How to Use Google BigQuery Subquery: Steps Explained

Venturing into Data Science and deciding on a tool to use to solve a given problem can be challenging at times, especially when you have a wide array of choices. In this age of data transformation, where organizations are constantly seeking out ways to improve the day-to-day handling of data being produced and looking for methods to minimize the cost of having these operations, it has become imperative to handle such data transformations in the Cloud as it is a lot easier to manage and is also cost-efficient.

In this article, you will gain information about Google BigQuery Subquery. You will also gain a holistic understanding of Google BigQuery, its key features, SQL, Subqueries, and the different types of Subqueries supported by Google BigQuery. Read along to find out in-depth information about undergoing GoogleBigQuery Subquery.

What is Google BigQuery?

Google BigQuery is a Cloud-based Data Warehouse that provides a Big Data Analytic Web Service for processing petabytes of data. It is intended for analyzing data on a large scale. 

It consists of two distinct components: Storage and Query Processing. It employs the Dremel Query Engine to process queries and is built on the Colossus File System for storage. These two components are decoupled and can be scaled independently and on-demand.

Google BigQuery is fully managed by Cloud service providers. We don’t need to deploy any resources, such as discs or virtual machines. It is designed to process read-only data.

For further information about Google Bigquery, follow the Official Documentation.

Key Features of Google BigQuery

Some of the key features of Google BigQuery are as follows:

1) Scalable Architecture

BigQuery has a scalable architecture and offers a petabyte scalable system that users can scale up and down as per load.

2) Faster Processing

Being a scalable architecture, BigQuery executes petabytes of data within the stipulated time and is more rapid than many conventional systems. BigQuery allows users to run analysis over millions of rows without worrying about scalability.

3) Fully-Managed

BigQuery is a product of Google Cloud Platform, and thus it offers fully managed and serverless systems.

What is SQL?

SQL stands for Structured Query Language and executes queries against the database for data analytics. According to ANSI (American National Standards Institute), it is the standard language for relational database management systems. 

With the help of SQL, you can:

• Create and Delete Database
• Create, Delete and Update Table
• Load/UnLoad data into Tables
• Set and Manage permissions on tables, procedures, and views
• And many more.

What are Subqueries?

A Subquery is an inner or nested query that users can use inside a SQL query or in a WHERE clause. The subquery specified will return the data used in the main query as a filter condition to retrieve the data from the main query.

Subqueries can be used with the SELECT, INSERT, UPDATE, and DELETE statements and the operators like =, <, >, >=, <=, IN, BETWEEN, etc.

To write the subquery, the user must follow some rules which are as follows:

1. The Subqueries must be enclosed within parenthesis.
2. A Subquery can have only one column in select.
3. A subquery cannot use the ORDER BY command. However, a GROUP BY function can be used instead of the ORDER BY function. 
4. Subqueries that return more than one row can only be used with IN operator.

What is the Need to Use Subqueries?

The instances where Subqueries are used are as follows: 

• There is a need to filter a table based on data from another table
• There is a need to refer a column from another table into the current table.

Prerequisites

• An active Google account with a subscription to Google Cloud Platform (GCP).
• Basic understanding of Google BigQuery.
• Understanding of basic SQL queries.
• Consider the following tables given below for the example purposes:

Products

product_id	product_name	product_owner
P01	PS4	Sony
P02	XBox	Microsoft
P03	Nintendo	Nintendo

Orders

order_id	order_date	prodcut_id	order_qty
O1	2022-01-02 01:12:23	P01	1
O1	2022-01-02 01:12:23	P02	1
O2	2022-01-04 02:12:23	P01	2
O3	2022-01-06 06:12:23	P03	2

Transactions

transaction_id	order_id	transaction_date	transaction_amt_usd
T01	O1	2022-01-02 01:12:23	800
T02	O2	2022-01-04 02:12:23	500
T03	O3	2022-01-06 06:12:23	1000

How to use Google BigQuery Subquery?

Google BigQuery uses ANSI SQL to query the data against the database, and hence it supports Subqueries within the main query. Let’s discuss the different types of subqueries that can be used in BigQuery:

• Google BigQuery Subquery: Expression Subqueries
• Google BigQuery Subquery: Table Subqueries
• Google BigQuery Subquery: Correlated Subqueries

1) Expression Subqueries

Expression Subqueries are those queries that return a single value and are used against expression. Expression subqueries can be correlated queries.

There are different types of Expression subqueries to look around:

• Scalar Subqueries
• EXISTS-based Subqueries
• IN-Based Subqueries
• ARRAY Subqueries

A) Scalar Subqueries

The subqueries resulting in a single column and single row are called scalar subqueries. Scalar subqueries are often used with SELECT or WHERE clauses.

Using multiple columns in Scalar queries results in analysis error, and also, if subquery results into multiple rows, that also results in a runtime error. To understand more about the scalar subqueries, consider the following example –

Consider the table Products and Orders, below query that represent a scalar subquery example:

SELECT order_id, (SELECT product_name 
                  FROM products 
                  WHERE orders.product_id = product.product_id) 
                  AS product_name
FROM orders;

Output:

+---------------------------+
| order_id   | product_name |
+---------------------------+
| O1         | PS4          |
| O1         | XBox         |
| O2         | PS4          |
| O3         | Nintendo     |
+---------------------------+

B) EXISTS-based Subqueries

EXISTS-based subqueries return TRUE if the subquery returns one or more rows and FALSE if it produces zero rows. In Exists-based subqueries, users can use any number of columns as required, which will not affect the query result.

SELECT EXISTS(SELECT product_id 
              FROM products 
              WHERE product_owner = 'PS4') AS result;

Output:

+--------+
| result |
+--------+
| TRUE   |
+--------+

C) IN-based Subqueries

IN-based subqueries return TRUE if the subquery returns the row and FALSE if it doesn’t return any value.

The value returned by IN-based subqueries must have a single column, and the data type of the returned value must be the same as the comparable value. Otherwise, it will throw an error.

SELECT "Nintendo" IN (SELECT product_name 
                      FROM products) as result;

Output:

+--------+
| result |
+--------+
| TRUE   |
+--------+

D) ARRAY Subqueries

ARRAY subqueries are the special case of expression subquery, and it returns ARRAY of values combined from different rows. If no rows are returned from the query, it returns an empty array.

SELECT ARRAY(SELECT product_id 
             FROM orders 
             WHERE order_qty = '1') as product_names
FROM orders LIMIT 1;            

Output:

+-----------------+
| product_names   |
+-----------------+
| [P01, P02]      |
+-----------------+

[Source_Destination_Blockj]

2) Table Subqueries

The Table subqueries are the types where the main query treats the result of the subquery as a temporary table and uses that to fetch the value. This query can only be used with FROM clause.

SELECT product_name FROM (SELECT product_name 
                          FROM products 
                          WHERE product_owner IN ('Sony'))

Output:

+----------------+
| product_name   |
+----------------+
| PS4            |
+----------------+

3) Correlated Subqueries

A Correlated subquery is another type of subquery that references a column from another table outside that subquery. Correlation prevents reusing of the subquery result. 

SELECT product_name, (SELECT order_id 
                      FROM orders 
                      WHERE orders.product_id = 'P02') AS order_id
FROM products;

Output:

+---------------------------+
| product_name   | order_id |
+---------------------------+
| XBOX           | O1       |
+---------------------------+

Conclusion

In this article, you have learned about BigQuery SubQuery. This article also provided information on Google BigQuery, its key features, SQL, Subqueries, and the different types of subqueries used in Google BigQuery in detail. For further information on BigQuery JSON Extract, BigQuery Create View Command, BigQuery Partition Tables, you can visit the former links.

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