ClickHouse：从同列名的select中访问源列

Posted 2023-03-25

【中文标题】ClickHouse：从同列名的select中访问源列

【英文标题】：ClickHouse: access to the source column from select with the same column name

【发布时间】：2019-01-29 16:25:43

【问题描述】：

当我在 SELECT 块中具有相同的列名时，我无法访问 WHERE 块中的源列值。

我需要在表 test.test 上绑定 MATERIALIZED VIEW，该表聚合记录 WHERE idx = 1 并将新记录推送到具有不同 idx 值的同一表 test.test。

create table test.test (
    idx UInt8,
    val Int64
) engine Memory()

insert into test.test (idx, val)
values 
    (toUInt8(1), toInt64(1)),
    (toUInt8(1), toInt64(2)),
    (toUInt8(1), toInt64(3))

-- Not working
select 2 as idx, sum(val) as val
from test.test
where idx = 1

-- Working fine, but not allowed with materialized view
select _idx as idx, val
from (

    select 2 as _idx, sum(val) as val
    from test.test as t
    where t.idx = 1

)

预计

┌─idx─┬─val─┐
│   2 │   6 │
└─────┴─────┘

实际

┌─idx─┬─val─┐
│   2 │   0 │
└─────┴─────┘

【参考方案1】：

试试这个查询（只要考虑到求和将应用于一包插入的数据，而不是应用于表 test.test 中的所有行。换句话说，视图将包含多个idx==2 的行）：

CREATE MATERIALIZED VIEW test.test_mv TO test.test AS
SELECT
    toUInt8(2) AS idx,
    val
FROM
(
    SELECT sum(val) AS val
    FROM test.test
    WHERE idx = 1
)

我建议使用更适合您的情况的 SummingMergeTree 表引擎：

CREATE MATERIALIZED VIEW IF NOT EXISTS test.test_mv2
ENGINE = SummingMergeTree
PARTITION BY idx
ORDER BY idx AS
SELECT
    idx,
    sumState(val) as sum
FROM test.test
GROUP BY idx;



SELECT
    idx,
    sumMerge(sum)
FROM test.test_mv2
GROUP BY idx;