当Count（*）结果为Null时，返回1而不是0

本文介绍了当Count（*）结果为Null时，返回1而不是0的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

我从SQL Server获得的代码

My code from SQL Server:

SELECT ESTAGIO.SK_ESTAGIO, ISNULL(count(ESTAGIO.SK_ESTAGIO), 0) as how_many
 from ESTAGIO
 left join ESTAGIARIO
 on ESTAGIARIO.SK_ESTAGIO = ESTAGIO.SK_ESTAGIO
 group by
    ESTAGIO.SK_ESTAGIO

当 ESTAGIARIO表中不存在 ESTAGIO.SK_ESTAGIO时，它返回1而不是0，我已经尝试使用ISNULL（）， NULLIF（）和COALESCE（）仍然找不到上面的查询在应为0时返回1的问题。

When "ESTAGIO.SK_ESTAGIO" doesn't exist in the table "ESTAGIARIO" it returns 1 instead of 0, I already tried to use ISNULL(), NULLIF() and COALESCE() and still couldn't find the problem that is making the query above returning 1 when it should be 0.

推荐答案

您正在计算错误的字段。这样做，从外部联接表 ESTAGIARIO 中获取字段（不是从 ESTAGIO 中获取）：

You are counting the wrong field. Do it like this, taking the field from the outer joined table ESTAGIARIO (not from ESTAGIO):

SELECT ESTAGIO.SK_ESTAGIO, Count(ESTAGIARIO.SK_ESTAGIO) as how_many
 from ESTAGIO
 left join ESTAGIARIO
 on ESTAGIARIO.SK_ESTAGIO = ESTAGIO.SK_ESTAGIO
 group by
    ESTAGIO.SK_ESTAGIO

顺便说一句，。

BTW, count can never return null.

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