FAQ: Common Metrics - 1 Day Retention 2

This community-built FAQ covers the “1 Day Retention 2” exercise from the lesson “Common Metrics”.

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This exercise can be found in the following Codecademy content:

SQL: Analyzing Business Metrics

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Hello there,
I’m doing the 1 Day Retention exercise and I have a question on the SELF-JOIN part.

This is the code that allows us to join the two tables to each other in order to compare the players on Day N to the player on Day N+1.

SELECT DATE(G1.created_at) AS Date, G1.user_id
FROM gameplays AS G1
JOIN gameplays AS G2
ON G1.user_id = G2.user_id
ORDER BY 1
LIMIT 100;

I added a condition to see the result for one user_id (128), and the date and user_id from G2 (to see exactly what’s happening), and the result of the query is enclosed (

.

So what I don’t understand is why or how come the result groups it by day?
What I mean is that we have first rows with G1.Date being 04/08 and only G2.Date changing to 04/08, 05/08, 08/08, etc. and then at the last row, G1.Date changes to 05/08 and G2.Date to 04/08.

I thought a JOIN compare the matching values and then represents the selected values, so in our case, we should have
G1. Date | user_id | G2.Date
1st row: 04/08 | 128 | 04/08
2nd row: 05/08 | 128 | 05/08
3rd row: 08/08 | 128 | 08/08
etc.

I hope it’s clear and thanks for your help!

Ziad

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