Скачать или смотреть How to Select All Values in Hive with Distinct of Two Columns

This guide explains how to select data from a Hive table while ensuring distinct values based on two specified columns. Learn the methodology and see an example query using HiveQL to achieve this.
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How to Select All Values in Hive with Distinct of Two Columns

As data continues to grow in volume and complexity, being able to manipulate and extract meaningful insights from it becomes crucial. One common requirement when dealing with datasets is to fetch distinct values based on specific columns. This guide will guide you through the process of selecting all the values from a Hive table while ensuring distinctness based on two columns: ce_id and filename.

The Problem at Hand

Imagine you have a Hive table structured as follows:

colAcolBce_idfilenamedtvj4gg40vj5gg55fr4gg40ty7yy32ae5ee43Your objective is to retrieve all records but ensure that the results are distinct based on ce_id and filename. You can easily envision the expected result, which would look something like this:

colAcolBce_idfilenamedtvj4gg40vj5gg55ty7yy32ae5ee43In this case, our task is to write an appropriate query to achieve this goal.

Solution: Using row_number()

The solution to selecting distinct rows based on two columns in Hive can be elegantly accomplished using the row_number() window function. This function allows us to partition our data by the specified columns and assign a sequential number to each row within those partitions.

Step-by-Step Breakdown of the Query

Here’s the structured SQL query you can use:

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

Let's Analyze the Query

Subquery:

First, we create a subquery that selects all columns from the table t.

We use row_number() OVER (PARTITION BY ce_id, filename ORDER BY dt) to assign a unique number to each row based on the partition created by ce_id and filename. The rows are ordered by the dt column, which means we will have the smallest dt first.

Filtering with WHERE:

The outer query then filters the results where seqnum = 1. This means we only keep the first row from each group identified by ce_id and filename, effectively achieving distinctness.

Important Note

It's essential to understand how the ORDER BY dt clause affects which row is selected. If you want a different row based on another condition, you can change the ordering specified in the ORDER BY clause.

Conclusion

By utilizing the row_number() function in Hive, you can effectively select distinct records based on multiple columns while maintaining simplicity and clarity in your SQL queries.

If you have ever faced the challenge of retrieving distinct values from large datasets, this method can significantly streamline your query operations. Happy querying!