Ensure Correct Numeric Ordering in Visit ID Comparison

Q: Ensure Correct Numeric Ordering in Visit ID Comparison

This question evaluates understanding of data typing and comparison semantics in SQL and Python, specifically how string versus numeric representations affect ordering and sequencing; it is categorized under Data Manipulation (SQL/Python) and examines both conceptual understanding and practical application.

Q: How do I practice SQL interview questions?

PracHub provides an interactive SQL console where you can write and test queries against real database schemas. Get instant feedback and compare your solution with the expected output.

Q: What difficulty level is this coding question?

This is a Medium difficulty Data Manipulation (SQL/Python) question, commonly asked during Technical Screen rounds at Amazon.

Q: What role is this question designed for?

This question is commonly asked for Data Scientist candidates at Amazon during technical interviews.

Question

visits

+----------+---------+-----------+---------------------+
| visit_id | user_id | page      | visit_ts            |
+----------+---------+-----------+---------------------+
| '1'      | 'u1'    | '/home'   | 2024-05-01 10:00:00 |
| '2'      | 'u2'    | '/home'   | 2024-05-01 10:05:00 |
| '10'     | 'u3'    | '/home'   | 2024-05-01 10:10:00 |
| '11'     | 'u1'    | '/about'  | 2024-05-02 11:00:00 |
| '12'     | 'u2'    | '/about'  | 2024-05-02 11:05:00 |

##### Scenario

Product analytics team has a Visits table where the primary key visit_id is stored as VARCHAR, even though it contains numeric strings. You need to analyze visit pairs for sequencing questions.

##### Question

Write an SQL query that returns all ordered pairs of visits (v1, v
2) such that v1.visit_id < v2.visit_id using lexicographic comparison on the VARCHAR visit_id column. Explain why directly using the < operator on VARCHAR IDs like '9' and '10' can yield unexpected ordering, and show how you would guarantee correct numeric ordering if required.

##### Hints

Think self-join: FROM visits t1 JOIN visits t2 ON t1.visit_id < t2.visit_id; discuss CAST/LPAD when numeric semantics are needed.

PracHub · Accepted Answer

This question evaluates understanding of data typing and comparison semantics in SQL and Python, specifically how string versus numeric representations affect ordering and sequencing; it is categorized under Data Manipulation (SQL/Python) and examines both conceptual understanding and practical application.

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