ClickHouse/docs/en/sql-reference/statements/select/limit-by.md

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LIMIT BY

LIMIT BY Clause

A query with the LIMIT n BY expressions clause selects the first n rows for each distinct value of expressions. The key for LIMIT BY can contain any number of expressions.

ClickHouse supports the following syntax variants:

• LIMIT [offset_value, ]n BY expressions
• LIMIT n OFFSET offset_value BY expressions

During query processing, ClickHouse selects data ordered by sorting key. The sorting key is set explicitly using an ORDER BY clause or implicitly as a property of the table engine (row order is only guaranteed when using ORDER BY, otherwise the row blocks will not be ordered due to multi-threading). Then ClickHouse applies LIMIT n BY expressions and returns the first n rows for each distinct combination of expressions. If OFFSET is specified, then for each data block that belongs to a distinct combination of expressions, ClickHouse skips offset_value number of rows from the beginning of the block and returns a maximum of n rows as a result. If offset_value is bigger than the number of rows in the data block, ClickHouse returns zero rows from the block.

:::note
LIMIT BY is not related to LIMIT. They can both be used in the same query. :::

If you want to use column numbers instead of column names in the LIMIT BY clause, enable the setting enable_positional_arguments.

Examples

Sample table:

CREATE TABLE limit_by(id Int, val Int) ENGINE = Memory;
INSERT INTO limit_by VALUES (1, 10), (1, 11), (1, 12), (2, 20), (2, 21);

Queries:

SELECT * FROM limit_by ORDER BY id, val LIMIT 2 BY id

┌─id─┬─val─┐
│  1 │  10 │
│  1 │  11 │
│  2 │  20 │
│  2 │  21 │
└────┴─────┘

SELECT * FROM limit_by ORDER BY id, val LIMIT 1, 2 BY id

┌─id─┬─val─┐
│  1 │  11 │
│  1 │  12 │
│  2 │  21 │
└────┴─────┘

The SELECT * FROM limit_by ORDER BY id, val LIMIT 2 OFFSET 1 BY id query returns the same result.

The following query returns the top 5 referrers for each domain, device_type pair with a maximum of 100 rows in total (LIMIT n BY + LIMIT).

SELECT
    domainWithoutWWW(URL) AS domain,
    domainWithoutWWW(REFERRER_URL) AS referrer,
    device_type,
    count() cnt
FROM hits
GROUP BY domain, referrer, device_type
ORDER BY cnt DESC
LIMIT 5 BY domain, device_type
LIMIT 100