Intersect on two array_agg columns in the same row

Question

I have a simple Postgres dataset that looks like this:

INSERT INTO mytable (day, person)
values
('Monday', 'A'),
('Monday', 'B'),
('Tuesday', 'A'),
('Thursday', 'B');

I then run a query that yields two array_aggs as follows:

SELECT *
FROM (select day as d1,
             array_agg(distinct person) as agg1
      from mytable
      group by day) AS AA
   cross join
     (select day as d2,
             array_agg(distinct person) as agg2
      from mytable
      group by day) AS BB

which yields this dataset:

Monday, {A,B}, Monday, {A,B}
Monday, {A,B}, Thursday, {B}
Monday, {A,B}, Tuesday, {A}
Thursday, {B}, Monday, {A,B}
Thursday, {B}, Thursday, {B}
Thursday, {B}, Tuesday, {A}
Tuesday, {A}, Monday, {A,B}
Tuesday, {A}, Thursday, {B}
Tuesday, {A}, Tuesday, {A}

I would like to add a fifth column to this query that identifies that number of repeat entries in agg1 and agg2 across each row.

So for example, the first row would be 2 and the second row would be 1. I was hoping to do it as follows, but this gives me a ambiguous syntax error:

SELECT *, count(select unnest(agg1) intersect select unnest(agg2))
FROM (select day as d1,
             array_agg(distinct person) as agg1
      from mytable
      group by day) AS AA
   cross join
     (select day as d2,
             array_agg(distinct person) as agg2
      from mytable
      group by day) AS BB

Answer 1

Postgresql has LATERAL.

Which can be used to do something with the content of fields on record level.

create table mytable (day varchar(30), person varchar(1));

INSERT INTO mytable (day, person)
values
('Monday', 'A'),
('Monday', 'B'),
('Tuesday', 'A'),
('Thursday', 'B');

SELECT *
FROM (
  select day as d1,
             array_agg(distinct person) as agg1
      from mytable
      group by day) AS AA
   cross join
     (select day as d2,
             array_agg(distinct person) as agg2
      from mytable
      group by day
) AS BB
CROSS JOIN LATERAL 
(
   SELECT COUNT(*) AS MatchingPersons
   FROM
   (
     SELECT unnest(agg1) person
     INTERSECT
     SELECT unnest(agg2)
   ) q
) lat 

d1       | agg1  | d2       | agg2  | matchingpersons
:------- | :---- | :------- | :---- | --------------:
Monday   | {A,B} | Monday   | {A,B} |               2
Thursday | {B}   | Monday   | {A,B} |               1
Tuesday  | {A}   | Monday   | {A,B} |               1
Monday   | {A,B} | Thursday | {B}   |               1
Thursday | {B}   | Thursday | {B}   |               1
Tuesday  | {A}   | Thursday | {B}   |               0
Monday   | {A,B} | Tuesday  | {A}   |               1
Thursday | {B}   | Tuesday  | {A}   |               0
Tuesday  | {A}   | Tuesday  | {A}   |               1

db<>fiddle here