count（*）vs count（column-name） - 这是更正确吗？ [英] count(*) vs count(column-name) - which is more correct?

问题描述

如果您 count（*） vs count（column-name）这两个例子？

我有一个倾向总是写 count（*），因为它似乎更适合我的思想与它的概念是一个集合函数，如果这是有道理的。

但我不知道如果它在技术上最好，因为我倾向于看到示例代码写的没有 * 更多

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

vs。 count（column-name）：

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

解决方案

• COUNT（*）对所有行计数


• COUNT（列） / li>
  
• COUNT（1）是相同的 COUNT（*） ，因为1是非空的表达式

您对 COUNT（*）或 COUNT（column）的使用应基于所需的输出仅。

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

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.

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

count(*):

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

vs. count(column-name):

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

解决方案

• COUNT(*) counts all rows
• COUNT(column) counts non-NULLs only
• COUNT(1) is the same as COUNT(*) because 1 is a non-null expressions

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

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