Question

description

Syntax

COUNT([DISTINCT] expr)

用于返回满足要求的行的数目

example

MySQL > select count(*) from log_statis group by datetime;
+----------+
| count(*) |
+----------+
| 28515903 |
+----------+

MySQL > select count(datetime) from log_statis group by datetime;
+-------------------+
| count(`datetime`) |
+-------------------+
|         28521682  |
+-------------------+

MySQL > select count(distinct datetime) from log_statis group by datetime;
+-------------------------------+
| count(DISTINCT `datetime`)    |
+-------------------------------+
|                       71045   |
+-------------------------------+

keyword

COUNT