Hive QL Retrieve Max Value: A Step-by-Step Guide

Introduction

Hive QL is a query language used to perform calculations and aggregations on data in Hive, a popular data warehousing and big data platform. In this article, we will explore how to use Hive QL to retrieve the maximum value for a specific column based on another column.

Understanding the Problem Statement

The problem statement presents a scenario where we have two columns: start_time and time_stamp. The start_time column represents the starting time of a job, while the time_stamp column records the timestamp when each work is done. We want to retrieve the start_time of jobs that have ended with the maximum time_stamp value.

Analyzing the Sample Data

Let’s analyze the provided sample data:

We want to retrieve the start_time of jobs that have ended with the maximum time_stamp value.

The Original Query

The original query attempted to use the following Hive QL command:

select 
JOB,
start_time,
max(time_stamp)
from table_1

However, this query does not produce the desired result. Let’s analyze why.

Why the Original Query Fails

The original query fails because it uses the max function to aggregate the time_stamp values for each job separately. This means that the max value is calculated for each row individually, rather than considering the maximum time_stamp value across all rows for a given job.

For example, for JOB_A, the max(time_stamp) will be 11:45:00 (the last timestamp for work_B), which is not the correct maximum value. We want to consider the maximum time_stamp value across all rows for JOB_A, which is 12:30:00 (the last timestamp for work_B).

The Correct Solution

To solve this problem, we can use a subquery to get the latest timestamp for each job, and then use that result to calculate the maximum time_stamp value.

The correct query using Hive QL is:

select 
c.JOB,
max(c.time_stamp) as max_time_stamp
from test1229 c
join (select * from test1229) d on d.JOB = c.JOB
where c.time_stamp > d.time_stamp
group by c.JOB

Let’s break down this query:

• The subquery (select * from test1229) gets all rows for the table, and assigns an alias d to each row.
• The join (select * from test1229) d on d.JOB = c.JOB joins each row with itself based on the JOB column. This creates a temporary result set where each row has two copies: one original row and one duplicate row that overlaps with the original row on the JOB column.
• The condition where c.time_stamp > d.time_stamp filters out rows where the timestamp is not greater than the previous timestamp for the same job. This ensures that we only consider the latest timestamp for each job.
• The group by c.JOB clause groups the result set by the JOB column, so that we can calculate the maximum time_stamp value separately for each job.

Understanding the Join Operation

The join operation in Hive QL is used to combine two or more tables based on a common column. In this case, we are joining each row with itself based on the JOB column.

join (select * from test1229) d on d.JOB = c.JOB

is equivalent to:

left join test1229 d on d.JOB = c.JOB

In this context, we are not performing a traditional inner join. Instead, we are using the join clause to create a temporary result set where each row has two copies: one original row and one duplicate row that overlaps with the original row on the JOB column.

Using Window Functions

Hive QL supports window functions, which allow us to perform calculations across rows without grouping them. One such function is ROW_NUMBER(), which assigns a unique number to each row within a partition of a result set.

For example:

select JOB,
       time_stamp,
       ROW_NUMBER() OVER (PARTITION BY JOB ORDER BY time_stamp DESC) as rn
from test1229

This query assigns a row number to each row within each job, based on the timestamp. The rn column will have the following values:

We can then use the rn column to filter out rows where the row number is not 1, effectively getting only the latest timestamp for each job.

Using Window Functions in Hive QL

Hive QL does support window functions, but they are limited compared to other databases like SQL Server or Oracle. For example, Hive QL does not support all types of window functions, such as LAG or LEAD.

However, we can use the ROW_NUMBER() function to achieve similar results.

select 
c.JOB,
max(c.time_stamp) as max_time_stamp
from test1229 c
join (select * from test1229) d on d.JOB = c.JOB
where row_number() over (partition by c.JOB order by c.time_stamp desc) = 1
group by c.JOB

This query uses the ROW_NUMBER() function to assign a unique number to each row within each job, based on the timestamp. It then filters out rows where the row number is not 1, effectively getting only the latest timestamp for each job.

Conclusion

In this article, we have explored how to use Hive QL to retrieve the maximum value for a specific column based on another column. We analyzed the sample data, understood why the original query failed, and provided the correct solution using a subquery and window functions.

Last modified on 2024-05-02