Trying to Add a DISTINCT COUNT Column in Table to INNER JOIN Query in SQLite

In this article, we will explore how to add a DISTINCT COUNT column in table to an INNER JOIN query in SQLite. We will dive deep into the inner workings of SQL queries and explain the concept of subqueries and join operations.

Understanding INNER JOIN

Before we proceed, it’s essential to understand what an INNER JOIN is. An INNER JOIN returns records that have matching values between two tables. In this case, we’re joining the player_season table with the player table based on the player_id column.

-- ER Diagram
+---------+
| player_id |
+---------+
| 1        |
| 2        |
| 3        |

+----------+
| team_id |
+----------+
| 1610612745|
| 1610612746|

+-----------+-------------+------------+
| player_id | season       | games_played|
+-----------+-------------+------------+
| 1         | Spring      | 10         |
| 1         | Summer      | 15         |
| 2         | Winter      | 12         |
| 3         | Fall        | 8          |
+-----------+-------------+------------+

-- INNER JOIN
SELECT player.player_name, player_season.*
FROM player_season
INNER JOIN player
ON player_season.player_id = player.player_id
WHERE player_season.team_id = 1610612745;

Understanding Subqueries

A subquery is a query nested inside another query. In this case, we’re using a subquery to count the number of games played by each player.

-- Subquery
SELECT 
DISTINCT COUNT (player_id)
FROM   player_game_log
AS games_played;

The Problem

The problem with our original query is that it’s not directly possible to add a DISTINCT COUNT column in table to an INNER JOIN query. We can’t simply add a new column to the player_season table and expect SQLite to magically fill it with the count of games played.

Solution: Adding the Subquery Directly

To solve this problem, we need to modify our approach. Instead of using a subquery, we can use a JOIN operation directly in the SELECT statement. This is done by adding the subquery as a correlated subquery that references the player_id column in the main query.

SELECT player.player_name,
       (SELECT DISTINCT COUNT(player_id)
        FROM   player_game_log
        WHERE  player_game_log.player_id = player.player_id) AS games_played,
       player_season.*
FROM   player_season
INNER JOIN player
ON player_season.player_id = player.player_id
WHERE player_season.team_id = 1610612745 

In this revised query, the subquery is added as a correlated subquery that references the player_id column in the main query. This allows us to count the number of games played by each player directly in the SELECT statement.

How it Works

Let’s break down how this works:

1. The subquery (SELECT DISTINCT COUNT(player_id) FROM player_game_log WHERE player_game_log.player_id = player.player_id) counts the number of rows in the player_game_log table where the player_id matches the current row being processed.
2. This count is then correlated to each row in the player_season table using the same player_id.
3. The resulting values are added as a new column called games_played.

Benefits

Using this approach has several benefits:

• Improved performance: By avoiding the need for an outer query, we can improve the overall performance of our query.
• Simplified logic: This approach simplifies our query logic by allowing us to combine multiple operations into a single statement.

Example Use Case

Suppose we have two tables: players and games. The players table contains information about each player, including their name and team ID. The games table contains information about each game played by each player, including the game date and score.

-- Players Table
+---------+
| player_id |
+---------+
| 1        |
| 2        |
| 3        |

+----------+
| team_id |
+----------+
| 1610612745|
| 1610612746|

-- Games Table
+-----------+------------+--------+
| player_id | game_date  | score  |
+-----------+------------+--------+
| 1         | 2022-01-01 | 100    |
| 1         | 2022-02-01 | 120    |
| 2         | 2022-03-01 | 90     |
| 3         | 2022-04-01 | 110    |
+-----------+------------+--------+

We want to get the name of each player and the total number of games played by each player.

SELECT p.player_name,
       (SELECT DISTINCT COUNT(g.player_id)
        FROM   games g
        WHERE  g.player_id = p.player_id) AS total_games_played
FROM players p;

In this example, we use a correlated subquery to count the number of rows in the games table where the player_id matches the current row being processed. The resulting values are added as a new column called total_games_played.

Conclusion

Adding a DISTINCT COUNT column in table to an INNER JOIN query can be challenging, but it’s not impossible. By using correlated subqueries and joining tables based on common columns, we can solve this problem efficiently and effectively.

Remember to always consider the performance implications of your queries and optimize them whenever possible. With practice and experience, you’ll become proficient in writing efficient SQL queries that meet your needs.

Last modified on 2023-12-18