LeetCode 511 - Game Play Analysis I

Database Language: SQL Server

Difficulty: Easy

Problem Description

Input

Table: Activity

| Column Name  | Type    |
| ------------ | ------- |
| player_id    | int     |
| device_id    | int     |
| event_date   | date    |
| games_played | int     |

(`player_id`, `event_date`) is the primary key (combination of columns with unique values) of this table.

This table shows the activity of players of some games.

Each row is a record of a player who logged in and played a number of games (possibly 0) before logging out on someday using some device.

Requirement

Write a solution to find the first login date for each player.

Return the result table in any order.

The result format is in the following example.

Examples

Example 1

Input

Activity table:

| player_id | device_id | event_date | games_played |
| --------- | --------- | ---------- | ------------ |
| 1         | 2         | 2016-03-01 | 5            |
| 1         | 2         | 2016-05-02 | 6            |
| 2         | 3         | 2017-06-25 | 1            |
| 3         | 1         | 2016-03-02 | 0            |
| 3         | 4         | 2018-07-03 | 5            |
Output
| player_id | first_login |
| --------- | ----------- |
| 1         | 2016-03-01  |
| 2         | 2017-06-25  |
| 3         | 2016-03-02  |

SQL Schema

CREATE TABLE [dbo].[Activity] (player_id INT NOT NULL, device_id INT, event_date DATE NOT NULL, games_played INT);
ALTER TABLE [dbo].[Activity] ADD CONSTRAINT PK_Ativity PRIMARY KEY (player_id, event_date);

TRUNCATE TABLE [dbo].[Activity];
INSERT INTO [dbo].[Activity] (player_id, device_id, event_date, games_played) VALUES ('1', '2', '2016-03-01', '5');
INSERT INTO [dbo].[Activity] (player_id, device_id, event_date, games_played) VALUES ('1', '2', '2016-05-02', '6');
INSERT INTO [dbo].[Activity] (player_id, device_id, event_date, games_played) VALUES ('2', '3', '2017-06-25', '1');
INSERT INTO [dbo].[Activity] (player_id, device_id, event_date, games_played) VALUES ('3', '1', '2016-03-02', '0');
INSERT INTO [dbo].[Activity] (player_id, device_id, event_date, games_played) VALUES ('3', '4', '2018-07-03', '5');

Solution

Given that the requirement is simply asking for the first login date for each player, this calls for the `MIN` aggregate function. The `MIN` aggregate function returns the minimum (lowest) value in a set. The query to get the first login date for each player is as follows:

# Final Solution Query
SELECT player_id, MIN(event_date) AS first_login
FROM Activity
GROUP BY player_id

Since the requirement wants the first login date to be called `first_login`, an alias is assigned to the `MIN(event_date)`. Also, the `GROUP BY player_id` is included in the query because not all columns in the `SELECT` statement are using an aggregate function (in this case, the `player_id` column). If the `GROUP BY player_id` is omitted, the following error will be generated:

Query 1 ERROR: Msg: 8120, Line 1, State: 1, Level: 16
Column 'Activity.player_id' is invalid in the select list because it is not contained in either
an aggregate function or the GROUP BY clause.

The query plan generated by SQL Server is as follows:

  |--Stream Aggregate(GROUP BY:([leetcode].[dbo].[Activity].[player_id])
    DEFINE:([Expr1002]=MIN([leetcode].[dbo].[Activity].[event_date])))
    |--Clustered Index Scan(OBJECT:([leetcode].[dbo].[Activity].[PK_Ativity]), ORDERED FORWARD)

Here's the fastest runtime for this query:

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