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Discover how to efficiently select cases from your SQL tables where IDs are unique and meet specific criteria. Learn techniques with real examples!
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This video is based on the question https://stackoverflow.com/q/64055348/ asked by the user 'Dylan Russell' ( https://stackoverflow.com/u/7350570/ ) and on the answer https://stackoverflow.com/a/64055727/ provided by the user 'Mikhail Berlyant' ( https://stackoverflow.com/u/5221944/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Query Unique Non-Duplicated Records in SQL: A Step-by-Step Guide

When working with SQL databases, you may encounter scenarios where you want to select records from a table while ensuring that certain IDs are not duplicated. This can provide valuable insights, especially when filtering records based on specific values. In this post, we’ll navigate a common problem in SQL: how to select records that meet certain conditions while ensuring the IDs are unique.

The Problem: Understanding the Requirements

Let's start by outlining the problem with an example. Consider the following table structure:

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From this data, we have two requirements:

First Requirement: We want all records where ncode equals 1 and code equals "ABC".

Second Requirement: We only want these records if the id is not duplicated elsewhere in the table. Specifically, if an id has an ncode greater than 1 anywhere in the table, it should be excluded from the results.

Given these requirements, if we apply a simple SQL query like:

[[See Video to Reveal this Text or Code Snippet]]

we'd satisfy the first condition, but this query would still return all records without checking for duplicates of id.

The Solution: Writing the SQL Query

Using Window Functions

To solve this problem efficiently in SQL, we can leverage window functions. By partitioning data based on id, we can count the occurrences of non-matching ncode. Here's how you can structure the query:

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Explanation of the Query

Subquery: The inner query calculates a new column cnt which counts how many times ncode greater than 1 appears, grouped by the same id.

Filtering: The outer query then filters results to only include records where code is "ABC", ncode equals 1, and cnt equals 0 (meaning no duplicated id exists with a higher ncode).

Ordering: Finally, results are ordered by id.

This results in the desired output:

[[See Video to Reveal this Text or Code Snippet]]

Google BigQuery Solution

If you are using Google BigQuery, the query syntax looks a bit different but achieves the same results. Here’s one way to do it:

[[See Video to Reveal this Text or Code Snippet]]

Breakdown of the BigQuery Query

Aggregating Data: It groups records by id and retrieves the ANY_VALUE of ncode and code.

Count Check: Using HAVING COUNT(1) = 1 ensures only ids that appear once in the grouped data are retrieved.

Conditional Filter: Finally, it checks that the code is "ABC" and ncode is 1.

Conclusion

By using window functions or appropriate aggregation techniques in SQL, we can effectively filter data to meet specific requirements, such as excluding duplicates. This approach ensures that we only receive the records we are interested in, thereby enhancing the quality of our data insights.

In scenarios where you're dealing with complex data like this, remember that understanding how to leverage SQL's powerful functionalities can save you a significant amount of time and effort in data analysis.



If you have any more questions or need further clarification on SQL queries or data handling, feel free to reach out in the comments below!