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* translate [select] section into Chinese to improve readability * trigger ci Co-authored-by: mikepop7 <> Co-authored-by: Ivan Blinkov <github@blinkov.ru>
1.6 KiB
1.6 KiB
toc_title |
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WITH |
WITH子句
本节提供对公共表表达式的支持 (CTE),所以结果 WITH
子句可以在其余部分中使用 SELECT
查询。
限制
- 不支持递归查询。
- 当在section中使用子查询时,它的结果应该是只有一行的标量。
- Expression的结果在子查询中不可用。
例
示例1: 使用常量表达式作为 “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
示例2: 从SELECT子句列表中逐出sum(bytes)表达式结果
WITH sum(bytes) as s
SELECT
formatReadableSize(s),
table
FROM system.parts
GROUP BY table
ORDER BY s
例3: 使用标量子查询的结果
/* 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
例4: 在子查询中重用表达式
作为子查询中表达式使用的当前限制的解决方法,您可以复制它。
WITH ['hello'] AS hello
SELECT
hello,
*
FROM
(
WITH ['hello'] AS hello
SELECT hello
)
┌─hello─────┬─hello─────┐
│ ['hello'] │ ['hello'] │
└───────────┴───────────┘