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How to select data with percentages from Clickhouse?

Given the following table:

CREATE TABLE main
(
    `job_id` UUID,
    `request_time` DateTime,
    `host_id` UInt8,
    `status_code` LowCardinality(String),
)
ENGINE = MergeTree
ORDER BY request_time
SETTINGS index_granularity = 8192

I’m trying to fetch all statuses by host, with their relative percentage. In order to do that, I need to count results grouped by host and status, and divide each counted field by its total host fields count.

For example, this query would have worked in MySQL:

SELECT
    main.host_id,
    main.status_code,
    COUNT() AS status_count,
    COUNT() / sub.host_total * 100 AS percent
FROM
    main
INNER JOIN (
    SELECT host_id, COUNT() AS host_total
    FROM main
    GROUP BY host_id
) AS sub ON (sub.host_id = main.host_id)
GROUP BY
    main.host_id,
    main.status_code

But ClickHouse throws:

DB::Exception: Unknown identifier: host_total; there are columns: host_id, status_code, count(): While processing host_id, status_code, count() AS status_count, (count() / host_total) * 100 AS percent. (UNKNOWN_IDENTIFIER)

Probably because correlated (dependent) subqueries are not supported.

I was suggested to use CTE, so I tried this:

WITH sub AS (
    SELECT host_id, COUNT() AS host_total
    FROM main
    GROUP BY host_id
)
SELECT
    main.host_id,
    main.status_code,
    COUNT() AS status_count,
    COUNT() / (SELECT host_total FROM sub WHERE sub.host_id = main.host_id) * 100 AS percent
FROM
    main
GROUP BY
    main.host_id,
    main.status_code

But still no luck:

DB::Exception: Missing columns: ‘main.host_id’ while processing query: ‘SELECT host_total FROM sub WHERE host_id = main.host_id’, required columns: ‘host_total’ ‘host_id’ ‘main.host_id’ ‘host_total’ ‘host_id’ ‘main.host_id’: While processing (SELECT host_total FROM sub WHERE sub.host_id = main.host_id) AS _subquery20: While processing count() / ((SELECT host_total FROM sub WHERE sub.host_id = main.host_id) AS _subquery20): While processing (count() / ((SELECT host_total FROM sub WHERE sub.host_id = main.host_id) AS _subquery20)) * 100 AS percent. (UNKNOWN_IDENTIFIER)

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Answer

CH throws an incorrect error. https://github.com/ClickHouse/ClickHouse/issues/4567

host_total should be in groupby section or under aggregate function

insert into main(request_time, host_id,status_code) values ( now(), 1, 200);
insert into main(request_time, host_id,status_code) values ( now(), 1, 500);
insert into main(request_time, host_id,status_code) values ( now(), 1, 200);
insert into main(request_time, host_id,status_code) values ( now(), 2, 500);
insert into main(request_time, host_id,status_code) values ( now(), 2, 200);
insert into main(request_time, host_id,status_code) values ( now(), 3, 500);

SELECT
    main.host_id,
    main.status_code,
    COUNT() AS status_count,
    round((COUNT() / any(sub.host_total)) * 100, 2) AS percent
FROM main
INNER JOIN
(
    SELECT
        host_id,
        COUNT() AS host_total
    FROM main
    GROUP BY host_id
) AS sub ON sub.host_id = main.host_id
GROUP BY
    main.host_id,
    main.status_code
ORDER BY
    main.host_id ASC,
    main.status_code ASC

┌─host_id─┬─status_code─┬─status_count─┬─percent─┐
│       1 │ 200         │            2 │   66.67 │
│       1 │ 500         │            1 │   33.33 │
│       2 │ 200         │            1 │      50 │
│       2 │ 500         │            1 │      50 │
│       3 │ 500         │            1 │     100 │
└─────────┴─────────────┴──────────────┴─────────┘

But there is better ways to solve it:

window functions

SELECT
    host_id,
    status_code,
    status_count,
    round((status_count / host_total) * 100, 2) AS percent
FROM
(
    SELECT
        host_id,
        status_code,
        status_count,
        sum(status_count) OVER (PARTITION BY host_id) AS host_total
    FROM
    (
        SELECT
            host_id,
            status_code,
            COUNT() AS status_count
        FROM main
        GROUP BY
            host_id,
            status_code
    )
)
ORDER BY
    host_id ASC,
    status_code ASC

┌─host_id─┬─status_code─┬─status_count─┬─percent─┐
│       1 │ 200         │            2 │   66.67 │
│       1 │ 500         │            1 │   33.33 │
│       2 │ 200         │            1 │      50 │
│       2 │ 500         │            1 │      50 │
│       3 │ 500         │            1 │     100 │
└─────────┴─────────────┴──────────────┴─────────┘

arrays

SELECT
    host_id,
    status_code,
    status_count,
    round((status_count / host_total) * 100, 2) AS percent
FROM
(
    SELECT
        host_id,
        sumMap([CAST(status_code, 'String')], [1]) AS ga,
        count() AS host_total
    FROM main
    GROUP BY host_id
)
ARRAY JOIN
    ga.1 AS status_code,
    ga.2 AS status_count

┌─host_id─┬─status_code─┬─status_count─┬─percent─┐
│       1 │ 200         │            2 │   66.67 │
│       1 │ 500         │            1 │   33.33 │
│       2 │ 200         │            1 │      50 │
│       2 │ 500         │            1 │      50 │
│       3 │ 500         │            1 │     100 │
└─────────┴─────────────┴──────────────┴─────────┘

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