group-sold-products-by-the-date.sql

/*

1484. Group Sold Products By The Date
Table Activities:
+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| sell_date   | date    |
| product     | varchar |
+-------------+---------+
There is no primary key for this table, it may contains duplicates.
Each row of this table contains the product name and the date it was sold in a market.
Write an SQL query to find for each date, the number of distinct products sold and their names.
The sold-products names for each date should be sorted lexicographically.
Return the result table ordered by sell_date.
The query result format is in the following example.
Activities table:
+------------+-------------+
| sell_date  | product     |
+------------+-------------+
| 2020-05-30 | Headphone   |
| 2020-06-01 | Pencil      |
| 2020-06-02 | Mask        |
| 2020-05-30 | Basketball  |
| 2020-06-01 | Bible       |
| 2020-06-02 | Mask        |
| 2020-05-30 | T-Shirt     |
+------------+-------------+
Result table:
+------------+----------+------------------------------+
| sell_date  | num_sold | products                     |
+------------+----------+------------------------------+
| 2020-05-30 | 3        | Basketball,Headphone,T-shirt |
| 2020-06-01 | 2        | Bible,Pencil                 |
| 2020-06-02 | 1        | Mask                         |
+------------+----------+------------------------------+
For 2020-05-30, Sold items were (Headphone, Basketball, T-shirt), we sort them lexicographically and separate them by comma.
For 2020-06-01, Sold items were (Pencil, Bible), we sort them lexicographically and separate them by comma.
For 2020-06-02, Sold item is (Mask), we just return it.

*/

# V0

# V1
# https://zhuanlan.zhihu.com/p/264969726
# IDEA : GROUP_CONCAT()
# MySQL GROUP_CONCAT() function returns a string with concatenated non-NULL value from a group. Returns NULL when there are no non-NULL values.
# https://www.w3resource.com/mysql/aggregate-functions-and-grouping/aggregate-functions-and-grouping-group_concat.php
SELECT sell_date, COUNT(DISTINCT product) AS num_sold, 
GROUP_CONCAT(DISTINCT product ORDER BY product) AS products
FROM Activities
GROUP BY sell_date
ORDER BY sell_date;

# V2
# Time:  O(nlogn)
# Space: O(n)
SELECT sell_date,
       COUNT(DISTINCT(product)) AS num_sold, 
       GROUP_CONCAT(DISTINCT product ORDER BY product ASC SEPARATOR ',') AS products
FROM Activities
GROUP BY sell_date
ORDER BY sell_date ASC;