Amazon Kinesis Data Analytics SQL Reference (original) (raw)

AVG

Returns the average of a group of values from a windowed query. A windowed query is defined in terms of time or rows. For information about windowed queries, see Windowed Queries. To return an exponentially weighted average of a stream of value expressions selected in a specified time window, see EXP_AVG.

When you use AVG, be aware of the following:

If you don't use the OVER clause, AVG is calculated as an aggregate function. In this case, the aggregate query must contain a GROUP BY clause on a monotonic expression based on ROWTIME that groups the stream into finite rows. Otherwise, the group is the infinite stream, and the query will never complete and no rows will be emitted. For more information, see Aggregate Functions.
A windowed query that uses a GROUP BY clause processes rows in a tumbling window. For more information, see Tumbling Windows (Aggregations Using GROUP BY).
If you use the OVER clause, AVG is calculated as an analytic function. For more information, see Analytic Functions.
A windowed query that uses an OVER clause processes rows in a sliding window. For more information, see Sliding Windows

Syntax

Tumbling Windowed Query


AVG(number-expression) ... GROUP BY monotonic-expression | time-based-expression

Sliding Windowed Query


AVG([DISTINCT | ALL] number-expression) OVER window-specification

Parameters

DISTINCT

Performs the aggregate function only on each unique instance of a value.

ALL

Performs the aggregate function on all values. ALL is the default.

number-expression

Specifies the value expressions evaluated for each row in the aggregation.

OVER window-specification

Divides records in a stream partitioned by the time range interval or the number of rows. A window specification defines how records in the stream are partitioned by the time range interval or the number of rows.

GROUP BY monotonic-expression | time-based-expression

Groups records based on the value of the grouping expression, returning a single summary row for each group of rows that has identical values in all columns.

Examples

Example Dataset

The examples following are based on the sample stock dataset that is part of the Getting Started Exercise in the_Amazon Kinesis Analytics Developer Guide_. To run each example, you need an Amazon Kinesis Analytics application that has the sample stock ticker input stream. To learn how to create an Analytics application and configure the sample stock ticker input stream, see Getting Started in the Amazon Kinesis Analytics Developer Guide.

The sample stock dataset has the schema following.


(ticker_symbol  VARCHAR(4),
sector          VARCHAR(16),
change          REAL,
price           REAL)

Example 1: Return the Average of Values Using the GROUP BY Clause

In this example, the aggregate query has a GROUP BY clause on ROWTIME that groups the stream into finite rows. The AVG function is then calculated from the rows returned by the GROUP BY clause.

Using STEP (Recommended)


CREATE OR REPLACE STREAM "DESTINATION_SQL_STREAM" (
    ticker_symbol VARCHAR(4), 
    avg_price     DOUBLE);  
    
CREATE OR REPLACE PUMP "STREAM_PUMP" AS 
  INSERT INTO "DESTINATION_SQL_STREAM" 
    SELECT STREAM 
        ticker_symbol,
        AVG(price) AS avg_price
    FROM "SOURCE_SQL_STREAM_001"
    GROUP BY ticker_symbol, 
        STEP("SOURCE_SQL_STREAM_001".ROWTIME BY INTERVAL '60' SECOND);

Using FLOOR


CREATE OR REPLACE STREAM "DESTINATION_SQL_STREAM" (
    ticker_symbol VARCHAR(4), 
    avg_price     DOUBLE);  

CREATE OR REPLACE PUMP "STREAM_PUMP" AS 
  INSERT INTO "DESTINATION_SQL_STREAM" 
    SELECT STREAM 
        ticker_symbol,
        AVG(price) AS avg_price
    FROM "SOURCE_SQL_STREAM_001"
    GROUP BY ticker_symbol, 
        FLOOR("SOURCE_SQL_STREAM_001".ROWTIME TO MINUTE);

Results

The preceding examples output a stream similar to the following.

Table showing stock ticker symbols NFS, WAS, PPL, ALY with corresponding dates and average prices.

Example 2: Return the Average of Values Using the OVER Clause

In this example, the OVER clause divides records in a stream partitioned by the time range interval of '1' hour preceding. The AVG function is then calculated from the rows returned by the OVER clause.


CREATE OR REPLACE STREAM "DESTINATION_SQL_STREAM" (
    ticker_symbol VARCHAR(4),
    avg_price     DOUBLE);
CREATE OR REPLACE PUMP "STREAM_PUMP" AS 
    INSERT INTO "DESTINATION_SQL_STREAM"
    SELECT STREAM ticker_symbol, 
        AVG(price) OVER (
            PARTITION BY ticker_symbol
            RANGE INTERVAL '1' HOUR PRECEDING) AS avg_price
    FROM "SOURCE_SQL_STREAM_001"

The preceding example outputs a stream similar to the following.

Table showing stock data with columns for timestamp, ticker symbol, and average price.

Usage Notes

Amazon Kinesis Analytics doesn't support AVG applied to interval types. This functionality is a departure from the SQL:2008 standard.

When used as an analytic function, AVG returns null if the window being evaluated contains no rows, or if all rows contain null values. For more information, see Analytic Functions. AVG also returns null for a PARTITION BY clause for which the partition within the window matching the input row contains no rows or all rows are null. For more information about PARTITION BY, see WINDOW Clause (Sliding Windows).

AVG ignores null values from the set of values or a numeric expression. For example, each of the following return the value of 2:

AVG(1, 2, 3) = 2
AVG(1,null, 2, null, 3, null) = 2