MySQL-使用LEFT JOIN和分组查询不返回计数为0的行

Question

I am running below mentioned query.

select c.id, c.code, c.name, count(a.iso_country) from countries c 
left join airports a on c.code = a.iso_country group by a.iso_country 
order by count(a.iso_country);

In my 'countries' table, I have 247 rows. In 'airports' table, 'iso_country' column maps to 'code' column in 'countries' table.

Below are the table definitions. Countries table -

CREATE TABLE `countries` (
`id` int(11) NOT NULL,
`code` varchar(2) NOT NULL,
`name` text,
`continent` text,
PRIMARY KEY (`id`),
UNIQUE KEY `code_UNIQUE` (`code`),
KEY `code_idx` (`code`)
)

Airports table -

CREATE TABLE `airports` (
`id` int(11) NOT NULL,
`type` text,
`name` text,
`continent` text,
`iso_country` varchar(2) DEFAULT NULL,
`iso_region` text,
PRIMARY KEY (`id`),
KEY `country_iso_code_fk_idx` (`iso_country`),
CONSTRAINT `country_fk` FOREIGN KEY (`iso_country`) REFERENCES `countries` 
(`code`) )

The issue I'm facing is - the query I mentioned above returns 242 countries - 241 countries with airports and 1 with null values for 'airports', but doesn't include other 5 countries who also don't have any airports. Please guide me what am I doing wrong in this query.

PS:- I am just a novice in SQL.

I'm running on MySQL 5.7 Community Edition. Thanks in advance.

Answer 1

You want a count of airports by country, including those where there are none, right?

Try this:

SELECT
    c.id, c.code, c.name, count(a.iso_country) AS airport_count
FROM
    countries c LEFT JOIN airports a ON c.code = a.iso_country
GROUP BY
    c.id, c.code, c.name
ORDER BY
    airport_count DESC;

Answer 2

Could it have something to do with NULL values? Your a.iso_country column is DEFAULT NULL and c.code is NOT NULL . Usually when comparing values, if either can be NULL , you want to use something like COALESCE to provide a second option in the case that the value is NULL . The reason for this being that NULL != '' and NULL != 0 . What if you join on COALESCE(a.iso_country, '') = COALESCE(c.code, '') ?

I'm also not sure how COUNT behaves when given a NULL . You may need to be careful there.

Answer 3

您可以在where子句中检查这种情况吗？在没有找到Airport的表中隐式检查null。

where (airports.name.is null)