COUNT
Function Description n
COUNT() is a type of aggregate function that calculates the number of records of the query result, and the result is a BIGINT value. Returns 0 when there is no matching row or COUNT(NULL).
Function Syntax
> COUNT(expr)
> COUNT(distinct column)
Parameter definition
| parameters | Instructions |
|---|---|
| expr | Any query result can be either a column name or a result of a function or mathematical operation. You can also use `*` to directly count the number of rows |
| distinct column | Deduplicate the repeated values in the column. |
Example
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 |
+-----------------------------+