ClickHouse/docs/en/sql-reference/statements/select/with.md

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/en/sql-reference/statements/select/with	WITH

WITH Clause

ClickHouse supports Common Table Expressions (CTE) and substitutes the code defined in the WITH clause in all places of use for the rest of SELECT query. Named subqueries can be included to the current and child query context in places where table objects are allowed. Recursion is prevented by hiding the current level CTEs from the WITH expression.

Please note that CTEs do not guarantee the same results in all places they are called because the query will be re-executed for each use case.

An example of such behavior is below

with cte_numbers as 
(
    select 
        num 
    from generateRandom('num UInt64', NULL) 
    limit 1000000
)
select
    count() 
from cte_numbers
where num in (select num from cte_numbers)

If CTEs were to pass exactly the results and not just a piece of code, you would always see 1000000

However, due to the fact that we are referring cte_numbers twice, random numbers are generated each time and, accordingly, we see different random results, 280501, 392454, 261636, 196227 and so on...

Syntax

WITH <expression> AS <identifier>

or

WITH <identifier> AS <subquery expression>

Examples

Example 1: Using constant expression as “variable”

WITH '2019-08-01 15:23:00' as ts_upper_bound
SELECT *
FROM hits
WHERE
    EventDate = toDate(ts_upper_bound) AND
    EventTime <= ts_upper_bound;

Example 2: Evicting a sum(bytes) expression result from the SELECT clause column list

WITH sum(bytes) as s
SELECT
    formatReadableSize(s),
    table
FROM system.parts
GROUP BY table
ORDER BY s;

Example 3: Using results of a scalar subquery

/* this example would return TOP 10 of most huge tables */
WITH
    (
        SELECT sum(bytes)
        FROM system.parts
        WHERE active
    ) AS total_disk_usage
SELECT
    (sum(bytes) / total_disk_usage) * 100 AS table_disk_usage,
    table
FROM system.parts
GROUP BY table
ORDER BY table_disk_usage DESC
LIMIT 10;

Example 4: Reusing expression in a subquery

WITH test1 AS (SELECT i + 1, j + 1 FROM test1)
SELECT * FROM test1;