes the current row by N. If N is not specified, defaults to 1. If FILL_VALUE is not specified, defaults to NULL. If there are fewer than N rows the follow the current row in the window, it returns FILL_VALUE. N must be a literal integer if specified. FILL_VALUE must be a scalar if specified. Supported on all non-semi-structured types.