PostgreSQL: How to find the last descendant in a linear “ancestor-descendant” relationship

问题

I have the following DB structure:

RELATIONSHIP_TABLE
- id << primary key
- id_ancestor << foreign key to the same table
- id_entry << foreign key to "ENTRY_TABLE"

ENTRY_TABLE
- id
- name
...

The hierarchy in table "RELATIONSHIP_TABLE" is linear. That means a record can be at most ancestor of one other record. Examples:

1. record1
2. record2 <- record3 <- record4
3. record5 <- record7 <- record9 <- record12

Every record within a particular hierarchy has the same "id_entry". Now, I would like to find the last descendant with a specific "id_entry". The result of the examples below would be:

1. record1
2. record4
3. record12

Does anyone know a solution?

Thanks in advance :)

QStormDS

回答1:

SELECT * 
FROM relationship_table rt
WHERE rt.id_entry = 42
AND NOT EXISTS (
   SELECT * FROM relationship_table nx
   WHERE nx.id_entry = 42      -- you can possibly omit this clause  
   AND nx.id_ancestor = rt.id  -- No children poining to rt ...
   )
 ;

来源：https://stackoverflow.com/questions/16792296/postgresql-how-to-find-the-last-descendant-in-a-linear-ancestor-descendant-re