How to write sql select query for following

Question

I have following tables

Table 1: person

columns: id,name,address,code

Table 2: carDetails

columns: id,person_id,car_brand
constraints: FL==>carDetails(person_id) reference person(id)
Note: carDetails is having multiple details for single person

Table 3: mobileDetails

columns: id,person_id,mobile_brand
constraints: FL==>mobileDetails(person_id) reference person(id)
Note: mobileDetails is having multiple details for single person

Similarly i have lot of details like car and mobile for person

What I want to select is:

person(id),
person(name),
Array of carDetails(brand) belonging to that particular person(id)
Array of mobileDetails(brand) belonging to that particular person(id)

Answer 1

You should write this query using subqueries for the aggregation:

select p.*, c.car_brands, m.mobile_brands
from person p left join
     (select c.person_id, array_agg(cd.car_brand) as car_brands
      from car_details c
      group by c.person_id
     ) c
     on c.person_id = p.id left join
     (select m.person_id, array_agg(m.mobile_brand) as mobile_brands
      from mobile_details m
      group by m.person_id
     ) m
     on m.person_id = m.id;

Two notes:

• You want to use left join , in case you have no data in one of the tables for some people.
• You want to aggregate before joining to avoid duplicates. Although you could add distinct to array_agg() that incurs a performance penalty.

If you are filtering the people, it is often more efficient to do this using a subquery or (equivalently) as lateral join:

select p.*,
       (select array_agg(cd.car_brand) as car_brands
        from car_details c
        where c.person_id = p.id
       ) as car_brands,
       (select array_agg(m.mobile_brand) as mobile_brands
        from mobile_details m
        where m.person_id = p.id
       ) as mobile_brands
from person p;