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1.7 KiB
1.7 KiB
| toc_title |
|---|
| WITH |
WITH Clause
This section provides support for Common Table Expressions (CTE), so the results of WITH clause can be used in the rest of SELECT query.
Limitations
- Recursive queries are not supported.
- When subquery is used inside WITH section, it’s result should be scalar with exactly one row.
- Expression’s results are not available in subqueries.
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 sum(bytes) expression result from 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 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: Re-using expression in subquery
As a workaround for current limitation for expression usage in subqueries, you may duplicate it.
WITH ['hello'] AS hello
SELECT
hello,
*
FROM
(
WITH ['hello'] AS hello
SELECT hello
)
┌─hello─────┬─hello─────┐
│ ['hello'] │ ['hello'] │
└───────────┴───────────┘