MySQL GROUP_CONCAT和多個查詢表

Question

我有七個要加入的表格：

Table   programs
Column  id    program_name    program_description
        1     Self Help       Self Help Description...
        2     Wellness        Wellness Description...
        3     Education       Education Description...
______________________________________________________

Table   county
Column  id    county_name
        1     Stark
        2     Portage
        3     Wayne
_________________________

Table   services
Column  id    service_name    service_description
        1     Counseling      Counseling Description...
        2     Group Therapy   Group Therapy Description...
        3     Evaluation      Evaluation Description...
__________________________________________________________

Table   population
Column  id    population_name
        1     Adults
        2     Children
        3     Youth
_____________________________

Table   program_county
Column  id    program_id    county_id
        1     1             2
        2     1             3
        3     2             1
        4     2             2
_____________________________________

Table   program_service
Column  id    program_id    service_id
        1     1             2
        2     1             3
        3     2             1
        4     2             2
        5     2             3
______________________________________

Table   program_population
Column  id    program_id    population_id
        1     1             3
        2     2             1
        3     2             3
        4     3             1
_________________________________________

我正在嘗試編寫一個查詢，該查詢將為每個程序返回一行，並在program_county，program_services和program_population表中檢索相關的行，並查找這些服務的名稱，人口和縣將它們分別顯示在一個字段中。 喜歡：

id   program_name    program_description         Counties           Services                                 Population Served
1    Self Help       Self Help Description...    Portage, Wayne     Group Therapy, Evaluation                Youth
2    Wellness        Wellness Description        Stark, Portage     Counseling, Group Therapy, Evaluation    Adults, Youth

我知道我必須使用聯接和GROUP_CONCAT，但是我很失落。

Answer 1

如果要查看所有程序，而不管它們是與縣，服務還是人口相關聯，就需要使用外部聯接。 同樣，您應該在要連接的值周圍使用coalesce()以正確處理NULL值。

這樣的事情應該起作用：

select programs.id, programs.program_name, programs.program_description,
  group_concat(distinct coalesce(county.county_name,'')) as "Counties",
  group_concat(distinct coalesce(services.service_name,'')) as "Services",
  group_concat(distinct coalesce(population.population_name,'')) as "Population Served"
from programs 
  left outer join program_county on program_county.program_id = programs.id
  left outer join county on county.id = program_county.county_id
  left outer join program_service on program_service.program_id = programs.id
  left outer join services on services.id = program_service.service_id
  left outer join program_population on program_population.program_id = programs.id
  left outer join population on population.id = program_population.population_id
group by programs.id, programs.program_name, programs.program_description
order by programs.id