Joining a table with dictionary to unique id column in different table - sql

Question

I have two tables, Table A and Table B that I am trying to extract information from and create a resulting table.
Table A
user_ids value
{user_123, user_234} apples
{user_456, user_123} oranges
{user_234} kiwi
Table B
id name
123 John Smith
234 Jane Doe
456 John Doe
I want to join the two tables in a way that will result in the following:
Table C
user_ids value user_names
{user_123, user_234} apples {John Smith, Jane Doe}
{user_456, user_123} oranges {John Doe, John Smith}
{user_234} kiwi {Jane Doe}
Any help would be really appreciated!

Answer 1

Others have already encouraged you to normalize your design and there are numerous posts on why this is recommended. Using your current shared dataset, the following was done using postgres where the user_ids was treated as a text array. I also tested with the user_ids as text as used cast(user_ids as text[]) to convert it to a text array
See fiddle and result below:
Schema (PostgreSQL v11)
CREATE TABLE table_a (
"user_ids" text[],
"value" VARCHAR(7)
);
INSERT INTO table_a
("user_ids", "value")
VALUES
('{user_123, user_234}', 'apples'),
('{user_456, user_123}', 'oranges'),
('{user_234}', 'kiwi');
CREATE TABLE table_b (
"id" INTEGER,
"name" VARCHAR(10)
);
INSERT INTO table_b
("id", "name")
VALUES
('123', 'John Smith'),
('234', 'Jane Doe'),
('456', 'John Doe');
The first CTE user_values creates a row for each user_id and value. The second CTE merged_values joins table_b on the pattern user_<user_id> if it exists and ensures unique results using DISTINCT. The final projection groups based on values and users array_agg to collect all user_ids or names into a single row.
Query #1
WITH user_values AS (
SELECT
unnest(a.user_ids) user_id,
a.value
FROM
table_a a
),
merged_values AS (
SELECT DISTINCT
a.user_id,
a.value,
b.name
FROM
user_values a
LEFT JOIN
table_b b ON a.user_id = CONCAT('user_',b.id)
)
SELECT
array_agg(user_id) user_ids,
value,
array_agg(name) "names"
FROM
merged_values
GROUP BY
value;
user_ids
value
names
user_123,user_456
oranges
John Smith,John Doe
user_123,user_234
apples
John Smith,Jane Doe
user_234
kiwi
Jane Doe
View on DB Fiddle