MYSQL'选择计数（distcolt col）'结果错误？

Question

This is my table

CREATE TABLE `tabletest`     
( `id` bigint(20) NOT NULL ,     
`type` int(11) NOT NULL DEFAULT '0' ,     
`parent_id` bigint(20) NOT NULL DEFAULT '0' ,    
 PRIMARY KEY (`id`),     
KEY `idx_parent_id_type` (`parent_id`,`type`)     
) ENGINE=InnoDB DEFAULT CHARSET=utf8

data:

type   parent_id    
101    0    
101    4    
101    6    
101    7    
101    9

this is sql 1:

select count(DISTINCT parent_id) from tabletest where type = 101

the result 1:

3

this is sql 2:

select count(DISTINCT parent_id,type) from tabletest where type = 101

the result 2:

5

why? because the key?

Answer 1

It's a known MySQL bug, you could check it via other engines (spark sql or something). Try changing your MySQL version. See this

Answer 2

Well, I replicate your test with your code and dataset and my results is (under MariaDB5.5.52) :

select count(DISTINCT parent_id) from tabletest where type = 101;

->5

select count(DISTINCT parent_id,type) from tabletest where type = 101

->5

What is your version of MySQL ?

Answer 3

尝试这个 ：

SELECT DISTINCT(A.type),COUNT(*) AS CountNumber FROM tabletest AS A WHERE A.type=101 GROUP BY A.type