分组并加入同一表 Sql - PostgreSql

Question

I have a table like this:

Id | Name       | ParentId | Hierarchy
1  | California | 1        | 1
2  | San Deigo  | 1        | 2
3  | Bay Area   | 1        | 2
4  | San Fran   | 1        | 2

Hierachy 1 indicates top level like California. Hierarchy 2 indicate second level like state/region in California eg San Diego. Hierarchy 3 indicates third level like street in San Diego. Is it possible to get districts with parent codes with hierarchy 2. It looks like it needs group by and join in same table.

Answer 1

This answers the original version of the question.

Is it possible to get districts with parent codes with hierarchy 2?

This is just a select:

select t.districtname, t.parencdoe 
from t
where t.hierarhy = 2;

Your data is malformed. You should have a parentid column that refers to the primary key of the table. The parentcode is not appropriate. It also seems redundant to have columns like regionname in all rows.

Answer 2

I have comeup with the following solution:

select p.name as Parent,
       c.name as Child      
FROM
    theTable p
INNER JOIN theTable c ON p.id = c.parentId
where p.hierarchy = 1