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'),
('1', '2', '2016-05-02', '6'),
('2', '3', '2017-06-25', '1'),
('3', '1', '2016-03-02', '0'),
('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:
Figure 1: LeetCode 511 - Game Play Analysis I - SQL Server Runtime
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