为什么“SELECT COUNT(DISTINCT< Column>)FROM< Table>”返回0? [英] Why "SELECT COUNT(DISTINCT <Column>) FROM <Table>" return 0?
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问题描述
我已经对大型表中的所有列执行了上面的查询在两种情况下,COUNT(DISTINCT)可以返回零。第一个是列/表达式的所有值计算为
NULL
。第二个是 WHERE
子句(或 JOIN
)过滤掉所有行。 如果您没有 WHERE
或 JOIN
,那么值都是 NULL
for < Columnb>
。
I have run the query above for all columns in a massive table (Billion rows) and everything is fine except a couple which are returning 0. How is this possible?
解决方案
COUNT(DISTINCT)
can return zero under two circumstances. The first is that all the values for the column/expression evaluate to NULL
. The second is that the WHERE
clause (or JOIN
) filters out all rows.
If you have no WHERE
or JOIN
, then the values are all NULL
for <Columnb>
.
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