Understanding Redshift LISTAGG Function: 3 Critical Aspects

The ever-increasing rise in data gives organizations more meaningful insights, but handling huge amounts of data leads to a curse of dimensionality and considerable lag in performance. Despite proper planning, traditional Data Warehouses suffer storage limitations, giving a financial burden to companies. However, Amazon provided a Cloud-based Data Warehouse product called ‘Amazon Redshift’ that not only eradicates data storage issues but also interacts seamlessly with Business intelligence (BI) tools to provide Analytical solutions. As it utilizes SQL at the backend, analysts can also make use of the Amazon Redshift LISTAGG function to aggregate strings along with a user-defined delimiter.

This article gives an overview of the Amazon Redshift LISTAGG function. It introduces Amazon Redshift and a glimpse of SQL commands, aggregate functions, and clauses. Furthermore, it also provides the syntax, arguments, usage, and example queries of the Amazon Redshift LISTAGG function.

Table of Contents

• Introduction to Amazon Redshift
• SQL Command in Amazon Redshift
• Aggregated SQL Function in Amazon Redshift
• SQL Clauses in Amazon Redshift
• Amazon Redshift LISTAGG Function
  • Syntax, Arguments, and Limitations of Amazon Redshift LISTAGG
  • Usage of Amazon Redshift LISTAGG
• Use Cases of Amazon Redshift LISTAGG
• Conclusion

Introduction to Amazon Redshift

Amazon Redshift is a Cloud-based Data Warehousing Solution rolled by Amazon Web Services (AWS). It can store petabytes of data and therefore eliminates the issue of data storage scarcity. With Amazon Redshift, analysts can query structured and semistructured data across Data Warehouses using standard Business Intelligence (BI) tools, SQL, and more. Since its inception in 2012, it has enabled organizations to handle large datasets by Massive Parallel Processing (MPP) and data compression techniques to decrease command execution time. As it integrates with Business Intelligence tools, data from Amazon Redshift is processed to build powerful reports and dashboards.

For more information on Amazon Redshift, click here.

SQL Commands in Amazon Redshift

There are many programming languages that interact with data, but SQL is considered the standard language for storing, modifying, and retrieving data in Amazon Redshift. SQL is a query language that resembles a set of English words arranged to perform desired operations on a huge number of records. Based on a given task, SQL queries can be classified into 5 parts as below:

• Data Definition Language: Data Definition Language (DDL) consists of commands that deal with the structure of tables present in the database. DDL includes commands like CREATE, ALTER, DROP, and TRUNCATE. All the DDL commands are auto-committed, thereby permanently saving all changes in working databases. 
• Data Manipulation Language: Data Manipulation Language (DML) deals with the manipulation of data in databases. DML includes commands INSERT, UPDATE, and DELETE. As DML commands are not auto-committed, hence they can be rolled back. 
• Data Query Language: Data Query Language (DQL) is used to fetch data from a database. It consists of a ‘SELECT’ command to choose desired attributes. A ‘WHERE’ clause is used along the DQL command to retrieve specific results from an entire table.
• Data Control Language: Data Control Language (DCL) commands consist of ‘GRANT’ and ‘REVOKE’ which gives database administrators authority to give various permissions by limiting the access to other users.
• Transaction Control Logic: Transaction Control Logic (TCL) commands deal with a set of tasks as a single execution unit. The TCL commands consist of COMMIT, SAVEPOINT, ROLLBACK, and SET TRANSACTION Each transaction begins with a specific task and ends when all the relative tasks are logically executed. If any of the tasks fails, the entire transaction process fails.

Aggregated SQL Function in Amazon Redshift

SQL Aggregate function houses various values together (vertically) multiple rows as input to return a single value of more significant meaning. Below are aggregate functions supported by SQL in Amazon Redshift:

• Count: SQL COUNT(*) function returns the count of the total number of records. However, COUNT(column_name) will return a count of non-null records for a given column.
• Max: SQL MAX statement returns the largest value in a column. For instance, applying max(salary) returns the highest value in the salary column.
• Min: SQL MIN statement returns the minimum value in a column. For instance, applying min (sales) returns the least value in the sales column.
• Sum: To find the total sum of a particular column, you can use the SQL Sum() function. It should be noted that null values in the sum function are by default treated as 0 compared to the count function.
• Avg: AVG is a SQL aggregate function that calculates the average of selected columns. Average can manually be calculated by dividing the sum and count function on the desired column. It can be applied only on features having numeric values and completely ignores null values.

SQL Clauses in Amazon Redshift

SQL has in-built functions to filter and analyze data quickly. When dealing with large databases, clauses power analysts to apply constraints and arrange the data to get better insights. Below are the most widely used clauses:

• WHERE: ‘WHERE’ clause is applied to perform a conditional query in the SQL statement. It can be used in UPDATE, and DELETE statements to perform the desired operation on data.
• AND and OR: While using the AND clause, a query must contain at least two conditions. It gives results if both conditions are satisfied. Although the ‘OR’ clause requires two conditions, it returns the result if at least one condition satisfies the mentioned requirements. Both AND and OR clauses parse multiple conditions using the WHERE clause.
• LIKE: A ‘LIKE’ clause utilizes two symbols ‘%’, and ‘_’ it is beneficial to find specific patterns in data. While the former symbol represents zero, one, or multiple characters, the latter is used for one single character.
• LIMIT: When working with a colossal amount of data in a table, a ‘LIMIT’ clause restricts the number of records. The ‘LIMIT’ clause returns records for a specified number, and it should neither be a Float nor an Exponential value.
• GROUP BY: Often a particular column has similar values spread across different rows. These identical data can be arranged in groups using a ‘GROUP BY’ clause. A ‘GROUP BY’ clause is either placed before the ‘ORDER BY’ clause or after the ‘WHERE’ clause.
• ORDER BY: To sort data in ascending or descending order, you can use the ‘ORDER BY’ clause. Any column having comparable data can be passed through an ‘ORDER BY’ clause. By default, it sorts data in ascending order.

Amazon Redshift LISTAGG Function

The Amazon Redshift LISTAGG function in the Database Management System (DBMS) is used to aggregate strings (character, binary, or graphic) from a group of data delimited by a configurable separator in columns in a database table. It should be noted that:

• LISTAGG concatenates strings very quickly by grouping elements.
• LISTAGG also allows the ordering of elements in a concatenated list.

Syntax, Arguments, and Limitations of Amazon Redshift LISTAGG

The Amazon Redshift LISTAGG aggregate function orders the rows for each group, then concatenates the values into a single string. Amazon Redshift LISTAGG is a compute-node-only function, as it returns an error if the query doesn’t reference a user-defined table or Amazon Redshift system table.

Syntax

LISTAGG([DISTINCT] aggregate_expression [, 'delimiter'])
[WITHIN GROUP (ORDER BY order_list)]

Arguments

• DISTINCT (Optional): It is a clause to remove duplicate values from the specified expression before concatenating to fetch unique records. When evaluating duplicates, trailing spaces are ignored, so the strings ‘a’ and ‘a ‘ are treated as duplicates.
• aggregate_expression: Any valid expression (such as a column name) that gives aggregate value (ignoring null and empty string) can be an aggregate expression.
• delimiter (Optional): Delimiter is the string constant (default: NULL) used to separate the concatenated values.
• WITHIN GROUP (ORDER BY order_list) (Optional): An optional clause that specifies the sort order of the aggregated values within a group.

Limitations

Amazon Redshift LISTAGG has a limitation in VARCHAR size, and returns an error if the result set has VARCHAR size larger than 64K, as shown below:

Invalid operation: Result size exceeds LISTAGG limit

Usage of Amazon Redshift LISTAGG

Certain cases may require using multiple Amazon Redshift LISTAGG functions that use WITHIN GROUP clauses. Such WITHIN GROUP clauses must use the same ORDER BY values and result in an error.

For instance, the below query differentiates the usage of Amazon Redshift LISTAGG with the ORDER BY clause:

Incorrect Syntax

select listagg(sellerid)
within group (order by dateid) as sellers,
listagg(dateid)
within group (order by sellerid) as dates
from winsales;

Correct Syntax

select listagg(sellerid)
within group (order by dateid) as sellers,
listagg(dateid)
within group (order by dateid) as dates
from winsales;

select listagg(sellerid)
within group (order by dateid) as selllers,
listagg group (order by dateid) as sellers,
listagg(dateid) as dates
from winsales;

Use Cases of Amazon Redshift LISTAGG

Listed below are some of the major use cases of Amazon Redshift LISTAGG function:

Case I: Consider a database that has a sales table, and seller ID is one numeric column. To query Amazon Redshift LISTAGG and ORDER BY clause on seller ID where eventid is 4337, use the below query:

select listagg(sellerid, ', ') within group (order by sellerid) from sales
where eventid = 4337;

Query Output:

listagg
380, 380, 1178, 1178, 1178, 2731, 8117, 12905, 32043, 32043, 32043, 32432, 32432, 38669, 38750, 41498, 45676, 46324, 47188, 47188, 48294.   

Case II: For the above case, we use the DISTINCT clause if the task is to find a list of unique seller IDs. Below is the modified query:

Case III: Suppose the task is to aggregate the seller ID column sorted date-wise from the winsales table. You can achieve it using the below query:

Case IV: If a user wants to aggregate dates from the winsales table in descending order by seller ID, ascending order by sales ID, and buyer ID equals ‘b,’ you can return a pipe-separated list using ‘|’ delimiter in the Amazon Redshift LISTAGG function using the below query:

Case V: If the task is to obtain a list of salesid sorted alphabetically by buyerid from winsales table, follow the below LISTAGG query:

Conclusion

This article introduced you to Amazon Redshift and its SQL commands. It also provided a comprehensive overview of the Amazon Redshift LISTAGG function (including its syntax, usage, limitations, and some of the popular use cases).

Amazon Redshift accompanies simple SQL at the backend while dealing with large datasets. It also connects with BI tools that can help you display key parameters on a dashboard for better decision-making. Moreover, Amazon Redshift has enabled analysts to query exabytes of data, giving a scalable solution for business growth.

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