Concatenate multiple rows in an array with SQL on PostgreSQL

Question

I have a table constructed like this :

oid | identifier | value
1   | 10         | 101
2   | 10         | 102
3   | 20         | 201
4   | 20         | 202
5         


        

Answer 1

Simple example: each course have many lessons, so if i run code below:

SELECT
  lessons.course_id AS course_id,
  array_agg(lessons.id) AS lesson_ids
FROM lessons
GROUP BY
  lessons.course_id
ORDER BY
  lessons.course_id

i'd get next result:

┌───────────┬──────────────────────────────────────────────────────┐
│ course_id │                   lesson_ids                         │
├───────────┼──────────────────────────────────────────────────────┤
│         1 │ {139,140,141,137,138,143,145,174,175,176,177,147,... │
│         3 │ {32,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,... │
│         5 │ {663,664,665,649,650,651,652,653,654,655,656,657,... │
│         7 │ {985,984,1097,974,893,971,955,960,983,1045,891,97... │
│       ...                                                        │
└───────────┴──────────────────────────────────────────────────────┘