MySQL 查询计数和加入

Question

我有以下疑问：

SELECT shipments.file_number, file_creation_date, city_name, container_sequence
FROM cities, countries, shipments
LEFT OUTER JOIN container_details ON shipments.file_number = container_details.file_number
WHERE
destination_ocean_port = city_code
AND cities.country_code = countries.country_code
AND cities.country_code = 'CN'
AND file_creation_date > '2015-1-1'
AND file_creation_date < '2016-1-1' 

ORDER BY shipments.file_number;

它会像这样返回：

正如您所见，在某些情况下，每个 file_number 有多个 container_sequence。我怎样才能得到它，所以它只返回每个 file_number 之一，同时显示每个 file_number 有多少个 container_sequences 的计数？

我知道它可能与 joins 和 count() 有关，但我无法弄清楚。

谢谢

Answer 1

您需要使用 count() group by 函数和 group by 子句来获得所需的输出。我还重写了连接以使用具有显式连接语法的内部连接。

SELECT shipments.file_number, file_creation_date, city_name, count(container_sequence) as number_of_containers
FROM cities 
INNER JOIN countries ON cities.country_code = countries.country_code
INNER JOIN shipments ON shipments.destination_ocean_port = cities.city_code
LEFT OUTER JOIN container_details ON shipments.file_number = container_details.file_number
WHERE 
    cities.country_code = 'CN'
AND file_creation_date > '2015-1-1'
AND file_creation_date < '2016-1-1' 
GROUP BY shipments.file_number, file_creation_date, city_name
ORDER BY shipments.file_number;

Answer 2

SELECT shipments.file_number, file_creation_date, city_name, count(container_sequence) as total_container_sequence
FROM cities, countries, shipments
LEFT OUTER JOIN container_details ON shipments.file_number = container_details.file_number
WHERE
destination_ocean_port = city_code
AND cities.country_code = countries.country_code
AND cities.country_code = 'CN'
AND file_creation_date > '2015-1-1'
AND file_creation_date < '2016-1-1' 
GROUP BY shipments.file_number
ORDER BY shipments.file_number;