Joining/Querying Self-Referential “Grandchildren” Tables
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
asked Nov 28 '18 at 2:12
robbieperry22robbieperry22
186117
add a comment |
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
asked Nov 28 '18 at 2:12
robbieperry22robbieperry22
186117
add a comment |
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
asked Nov 28 '18 at 2:12
robbieperry22robbieperry22
186117
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
asked Nov 28 '18 at 2:12
robbieperry22robbieperry22
186117
asked Nov 28 '18 at 2:12
robbieperry22robbieperry22
186117
asked Nov 28 '18 at 2:12
robbieperry22robbieperry22
186117
asked Nov 28 '18 at 2:12
robbieperry22robbieperry22
186117
asked Nov 28 '18 at 2:12
robbieperry22robbieperry22
186117
186117
add a comment |
add a comment |
1 Answer
1
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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.
answered Nov 28 '18 at 6:04
Laurenz AlbeLaurenz Albe
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
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
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.
answered Nov 28 '18 at 6:04
Laurenz AlbeLaurenz Albe
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
add a comment |
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.
answered Nov 28 '18 at 6:04
Laurenz AlbeLaurenz Albe
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
add a comment |
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.
answered Nov 28 '18 at 6:04
Laurenz AlbeLaurenz Albe
50k102950
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.
answered Nov 28 '18 at 6:04
Laurenz AlbeLaurenz Albe
50k102950
answered Nov 28 '18 at 6:04
Laurenz AlbeLaurenz Albe
50k102950
answered Nov 28 '18 at 6:04
Laurenz AlbeLaurenz Albe
50k102950
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
add a comment |
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
add a comment |
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