# COUNT

## 函数说明 n

`COUNT()` 是聚合函数的一种，计算了查询结果的记录数，结果是一个 BIGINT 值。当没有匹配的行或 `COUNT(NULL)` 时返回 0。

## 函数语法

``````> COUNT(expr)
``````
``````> COUNT(distinct column)
``````

## 参数释义

expr 任何查询结果，既可以是列名，也可以是一个函数或者数学运算的结果。当不带 distinct 参数时也可以使用`*`，直接统计行数
distinct column 对列中重复值去重。

## 示例

``````drop table if exists tbl1,tbl2;
create table tbl1 (col_1a tinyint, col_1b smallint, col_1c int, col_1d bigint, col_1e char(10) not null);
insert into tbl1 values (0,1,1,7,"a");
insert into tbl1 values (0,1,2,8,"b");
insert into tbl1 values (0,1,3,9,"c");
insert into tbl1 values (0,1,4,10,"D");
insert into tbl1 values (0,1,5,11,"a");
insert into tbl1 values (0,1,6,12,"c");

> select count(col_1b) from tbl1;
+---------------+
| count(col_1b) |
+---------------+
|             6 |
+---------------+

> select count(*) from tbl1 where col_1d<10;
+----------+
| count(*) |
+----------+
|        3 |
+----------+

mysql> select count(distinct col_1b) from tbl1;
+------------------------+
| count(distinct col_1b) |
+------------------------+
|                      1 |
+------------------------+
``````