GROUP BY in Riak TS

The GROUP BY statement is used with SELECT to pick out and condense rows sharing the same value and return a single row. GROUP BY is useful for aggregating an attribute of a device over a time period; for instance, you could use it to pull average values for every 30 minute period over the last 24 hours.

This document will show you how to run various queries using GROUP BY. See the guidelines for more information on limitations and rules for queries in TS.

GROUP BY Basics

GROUP BY returns a single row for each unique combination of values for columns specified in the GROUP BY statement. There is no guaranteed order for the returned rows.

The SELECT statement must contain only the columns specified in GROUP BY. Columns not used as groups can appear as function parameters. The GROUP BY statement works on all rows, not just the values in the partition key, so all columns are available.

The aggregate function may be used with the GROUP BY statement. If used, SELECT may contain the columns specified in either GROUP BY or the aggregate function.

WARNING

Before you run GROUP BY you must ensure the node issuing the query has adequate memory to receive the response. If the returning rows do not fit into the memory of the requesting node, the node is likely to fail.

GROUP BY Examples

The following table defines a schema for tasks, including which project they are part of and when they were completed.

CREATE TABLE tasks (
    name VARCHAR NOT NULL,
    project VARCHAR NOT NULL,
    completed TIMESTAMP NOT NULL,
    subtask VARCHAR NOT NULL,
    duration SINT64 NOT NULL,
    PRIMARY KEY((name,project,quantum(completed,1,'m')),name,project,completed)
);

Basic GROUP BY

The query below returns one column per unique project and counts how many rows have the same project.

SELECT project, COUNT(name)
FROM tasks
WHERE completed > 1 AND completed < 1000 AND name = 'wheels' AND project = 'Mars Rover'
GROUP BY project;

More than one group

You can group as many columns as you choose, and the order of the grouping has no effect.

The query below returns one column per unique project, name combination, and counts how many rows have the same project, name combination.

SELECT project, COUNT(name)
FROM tasks
WHERE completed > 1 AND completed < 1000 AND name = 'wheels' AND project = 'Mars Rover'
GROUP BY project, name;

Timestamps in a group

Warning

Using a timestamp in the group can produce large number of groups due to the likelihood of the timestamps being unique.

You can include the timestamp (or quantized column) in the group.

The query below returns one column per unique project, name, timestamp combination.

SELECT project, name, completed
FROM tasks
WHERE completed > 1 AND completed < 1000 AND name = 'wheels' AND project = 'Mars Rover'
GROUP BY project, name, completed;

Using combination aggregate functions

If you use a combination aggregate function (COUNT(), SUM(), MEAN(), AVG(), STDDEV(), STDDEV_SAMP(), and STDDEV_POP()) with the SELECT statement, you can specify the column in the aggregate function alone, or in both the SELECT and the aggregate function.

This is allowed because the values returned can be grouped into one value: the result of the COUNT function.

The query below returns one column per unique project, name combination and counts how many rows have the same name.

SELECT COUNT(name)
FROM tasks
WHERE completed > 1 AND completed < 1000 AND name = 'wheels' AND project = 'Mars Rover'
GROUP BY project, name;

The query below returns one column per unique project and counts how many rows have the same timestamp.

SELECT project, COUNT(completed)
FROM tasks
WHERE completed > 1 AND completed < 1000 AND name = 'wheels' AND project = 'Mars Rover'
GROUP BY project;

GROUP BY columns not in the partition key

Since the GROUP BY statement works on all rows, all columns are available, which means that using GROUP BY on the partition key values is not very useful because the partition key limits the result set.

If we create the following table:

CREATE TABLE tasks2 (
userid VARCHAR NOT NULL,
visits SINT64,
a_time TIMESTAMP NOT NULL, PRIMARY KEY(userid, a_time));

And and try to run the GROUP BY statement including userid in the SELECT statement:

SELECT userid, SUM(visits)
FROM tasks2
WHERE userid = 'roddy' AND a_time > 1 AND a_time < 10000
GROUP BY userid;

The result set would only have the group ‘roddy’ because it is required by the WHERE clause.

If, however, we combine two column names from the partition key in the group using SUM without specifying userid, GROUP BY will return multiple result rows for the userid ‘roddy’ with one column per visit.

In