MySQL 中的 COUNT(id) 与 COUNT(*)

COUNT(id) vs. COUNT(*) in MySQL(MySQL 中的 COUNT(id) 与 COUNT(*))

问题描述

假设表中有一个主要字段id"(如速度等)，以下查询之间是否存在差异?

Is there a difference between the following queries, assuming there is a primary field 'id' in the table (as in speed, etc)?

SELECT COUNT(id) 
  FROM table

对比

SELECT COUNT(*) 
  FROM table

推荐答案

我知道问题是关于 MySQL，但就其价值而言，建议将 count(*) 用于 Oracle:这表明这是特定于数据库的(请参阅上面来自 BalusC 的评论).由于许多数据库(MS-SQL、MySQL)都有保存各种类型元数据的信息模式表，如果一种语法只是查找一个容易获得的值，而另一种则直接访问该表，那么肯定会有差异.在一天结束时:尝试不同的选项，看看 EXPLAIN 告诉你在幕后发生了什么.

I know the question is about MySQL, but for what it's worth, count(*) is recommended for Oracle: which goes to show that this is database specific (see comment above from BalusC). Since a lot of databases (MS-SQL, MySQL) have information schema tables that hold various types of metadata, there are bound to be differences if one syntax is simply looking up a readily-available value, and another is going straight to the table. At the end of day: try different options, an see what EXPLAIN is telling you is going on behind the scenes.

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