Microsoft SQL Server Conditional Joining based on 2 columns - sql

Question

I am looking to join 3 tables, all with the same data except one column is a different name (different date for each of the the 3 tables). The three tables look like the following. The goal is if a condition exists in table 1 AND/OR table 2 determine if a condition does or does not exist in table 3 for each individual id/condition. I'm currently left joining table 2 to table 1 but I'm aware that is not accounting for if a condition in table 2 exists that is not in table it is not being accounted for, anyways, any help would into this would be useful.
Table 1
id place Condition_2018
123 ABC flu
456 ABC heart attack
Table 2
id place Condition_2019
123 ABC flu
789 def copd
Table 3
id place Condition_2020
456 ABC heart attack
789 def copd
123 ABC flu
OUTPUT:
Table 2
id place Condition_2018 Condition_2019 Condition_2020
123 ABC flu flu flu
456 ABC heart attack null heart attack
789 def NULL copd copd
Thank you!

Answer 1

How about this (SQL Server syntax)...
SELECT
x.id
, x.place
, x.Condition_2018
, x.Condition_2019
, t3.Condition_2020
FROM (
SELECT
COALESCE(t1.id, t2.id) AS id
, COALESCE(t1.place, t2.place) AS place
, t1.Condition_2018
, t2.Condition_2019
FROM Table1 AS t1
FULL OUTER JOIN Table2 AS t2 ON t1.id = t2.id AND t1.place = t2.place
) AS x LEFT JOIN Table3 AS t3 ON x.id = t3.id AND x.place = t3.place

Answer 2

If your database supports full join, you can just do:
select
id,
place,
t1.condition_2018,
t2.condition_2019,
t3.condition_2020
from table1 t1
full join table2 t2 using(id, place)
full join table3 t3 using(id, place)
Otherwise, it is a bit more complicated: union all and aggregation is one method:
select
id,
place,
max(condition_2018) condition_2018,
max(condition_2019) condition_2019,
max(condition_2020) condition_2020
from (
select id, place, condition_2018, null condition_2019, null condition 2020 from table1
union all
select id, place, null, condition_2019, null from table2
select id, place, null, null, condition_2020 from table3
) t
group by id, place

Answer 3

You seem to want everything in Table3 and matches in the other two tables. That is just left joins:
select t3.id, t3.place,
t1.condition_2018, t2.condition_2019,
t3.condition_2020
from table3 t3 left join
table2 t2
on t3.id = t2.id and t3.place = t2.place left join
table1 t1
on t3.id = t1.id and t3.place = t1.place;

Answer 4

You need a full outer join of table1 and table2 and a left join to table3:
select
coalesce(t1.id, t2.id) id,
coalesce(t1.place, t2.place) place,
t1.Condition_2018,
t2.Condition_2019,
t3.Condition_2020
from table1 t1 full outer join table2 t2
on t2.id = t1.id
left join table3 t3
on t3.id = coalesce(t1.id, t2.id)
See the demo.
Results:
> id | place | Condition_2018 | Condition_2019 | Condition_2020
> --: | :---- | :------------- | :------------- | :-------------
> 123 | ABC | flu | flu | flu
> 456 | ABC | heart attack | null | heart attack
> 789 | def | null | copd | copd