Learn Hive Query to Unlock the Power of Big Data Analytics

Introduction

Given the number of large datasets that data engineers handle on a daily basis, it is no doubt that a dedicated tool is required to process and analyze such data. Some tools like Pig, One of the most widely used tools to solve such a problem is Apache Hive which is built on top of Hadoop.

Apache Hive is a data warehousing built on top of Apache Hadoop. Using Apache Hive, you can query distributed data storage, including the data residing in Hadoop Distributed File System (HDFS), which is the file storage system provided in Apache Hadoop. Hive also supports the ACID properties of relational databases with ORC file format, which is optimized for faster querying. But the real reason behind the prolific use of Hive for working with Big Data is that it is an easy-to-use querying language.

Simple Selects

In Hive, querying data is performed by a SELECT statement. A select statement has 6 key components;

1. SELECT column names
2. FROM table-name
3. GROUP BY column names
4. WHERE conditions
5. HAVING conditions
6. ORDER by column names

In practice, very few queries will have all of these clauses in them, simplifying many queries. On the other hand, conditions in the WHERE clause can be very complex, and if you need to JOIN two or more tables together, then more clauses (JOIN and ON) are needed.

All of the clause names above have been written in uppercase for clarity. HQL is not case-sensitive. Neither do you need to write each clause on a new line, but it is often clearer to do so for all but the simplest of queries.

Over here, we will start with the very simple ones and work our way up to the more complex ones.

Simple Selects ‐ Selecting Columns

Amongst all the hive queries, the simplest query is effectively one which returns the contents of the whole table. Following is the syntax to do that –

SELECT *
FROM geog_all;
CREATE VIEW [IF NOT EXISTS] [db_name.]view_name [(column_name [COMMENT column_comment], ...)  ]

It is better to practice and generally more efficient to explicitly list the column names that you want to be returned. This is one of the optimization techniques that you can use while querying in Hive.

SELECT anonid, fueltypes, acorn_type
FROM geog_all;

Simple Selects – Selecting Rows

In addition to limiting the columns returned by a query, you can also limit the rows returned. The simplest case is to say how many rows are wanted using the Limit clause.

SELECT anonid, fueltypes, acorn_type
FROM geog_all
LIMIT 10;

This is useful if you just want to get a feel for what the data looks like. Usually, you will want to restrict the rows returned based on some criteria. i.e., certain values or ranges within one or more columns.

SELECT anonid, fueltypes, acorn_type
FROM geog_all
WHERE fueltypes = "ElecOnly";

The Expression in the where clause can be more complex and involve more than one column.

SELECT anonid, fueltypes, acorn_type
FROM geog_all
WHERE fueltypes = "ElecOnly" AND acorn_type > 42;

SELECT anonid, fueltypes, acorn_type
FROM geog_all
WHERE fueltypes = "ElecOnly" AND acorn_type > 42 AND nuts1 <> "--";

Notice that the columns used in the conditions of the WHERE clause don’t have to appear in the Select clause. Other operators can also be used in the where clause. For complex expressions, brackets can be used to enforce precedence.

SELECT anonid, fueltypes, acorn_type, nuts1, ldz
FROM geog_all
WHERE
fueltypes = "ElecOnly"
AND acorn_type BETWEEN 42 AND 47
AND (nuts1 NOT IN ("UKM", "UKI") OR ldz = "--");

Creating New Columns

It is possible to create new columns in the output of the query. These columns can be from combinations from the other columns using operators and/or built-in Hive functions.

SELECT anonid, eprofileclass, acorn_type, (eprofileclass * acorn_type) AS multiply, (eprofileclass + acorn_type) AS added
FROM edrp_geography_data b;

A full list of the operators and functions available within the Hive can be found in the documentation.

When you create a new column, it is usual to provide an ‘alias’ for the column. This is essentially the name you wish to give to the new column. The alias is given immediately after the expression to which it refers. Optionally you can add the AS keyword for clarity. If you do not provide an alias for your new columns, Hive will generate a name for you.

Although the term alias may seem a bit odd for a new column that has no natural name, alias’ can also be used with any existing column to provide a more meaningful name in the output.

Tables can also be given an alias, this is particularly common in join queries involving multiple tables where there is a need to distinguish between columns with the same name in different tables. In addition to using operators to create new columns, there are also many Hive built‐in functions that can be used.

Hive Functions

You can use various Hive functions for data analysis purposes. Following are the functions to do that.

Simple Functions

Let’s talk about the functions which are popularly used to query columns that contain string data type values.

Concat can be used to add strings together.

SELECT anonid, acorn_category,
acorn_group,
acorn_type,
concat (acorn_category, ",", acorn_group, ",", acorn_type)  AS acorn_code
FROM geog_all;

substr can be used to extract a part of a string

SELECT anon_id,
advancedatetime,
substr (advancedatetime, 1, 2) AS day,
substr (advancedatetime, 3, 3) AS month,
substr (advancedatetime, 6, 2) AS year
FROM elec_c;

Examples of length, instr, and reverse

SELECT anonid,
     acorn_code,
     length (acorn_code),
     instr (acorn_code, ',') AS a_catpos,
     instr (reverse (acorn_code), "," ) AS reverse_a_typepo

Where needed, functions can be nested within each other, cast and type conversions.

SELECT anonid,
substr (acorn_code, 7, 2) AS ac_type_string,
cast (substr (acorn_code, 7, 2) AS INT) AS ac_type_int,
substr (acorn_code, 7, 2) +1 AS ac_type_not_sure
FROM geog_all;

Aggregation Functions

Aggregate functions are used to perform some kind of mathematical or statistical calculation across a group of rows. The rows in each group are determined by the different values in a specified column or columns. A list of all of the available functions is available in the apache documentation.

SELECT anon_id,
              count (eleckwh) AS total_row_count,
              sum (eleckwh) AS total_period_usage,
              min (eleckwh) AS min_period_usage,
              avg (eleckwh) AS avg_period_usage,
             max (eleckwh) AS max_period_usage
       FROM elec_c
GROUP BY anon_id;

In the above example, five aggregations were performed over the single column anon_id. It is possible to aggregate over multiple columns by specifying them in both the select and the group by clause. The grouping will take place based on the order of the columns listed in the group by clause. What is not allowed is specifying a non‐aggregated column in the select clause that is not mentioned in the group by clause.

SELECT anon_id,
              substr (advancedatetime, 6, 2) AS reading_year,
              count (eleckwh) AS total_row_count,
              sum (eleckwh) AS total_period_usage,
              min (eleckwh) AS min_period_usage,
              avg (eleckwh) AS avg_period_usage,
              max (eleckwh) AS max_period_usage
       FROM elec_c
GROUP BY anon_id, substr (advancedatetime, 6, 2);

Unfortunately, the group by clause will not accept alias’.

SELECT anon_id,
              substr (advancedatetime, 6, 2) AS reading_year,
              count (eleckwh) AS total_row_count,
              sum (eleckwh) AS total_period_usage,
              min (eleckwh) AS min_period_usage,
              avg (eleckwh) AS avg_period_usage,
              max (eleckwh) AS max_period_usage
      FROM elec_c
GROUP BY anon_id, substr (advancedatetime, 6, 2)
ORDER BY anon_id, reading_year;

But the Order by clause does.

The Distinct keyword provides a set of a unique combination of column values within a table without any kind of aggregation.

SELECT DISTINCT eprofileclass, fueltypes
FROM geog_all;

Date Functions

In the elec_c and gas_c tables, the advance DateTime column, although it contains timestamp-type information, it is defined as a string type. For much of the time, this can be quite convenient. However, there will be times when we really do need to be able to treat the column as a Timestamp. Perhaps the most obvious example is when you need to sort rows based on the advanced time column.

Hive provides a variety of date-related functions to allow you to convert strings into timestamps and to additionally extract parts of the Timestamp.

unix_timestamp returns the current date and time – as an integer!

from_unixtime takes an integer and converts it into a recognizable Timestamp string

SELECT unix_timestamp () AS currenttime
FROM sample_07
LIMIT 1;

SELECT from_unixtime (unix_timestamp ()) AS currenttime
FROM sample_07
LIMIT 1;

There are various date part functions that will extract the relevant parts from a Timestamp string.

SELECT anon_id,
             from_unixtime (UNIX_TIMESTAMP (reading_date, 'ddMMMyy'))
                  AS proper_date,
            year (from_unixtime (UNIX_TIMESTAMP (reading_date, 'ddMMMyy')))
                 AS full_year,
            month (from_unixtime (UNIX_TIMESTAMP (reading_date, 'ddMMMyy')))
                AS full_month,
            day (from_unixtime (UNIX_TIMESTAMP (reading_date, 'ddMMMyy')))
               AS full_day,
           last_day (from_unixtime (UNIX_TIMESTAMP (reading_date, 'ddMMMyy')))
              AS last_day_of_month,
           date_add ( (from_unixtime (UNIX_TIMESTAMP (reading_date, 'ddMMMyy'))),10)
              AS added_days
FROM elec_days_c
ORDER BY proper_date;

Conclusion

In the article, we covered some basic Hive functions and queries. We saw that running queries on distributed data is not much different from running queries in MySQL. We covered some same basic queries like inserting records, working with simple functions, and working with aggregation functions in Hive.

Key Takeaways

• Hive Query Language is the language supported by Hive.
• HQL makes it easy for developers to query on Big data.
• HQL is similar to SQL, making it easy for developers to learn this language.

I recommend you go through these articles to get acquainted with tools for big data:

• Getting Started with Apache Hive – A Must-Know Tool For all Big Data and Data Engineering Professionals
• Introduction to the Hadoop Ecosystem for Big Data and Data Engineering
• PySpark for Beginners – Take your First Steps into Big Data Analytics (with Code)

Frequently Asked Questions