count(*) vs count(column-name) - 哪个更正确? [英] count(*) vs count(column-name) - which is more correct?

问题描述

如果你在这两个例子中使用 count(*) 和 count(column-name) 会有区别吗?

Does it make a difference if you do count(*) vs count(column-name) as in these two examples?

我倾向于总是写 count(*) 因为它似乎更适合我的想法，它是一个聚合函数的概念，如果有道理的话.

I have a tendency to always write count(*) because it seems to fit better in my mind with the notion of it being an aggregate function, if that makes sense.

但我不确定它在技术上是否最好，因为我经常看到没有 * 的示例代码.

But I'm not sure if it's technically best as I tend to see example code written without the * more often than not.

计数(*):

select customerid, count(*), sum(price) 
from items_ordered
group by customerid
having count(*) > 1;

对比计数(列名):

SELECT customerid, count(customerid), sum(price)
FROM items_ordered
GROUP BY customerid
HAVING count(customerid) > 1;

推荐答案

• COUNT(*) 计算所有行
• COUNT(column) 只计算非 NULL 值
• COUNT(1) 与 COUNT(*) 因为 1 是非空表达式
• COUNT(*) counts all rows
• COUNT(column) counts non-NULLs only
• COUNT(1) is the same as COUNT(*) because 1 is a non-null expressions

您对COUNT(*) 或COUNT(column) 的使用应该基于仅所需的输出.

Your use of COUNT(*) or COUNT(column) should be based on the desired output only.

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