Question

I have a table with columns “one” and “two”:

a | x  
a | y  
a | z  
b | x  
b | z  
c | y  

I want to write a query to complement it with missing nested values

b | null | y  
c | null | x  
c | null | z  

Then I will select it with array_agg(two) group by one, such that

a {1 1 1}  
b {1 0 1}  
c {0 1 0}

And eventually export it in a CSV file with COPY query

What query should I write for the first step?

Answer 1

You can use a CROSS JOIN to build all the possible pairs of elements then a LEFT JOIN to check if each pair of elements exists:

SELECT
    T1.one,
    T2.two,
    CASE WHEN your_table.one IS NULL THEN 0 ELSE 1 END AS is_present
FROM (SELECT DISTINCT one FROM your_table) T1
CROSS JOIN (SELECT DISTINCT two FROM your_table) T2
LEFT JOIN your_table
ON T1.one = your_table.one AND T2.two = your_table.two

You can then add a GROUP BY T1.one and an ARRAY_AGG(...) to this query.