同一表中的SQL嵌套组

Question

I have data that looks something like this:

id      key         value
--------------------------------
1       country     us
1       state       nj
1       city        trenton
2       country     us
2       state       nj
2       city        springfield
3       country     us
3       state       nj
3       city        belmar
4       country     us
4       state       ny
4       city        new york
4       country     us
4       state       ny
4       city        white plains
4       country     canada
4       state       ontario
4       city        toronto
5       country     us
5       state       nj
5       city        trenton

I'm trying to write a SQL query to group by unique country, state, city

country     state       city
--------------------------------------
us          nj          trenton
us          nj          springfield
us          nj          belmar
us          ny          new york
us          ny          white plains
canada      ontario     toronto

I'm able to get DISTINCT country, state, and city, but I'm not sure how to join them together with the ID column.

Answer 1

It's worth stating for the record that this is a data mess and begs for "normalization". Having said that, you might try:

    SELECT cn.value as country, st.value as state, ct.value as city
    FROM table cn
    LEFT JOIN table st ON cn.id = st.id
    LEFT JOIN table st ON cn.id = ct.id
    WHERE cn.key = 'country'
    AND st.key = 'state'
    AND ct.key = 'city'
    ORDER BY cn.value , st.value , ct.value 

Answer 2

You can use conditional aggregation for this:

select t.id,
       max(case when t.key = 'country' then t.value end) as country,
       max(case when t.key = 'state' then t.value end) as state,
       max(case when t.key = 'city' then t.value end) as city
from t
group by id;

This has the advantage of keeping all id s, even when not all three values are populated.