SQL - How to pick the best available value for each column for each ID from multiple tables? - sql

Question

I have two tables with the same variables referring to attributes of a person.
How can I combine data from two such tables picking the best available value for each column from each table for each field?
Requirements:
For each field, I would like to fill it with a value from either one of the tables, giving a preference to table 1.
Values can be NULL in either table
In the combined table, the value for column 1 could come from table 2 (in case table 1 is missing a value for that person) and the value for column 2 could from table 1 (because both tables had a value, but the value from table 1 is preferred).
In my real example, I have many columns, so an elegant solution with less code duplication would be preferred.
Some users may exist in only one of the tables.
Example:
Table 1:
user_id | age | income
1 | NULL| 58000
2 | 22 | 60000
4 | 19 | 35000
Table 2:
user_id | age | income
1 | 55 | 55000
2 | 19 | NULL
3 | 22 | 33200
Desired output:
user_id | age | income
1 | 55 | 58000
2 | 22 | 60000
3 | 22 | 33200
4 | 19 | 35000

Answer 1

I think that's a full join and priorization logic with colaesce():
select user_id,
coalesce(t1.age, t2.age) as age,
coalesce(t1.income, t2.income) as income
from table1 t1
full join table2 t2 using(user_id)

Answer 2

Use full outer join if user_id in each table is unique.
SELECT
COALESCE(t1.user_id, t2.user_id) AS user_id,
GREATEST(t1.age, t2.age) AS age,
GREATEST(t1.income, t2.income) AS income
FROM t1
FULL OUTER JOIN t2 ON t1.user_id = t2.user_id

Answer 3

try like below using coalesce()
select t1.user_id, coalesce(t1.age,t2.age),
t1.income>t2.income then t1.income else t2.income end as income
table1 t1 join table2 t2 on t1.usesr_id=t2.user_id

Answer 4

You can use below code:
With TableA(Id,age,income) as
( --Select Common Data
select table_1.id,
--Select MAX AGE
case
when table_1.age> table_2.age or table_2.age is null then table_1.age else table_2.age
end,
--Select MAX Income
case
when table_1.income>table_2.income or table_2.income is null then table_1.income else table_2.income
end
from table_1 inner join table_2 on table_2.id=table_1.id
union all
-- Select Specific Data of Table 2
select table_2.id,table_2.age,table_2.income
from table_2
where table_2.id not in (select table_1.id from table_1)
union all
-- Select Specific Data of Table 1
select table_1.id,table_1.age,table_1.income
from table_1
where table_1.id not in (select table_2.id from table_2)
)select * from TableA