PostgreSQL Interview Question: what is string_agg() and when do you use it?

Title — string_agg()

Hi everyone, this post is about the Postgres useful function. Postgres offers a variety of useful functions thanks to committers and I am always grateful for such great community. I wanted to share one useful function, string_agg(), that I recently found. If you are a product manager or analysts who want to aggregate and analyze dataset, this entry is helpful.

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Problem Setting

Suppose your company are selling some products. Each row of this table contains the product name and the date it was sold in a market. You want to analyze product trends by season and day in a week.

Table Activities:

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| sell_date   | date    |
| product     | varchar |
+-------------+---------+