Joining/Querying Self-Referential “Grandchildren” Tables












0















Say I have a table as following:



countries



| ID    | Name    | Population | Continent |


Countries is self-referential, and has an association table:



alliances



| country_id | ally_id    |


I understand that I need to use the 'AS' keyword to join the table, say, as c1, c2, etc. But I can't quite wrap my head around how to go about this for grandchildren, great grandchildren, etc.



How would I write SQL, for example, to get the countries where they have an ally' who's ally's population is greater than 50 000 000?



I'm generating this SQL based off of models defined in code, so need to be able to support this kind of behaviour up to a user-defined depth.



Thanks!






sql postgresql







share|improve this question



























    0















    Say I have a table as following:



    countries



    | ID    | Name    | Population | Continent |


    Countries is self-referential, and has an association table:



    alliances



    | country_id | ally_id    |


    I understand that I need to use the 'AS' keyword to join the table, say, as c1, c2, etc. But I can't quite wrap my head around how to go about this for grandchildren, great grandchildren, etc.



    How would I write SQL, for example, to get the countries where they have an ally' who's ally's population is greater than 50 000 000?



    I'm generating this SQL based off of models defined in code, so need to be able to support this kind of behaviour up to a user-defined depth.



    Thanks!






    sql postgresql







    share|improve this question

























      0












      0








      0








      Say I have a table as following:



      countries



      | ID    | Name    | Population | Continent |


      Countries is self-referential, and has an association table:



      alliances



      | country_id | ally_id    |


      I understand that I need to use the 'AS' keyword to join the table, say, as c1, c2, etc. But I can't quite wrap my head around how to go about this for grandchildren, great grandchildren, etc.



      How would I write SQL, for example, to get the countries where they have an ally' who's ally's population is greater than 50 000 000?



      I'm generating this SQL based off of models defined in code, so need to be able to support this kind of behaviour up to a user-defined depth.



      Thanks!






      sql postgresql







      share|improve this question














      Say I have a table as following:



      countries



      | ID    | Name    | Population | Continent |


      Countries is self-referential, and has an association table:



      alliances



      | country_id | ally_id    |


      I understand that I need to use the 'AS' keyword to join the table, say, as c1, c2, etc. But I can't quite wrap my head around how to go about this for grandchildren, great grandchildren, etc.



      How would I write SQL, for example, to get the countries where they have an ally' who's ally's population is greater than 50 000 000?



      I'm generating this SQL based off of models defined in code, so need to be able to support this kind of behaviour up to a user-defined depth.



      Thanks!






      sql postgresql




      sql postgresql






      share|improve this question













      share|improve this question











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      asked Nov 28 '18 at 2:12









      robbieperry22robbieperry22

      186117




      186117
























          1 Answer
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          The query for your example is



          SELECT c1.name
          
FROM countries AS c1
          
   JOIN alliances AS a ON c1.id = a.country_id
          
   JOIN countries AS c2 ON a.ally_id = c2.id
          
WHERE c2.population > 50000000;


          If this was a homework question (and it looks like one) you have found the sucker who does it for you.






          share|improve this answer
























          • Thanks for the note. And actually no haha, it's not homework. I just used that example for explanatory purposes. I'm developing a library at work in Golang, that handles basic crud operations from user-defined data structures.

            – robbieperry22
            Nov 28 '18 at 17:35











          Your Answer






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          1 Answer
          1






          active

          oldest

          votes








          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          1














          The query for your example is



          SELECT c1.name
          
FROM countries AS c1
          
   JOIN alliances AS a ON c1.id = a.country_id
          
   JOIN countries AS c2 ON a.ally_id = c2.id
          
WHERE c2.population > 50000000;


          If this was a homework question (and it looks like one) you have found the sucker who does it for you.






          share|improve this answer
























          • Thanks for the note. And actually no haha, it's not homework. I just used that example for explanatory purposes. I'm developing a library at work in Golang, that handles basic crud operations from user-defined data structures.

            – robbieperry22
            Nov 28 '18 at 17:35
















          1














          The query for your example is



          SELECT c1.name
          
FROM countries AS c1
          
   JOIN alliances AS a ON c1.id = a.country_id
          
   JOIN countries AS c2 ON a.ally_id = c2.id
          
WHERE c2.population > 50000000;


          If this was a homework question (and it looks like one) you have found the sucker who does it for you.






          share|improve this answer
























          • Thanks for the note. And actually no haha, it's not homework. I just used that example for explanatory purposes. I'm developing a library at work in Golang, that handles basic crud operations from user-defined data structures.

            – robbieperry22
            Nov 28 '18 at 17:35














          1












          1








          1







          The query for your example is



          SELECT c1.name
          
FROM countries AS c1
          
   JOIN alliances AS a ON c1.id = a.country_id
          
   JOIN countries AS c2 ON a.ally_id = c2.id
          
WHERE c2.population > 50000000;


          If this was a homework question (and it looks like one) you have found the sucker who does it for you.






          share|improve this answer













          The query for your example is



          SELECT c1.name
          
FROM countries AS c1
          
   JOIN alliances AS a ON c1.id = a.country_id
          
   JOIN countries AS c2 ON a.ally_id = c2.id
          
WHERE c2.population > 50000000;


          If this was a homework question (and it looks like one) you have found the sucker who does it for you.







          share|improve this answer












          share|improve this answer



          share|improve this answer










          answered Nov 28 '18 at 6:04









          Laurenz AlbeLaurenz Albe

          50k102950




          50k102950













          • Thanks for the note. And actually no haha, it's not homework. I just used that example for explanatory purposes. I'm developing a library at work in Golang, that handles basic crud operations from user-defined data structures.

            – robbieperry22
            Nov 28 '18 at 17:35



















          • Thanks for the note. And actually no haha, it's not homework. I just used that example for explanatory purposes. I'm developing a library at work in Golang, that handles basic crud operations from user-defined data structures.

            – robbieperry22
            Nov 28 '18 at 17:35

















          Thanks for the note. And actually no haha, it's not homework. I just used that example for explanatory purposes. I'm developing a library at work in Golang, that handles basic crud operations from user-defined data structures.

          – robbieperry22
          Nov 28 '18 at 17:35





          Thanks for the note. And actually no haha, it's not homework. I just used that example for explanatory purposes. I'm developing a library at work in Golang, that handles basic crud operations from user-defined data structures.

          – robbieperry22
          Nov 28 '18 at 17:35




















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