基本上， count(1)产生的结果与 count(*)相同:，它计算由 group by 子句定义的组中的记录数.

Basically, count(1) produces just the same result as count(*): that is, it counts the number of records in the group defined by the group by clause.

为什么? count(< expr>)对< expr> 的每个非 null 值进行计数.这里给它一个常量值 1 ，它永远不会为 null -因此它计算所有行.

Why? count(<expr>) counts every non-null value of <expr>. Here it is given a constant value, 1, that is never null - so it counts all rows.

我不推荐 count(1)；检查每一行是否为空需要仅计算所有行的数据库工作.一些数据库可能会将 count(1)优化为 count(*)(显然，此固定表达式永远不会为 null )，但是当您可以优化自己的时候又何必呢?只需始终使用 count(*).

I wouldn't recommend count(1); checking for nullity of each and every row requires more work for the database that just counting all rows. Some databases might optimize count(1) as count(*) under the hood (as it is obvious that this fixed expression will never be null), but why bother, when you can optimize yourself already? Just use count(*) consistently.