How to join where some values only exist in one table?

Question

I have two tables, which look something like this:

Table_1:

Shop_ID | Offer_ID | Metric_1
--------|----------|---------
AAA     | 111      | 1
AAA     | 222      | 2
BBB     | 222      | 3

Table 2:

Shop_ID | Offer_ID | Metric_2
--------|----------|---------
AAA     | 111      | 1
AAA     | 222      | 2
BBB     | 111      | 3
CCC     | 111      | 4
CCC     | 222      | 5

I want to join them into one combined dataset, like this. The result needs to show both of the two metrics for all shops and their offers, even when the row containing that combination of store & offer is only present in one of the tables.

Shop_ID | Offer_ID | Metric_1 | Metric_2
--------|----------|----------|---------
AAA     | 111      | 1        | 1
AAA     | 222      | 2        | 2
BBB     | 111      | NULL     | 3
BBB     | 222      | 3        | NULL
CCC     | 111      | NULL     | 4
CCC     | 222      | NULL     | 5

Does anyone know how to do this, please?

Answer 1

You want a FULL JOIN :

select coalesce(t1.shop_id, t2.shop_id), coalesce(t1.offer_id, t2.offer_id),
       t1.metric_1, t2.metric_2
from table_1 t1 full join
     table_2 t2
     on t1.shop_id = t2.shop_id and t1.offer_id = t2.offer_id;

Or a bit simpler using USING :

select shop_id, offer_id, t1.metric_1, t2.metric_2
from table_1 t1 full join
     table_2 t2
     using (shop_id, offer_id);