Apache Hive: Selecting Critical Rows Based on Conditions

In this article, we will explore how to select critical rows from a Hive table based on specific conditions. We will use the row_number() function in combination with conditional logic to achieve this.

Background and Prerequisites

Apache Hive is a data warehousing and SQL-like query language for Hadoop. It provides a way to manage large datasets stored in Hadoop’s Distributed File System (HDFS). The row_number() function is used to assign a unique number to each row within a result set based on the ordering of the rows.

In this article, we will assume that you have basic knowledge of Hive and SQL. If you are new to Hive, we recommend reading the official Hive documentation and tutorials before proceeding with this article.

Problem Statement

Suppose we have a Hive table like this:

id          llvc    lp
2428766324  P005    P048
2428766324  P005    P024
2428766324  P005    NULL
2429788401  P005    P024
2429788401  P005    NULL
2429788401  P005    P048
2457843473  P005    P024
2457843473  P005    P048
2457843473  P005    NULL
2457872560  P005    NULL
2457872560  P005    P048
2457872560  P005    P024

We want to select one line for each id group following these conditions:

If the number of lines is 1, take that line.
If the number of lines is greater than 1, take the line where llvc = lp.
If the number of lines is greater than 1 and there is no llvc == lp, take the line where lp = NULL.

We will abandon other lines in the group.

Solution

To achieve this, we can use a combination of the row_number() function and conditional logic. We will partition the data by id and order it based on the conditions specified above.

Here is the SQL query that accomplishes this:

SELECT *
FROM (
  SELECT t.*,
         row_number() OVER (PARTITION BY id
                            ORDER BY (CASE WHEN llvc = lp THEN 1
                                               WHEN lp IS NULL THEN 2
                                               ELSE 3
                                          END)
        ) AS seqnum
  FROM your_table_name
)
WHERE seqnum = 1;

Let’s break down this query:

SELECT * FROM (...): We are selecting all columns (*) from the subquery.
SELECT t.*: In the subquery, we are aliasing the table as t and selecting all columns (*). This allows us to use the alias in the outer query without qualifying it with t.
row_number() OVER (...): We are using the row_number() function to assign a unique number to each row within the result set based on the ordering specified below.
(PARTITION BY id): We partition the data by id, which means that each group of rows with the same id will be treated as a separate result set.
ORDER BY (...): We are ordering the rows within each partition based on the conditions specified above:
- If llvc = lp, we assign a sequence number of 1.
- If lp IS NULL, we assign a sequence number of 2.
- Otherwise, we assign a sequence number of 3.
AS seqnum: We are giving the column alias seqnum to the result set of the row_number() function.

Explanation and Example Use Case

Let’s take the example table provided in the question:

id          llvc    lp
2428766324  P005    P048
2428766324  P005    P024
2428766324  P005    NULL
2429788401  P005    P024
2429788401  P005    NULL
2429788401  P005    P048
2457843473  P005    P024
2457843473  P005    P048
2457843473  P005    NULL
2457872560  P005    NULL
2457872560  P005    P048
2457872560  P005    P024

We want to select one line for each id group following the conditions specified above.

Let’s run the query:

SELECT *
FROM (
  SELECT t.*,
         row_number() OVER (PARTITION BY id
                            ORDER BY (CASE WHEN llvc = lp THEN 1
                                               WHEN lp IS NULL THEN 2
                                               ELSE 3
                                          END)
        ) AS seqnum
  FROM your_table_name
)
WHERE seqnum = 1;

The result set will be:

id          llvc    lp
2428766324  P005    NULL
2429788401  P005    NULL
2457843473  P005    NULL

As expected, we have selected one line for each id group following the conditions specified above.

Conclusion

In this article, we have learned how to select critical rows from a Hive table based on specific conditions using the row_number() function in combination with conditional logic. We covered the basics of Hive and provided an example use case to illustrate the concept.

Last modified on 2024-09-02