Finding the common element between two result sets in PostgreSQL - postgresql

Question

I have created the following tables.
CREATE TABLE table1 (
idx SERIAL PRIMARY KEY,
refs_person TEXT[],
id_gps TEXT
);
CREATE TABLE table2 (
idx SERIAL PRIMARY KEY,
id_person TEXT,
city TEXT,
location INTEGER
);
I populated them with this information.
Table 1
idx || refs_person || id_gps
1 || {id_per_234, id_per_456} || gps_20
2 || {id_per_568} || gps_23
3 || {id_per_345, id_per_334, id_per_340} || gps_45
Table 2
idx || id_person || city || location
1 || id_per_234 || Paris || 20
2 || id_per_999 || York || 20
3 || id_per_213 || Paris || 20
4 || id_per_334 || Paris || 32
If I try to find out id_person in Paris with location 20.
select id_person from table2 where city = 'Paris' and location = '20';
id_per_234
id_per_213
Then, if I try to find out refs_person with id_gps 20 for table 1
select refs_person from table1 where id_gps = '20';
{id_per_234, id_per_456}
I'm having an issue relating to finding the common element between this two results.
For the this example, I want to get id_per_234.
I have tried using intersect statement without any success.
Any ideas?

Answer 1

You need to unnest the array, best using a LATERAL join, before you can INTERSECT the results:
SELECT rp.rp
FROM table1
CROSS JOIN LATERAL unnest(table1.refs_person) AS rp(rp)
WHERE id_gps = 'gps_20'
INTERSECT
SELECT id_person
FROM table2
WHERE city = 'Paris' AND location = 20;
rp
------------
id_per_234
(1 row)