Over Aggregation #
OVER aggregates compute an aggregated value for every input row over a range of ordered rows. In contrast to
GROUP BY aggregates,
OVER aggregates do not reduce the number of result rows to a single row for every group. Instead
OVER aggregates produce an aggregated value for every input row.
The following query computes for every order the sum of amounts of all orders for the same product that were received within one hour before the current order.
SELECT order_id, order_time, amount, SUM(amount) OVER ( PARTITION BY product ORDER BY order_time RANGE BETWEEN INTERVAL '1' HOUR PRECEDING AND CURRENT ROW ) AS one_hour_prod_amount_sum FROM Orders
The syntax for an
OVER window is summarized below.
SELECT agg_func(agg_col) OVER ( [PARTITION BY col1[, col2, ...]] ORDER BY time_col range_definition), ... FROM ...
You can define multiple
OVER window aggregates in a
SELECT clause. However, for streaming queries, the
OVER windows for all aggregates must be identical due to current limitation.
ORDER BY #
OVER windows are defined on an ordered sequence of rows. Since tables do not have an inherent order, the ORDER BY
clause is mandatory. For streaming queries, Flink currently only supportsOVER` windows that are defined with an ascending time attributes order. Additional orderings are not supported.
PARTITION BY #
OVER windows can be defined on a partitioned table. In presence of a
PARTITION BY clause, the aggregate is computed for each input row only over the rows of its partition.
Range Definitions #
The range definition specifies how many rows are included in the aggregate. The range is defined with a
BETWEEN clause that defines a lower and an upper boundary. All rows between these boundaries are included in the aggregate. Flink only supports
CURRENT ROW as the upper boundary.
There are two options to define the range,
ROWS intervals and
RANGE intervals #
RANGE interval is defined on the values of the ORDER BY column, which is in case of Flink always a time attribute. The following RANGE interval defines that all rows with a time attribute of at most 30 minutes less than the current row are included in the aggregate.
RANGE BETWEEN INTERVAL '30' MINUTE PRECEDING AND CURRENT ROW
ROW intervals #
ROWS interval is a count-based interval. It defines exactly how many rows are included in the aggregate. The following
ROWS interval defines that the 10 rows preceding the current row and the current row (so 11 rows in total) are included in the aggregate.
ROWS BETWEEN 10 PRECEDING AND CURRENT ROW WINDOW
WINDOW clause can be used to define an
OVER window outside of the
SELECT clause. It can make queries more readable and also allows us to reuse the window definition for multiple aggregates.
SELECT order_id, order_time, amount, SUM(amount) OVER w AS sum_amount, AVG(amount) OVER w AS avg_amount FROM Orders WINDOW w AS ( PARTITION BY product ORDER BY order_time RANGE BETWEEN INTERVAL '1' HOUR PRECEDING AND CURRENT ROW)