AWS Big Data Blog

­­Use fuzzy string matching to approximate duplicate records in Amazon Redshift

November 2023: This post was reviewed and updated to use AWS Lambda user-defined function (UDF) instead of Python user-defined function (UDF).

Amazon Redshift is a fully managed, petabyte-scale data warehouse service in the cloud. Amazon Redshift enables you to run complex SQL analytics at scale and performance on terabytes to petabytes of structured and unstructured data, and make the insights widely available through popular business intelligence (BI) and analytics tools.

It’s common to ingest multiple data sources into Amazon Redshift to perform analytics. Often, each data source will have its own processes of creating and maintaining data, which can lead to data quality challenges within and across sources.

One challenge you may face when performing analytics is the presence of imperfect duplicate records within the source data. Answering questions as simple as “How many unique customers do we have?” can be very challenging when the data you have available is like the following table.

Name Address Date of Birth
Cody Johnson 8 Jeffery Brace, St. Lisatown 1/3/1956
Cody Jonson 8 Jeffery Brace, St. Lisatown 1/3/1956

Although humans can identify that Cody Johnson and Cody Jonson are most likely the same person, it can be difficult to distinguish this using analytics tools. This identification of duplicate records also becomes nearly impossible when working on large datasets across multiple sources.

This post presents one possible approach to addressing this challenge in an Amazon Redshift data warehouse. We import an open-source fuzzy matching Python library to Amazon Redshift, create a simple fuzzy matching user-defined function (UDF), and then create a procedure that weights multiple columns in a table to find matches based on user input. This approach allows you to use the created procedure to approximately identify your unique customers, improving the accuracy of your analytics.

This approach doesn’t solve for data quality issues in source systems, and doesn’t remove the need to have a wholistic data quality strategy. For addressing data quality challenges in Amazon Simple Storage Service (Amazon S3) data lakes and data pipelines, AWS has announced AWS Glue Data Quality (preview). You can also use AWS Glue DataBrew, a visual data preparation tool that makes it easy for data analysts and data scientists to clean and normalize data to prepare it for analytics.


To complete the steps in this post, you need the following:

The following AWS CloudFormation stack will deploy a new Redshift Serverless endpoint and an IAM role for use in this post, including all VPC entities required.


All SQL commands shown in this post are available in the following notebook, which can be imported into the Amazon Redshift Query Editor V2.

Overview of the dataset being used

The dataset we use is mimicking a source that holds customer information. This source has a manual process of inserting and updating customer data, and this has led to multiple instances of non-unique customers being represented with duplicate records.

The following examples show some of the data quality issues in the dataset being used.

In this first example, all three customers are the same person but have slight differences in the spelling of their names.

id name age address_line1 city postcode state
1 Cody Johnson 80 8 Jeffrey Brace St. Lisatown 2636 South Australia
101 Cody Jonson 80 8 Jeffrey Brace St. Lisatown 2636 South Australia
121 Kody Johnson 80 8 Jeffrey Brace St. Lisatown 2636 South Australia

In this next example, the two customers are the same person with slightly different addresses.

id name age address_line1 city postcode state
7 Angela Watson 59 3/752 Bernard Follow Janiceberg 2995 Australian Capital Territory
107 Angela Watson 59 752 Bernard Follow Janiceberg 2995 Australian Capital Territory

In this example, the two customers are different people with the same address. This simulates multiple different customers living at the same address who should still be recognized as different people.

id name age address_line1 city postcode state
6 Michael Hunt 69 8 Santana Rest St. Jessicamouth 2964 Queensland
106 Sarah Hunt 69 8 Santana Rest St. Jessicamouth 2964 Queensland

Load the dataset

First, create a new table in your Redshift Serverless endpoint and copy the test data into it by doing the following:

  1. Open the Query Editor V2 and log in using the admin user name and details defined when the endpoint was created.
  2. Run the following CREATE TABLE statement:
    create table customer (
        id smallint, 
        urid smallint,
        name varchar(100),
        age smallint,
        address_line1 varchar(200),
        city varchar(100),
        postcode smallint,
        state varchar(100)

    Screenshot of CREATE TABLE statement for customer table being run successfully in Query Editor V2

  3. Run the following COPY command to copy data into the newly created table:
    copy customer (id, name, age, address_line1, city, postcode, state)
    from ' s3://redshift-blogs/fuzzy-string-matching/customer_data.csv'
    IAM_ROLE default
    FORMAT csv
    REGION 'us-east-1'
  4. Confirm the COPY succeeded and there are 110 records in the table by running the following query:
    select count(*) from customer;

    Screenshot showing the count of records in the customer table is 110. Query is run in Query Editor V2

Fuzzy matching

Fuzzy string matching, more formally known as approximate string matching, is the technique of finding strings that match a pattern approximately rather than exactly. Commonly (and in this solution), the Levenshtein distance is used to measure the distance between two strings, and therefore their similarity. The smaller the Levenshtein distance between two strings, the more similar they are.

In this solution, we exploit this property of the Levenshtein distance to estimate if two customers are the same person based on multiple attributes of the customer, and it can be expanded to suit many different use cases.

This solution uses TheFuzz, an open-source Python library that implements the Levenshtein distance in a few different ways. We use the partial_ratio function to compare two strings and provide a result between 1–100. If one of the strings matches perfectly with a portion of the other, the partial_ratio function will return 100.

Weighted fuzzy matching

By adding a scaling factor to each of our column fuzzy matches, we can create a weighted fuzzy match for a record. This is especially useful in two scenarios:

  • We have more confidence in some columns of our data than others, and therefore want to prioritize their similarity results.
  • One column is much longer than the others. A single character difference in a long string will have much less impact on the Levenshtein distance than a single character difference in a short string. Therefore, we want to prioritize long string matches over short string matches.

The solution in this post applies weighted fuzzy matching based on user input defined in another table.

Create a table for weight information

This reference table holds two columns; the table name and the column mapping with weights. The column mapping is held in a SUPER datatype, which allows JSON semistructured data to be inserted and queried directly in Amazon Redshift. For examples on how to query semistructured data in Amazon Redshift, refer to Querying semistructured data.

In this example, we apply the largest weight to the column address_line1 (0.5) and the smallest weight to the city and postcode columns (0.1).

Using the Query Editor V2, create a new table in your Redshift Serverless endpoint and insert a record by doing the following:

  1. Run the following CREATE TABLE statement:
    CREATE TABLE ref_unique_record_weight_map(table_name varchar(100), column_mapping SUPER);
  2. Run the following INSERT statement:
    INSERT INTO ref_unique_record_weight_map VALUES (
            "colname": "name",
            "colweight": 0.3
            "colname": "address_line1",
            "colweight": 0.5
            "colname": "city",
            "colweight": 0.1
            "colname": "postcode",
            "colweight": 0.1
  3. Confirm the mapping data has inserted into the table correctly by running the following query:
    select * from ref_unique_record_weight_map;

    Screenshot showing the result of querying the ref_unique_record_weight_map table in Query Editor V2

  4. To check all weights for the customer table add up to 1 (100%), run the following query:
    select  cm.table_name, 
            sum(colmap.colweight) as total_column_weight 
    from    ref_unique_record_weight_map cm, cm.column_mapping.colmap colmap 
    where   cm.table_name = 'customer'
    group by cm.table_name;

    Screenshot showing the total weight applied to the customer table is 1.0

User-defined functions

With Amazon Redshift, you can create custom scalar user-defined functions (UDFs) using AWS Lambda written in any programming language supported by Lambda. A Lambda UDF incorporates a Lambda program that runs when the function is called and returns a single scalar value. Lambda UDFs are defined and managed in Lambda, and you can control the access privileges to invoke these UDFs in Amazon Redshift. You can invoke multiple Lambda functions in the same query or invoke the same function multiple times.

In this solution, we create a Lambda UDF to take two input values and provide their similarity as a response.

Create a Lambda Layer

A Lambda layer is a .zip file archive that contains supplementary code or data. Layers usually contain library dependencies, a custom runtime, or configuration files. In this use-case, we will use a Lambda Layer to contain the fuzzy matching.

If you have deployed the CloudFormation stack from above, then the Lambda Layer has been deployed for you with the name thefuzz-lambda-layer. To deploy this manually, follow instructions from the Lambda Layers documentation to upload the zip file as a Lambda Layer.

Create a Lambda function

Create a Lambda function in Python to do the following:

  1. Take two input values that can be of any data type as long as they are the same data type (such as two integers or two varchars).
  2. Return an integer value comparing the partial ratio between the two input values.
  3. Open the Lambda console page and create a new function called redshift_lambda_unique_record with a Python runtime.
  4. Add a Lambda Layer to your function by clicking the Add Layer button on the bottom right of the Lambda function page. Then choose your custom Lambda Layer to be added.
  5. Next add the following code in the Lambda Function code source and deploy the function.
    import json
    from thefuzz import fuzz
    def lambda_handler(event, context):
            response = dict()
            records = event["arguments"]
            results = []
            for record in records:
                value_1 = record[0]
                value_2 = record[1]
                    comparison = fuzz.partial_ratio(value_1, value_2)
            response['success'] = True   
            response['results'] = results
        except Exception as e:
            response['success'] = False  
            response['error_msg'] = str(e)
        return json.dumps(response)

Create a Lambda user defined function

Run the following code to create a Lambda UDF in Redshift that will invoke the Lambda Function you defined above.

CREATE OR REPLACE EXTERNAL FUNCTION unique_record (value_1 anyelement, value_2 anyelement)
LAMBDA 'redshift_lambda_unique_record'
IAM_ROLE default;

Stored procedures

Stored procedures are commonly used to encapsulate logic for data transformation, data validation, and business-specific logic. By combining multiple SQL steps into a stored procedure, you can reduce round trips between your applications and the database.

In this solution, we create a stored procedure that applies weighting to multiple columns. Because this logic is common and repeatable regardless of the source table or data, it allows us to create the stored procedure once and use it for multiple purposes.

Create a stored procedure

Run the following code snippet to create a new Amazon Redshift stored procedure called find_unique_id. This procedure will do the following:

  1. Take one input value. This value is the table you would like to create a golden record for (in our case, the customer table).
  2. Declare a set of variables to be used throughout the procedure.
  3. Check to see if weight data is in the staging table created in previous steps.
  4. Build a query string for comparing each column and applying weights using the weight data inserted in previous steps.
  5. For each record in the input table that doesn’t have a unique record ID (URID) yet, it will do the following:
    1. Create a temporary table to stage results. This temporary table will have all potential URIDs from the input table.
    2. Allocate a similarity value to each URID. This value specifies how similar this URID is to the record in question, weighted with the inputs defined.
    3. Choose the closest matched URID, but only if there is a >90% match.
    4. If there is no URID match, create a new URID.
    5. Update the source table with the new URID and move to the next record.

This procedure will only ever look for new URIDs for records that don’t already have one allocated. Therefore, rerunning the URID procedure multiple times will have no impact on the results.

CREATE OR REPLACE PROCEDURE find_unique_id(table_name varchar(100)) AS $$
    unique_record RECORD;
    column_info RECORD;

    column_fuzzy_comparison_string varchar(MAX) := '0.0';
    max_simularity_value decimal(5,2) := 0.0;

    table_check varchar(100);
    temp_column_name varchar(100);
    temp_column_weight decimal(5,2);
    unique_record_id smallint := 0;
        Check the ref_unique_record_weight_map table to see if there is a mapping record for the provided table.
        If there is no table, raise an exception
    SELECT INTO table_check cm.table_name from ref_unique_record_weight_map cm where cm.table_name = quote_ident(table_name);
        RAISE EXCEPTION 'Input table ''%'' not found in mapping object', table_name;
    END IF;

        Build query to be used to compare each column using the mapping record in the ref_unique_record_weight_map table.
        For each column specified in the mapping object, append a weighted comparison of the column
    FOR column_info IN (
        select  colmap.colname::varchar(100) column_name, 
                colmap.colweight column_weight 
        from    ref_unique_record_weight_map cm, cm.column_mapping.colmap colmap 
        where   cm.table_name = quote_ident(table_name)
    ) LOOP
        temp_column_name = column_info.column_name;
        temp_column_weight = column_info.column_weight;
        column_fuzzy_comparison_string = column_fuzzy_comparison_string || 
            ' + unique_record(t1.' || 
            temp_column_name || 
            '::varchar, t2.' || 
            temp_column_name || 
            '::varchar)*' || 

    /* Drop temporary table if it exists */
    EXECUTE 'DROP TABLE IF EXISTS #unique_record_table';

        For each record in the source table that does not have a Unique Record ID (URID):
            1. Create a new temporary table holding all possible URIDs for this record (i.e. all URIDs that have are present). 
                Note: This temporary table will only be present while the simularity check is being calculated
            2. Update each possible URID in the temporary table with it's simularity to the record being checked
            3. Find the most simular record with a URID
                3a. If the most simular record is at least 90% simular, take it's URID (i.e. this is not a unique record, and matches another in the table)
                3b. If there is no record that is 90% simular, create a new URID (i.e. this is a unique record)
            4. Drop the temporary table in preparation for the next record
    FOR unique_record in EXECUTE 'select * from ' || table_name || ' where urid is null order by id asc' LOOP

        /* Create temporary table */
        EXECUTE '
            CREATE TABLE #unique_record_table AS 
            SELECT id, urid, 0.0::decimal(5,2) as simularity_value 
            FROM ' || table_name || '
            where urid is not null

        /* Update simularity values in temporary table */
        EXECUTE '
            UPDATE #unique_record_table  
            SET simularity_value = round(calc_simularity_value,2)::decimal(5,2)
            FROM (
                SELECT ' || column_fuzzy_comparison_string || ' as calc_simularity_value,
               as upd_id
                FROM ' || table_name || ' t1
                INNER JOIN ' || table_name || ' t2
                ON <>
                AND = ' || quote_literal( || '
                ) t
            WHERE t.upd_id = id

        /* Find largest simularity value */
        SELECT INTO max_simularity_value simularity_value FROM (
            SELECT  MAX(simularity_value) as simularity_value 
            FROM    #unique_record_table

        /* If there is a >90% similar match, choose it's URID. Otherwise, create a new URID */
        IF max_simularity_value > 90 THEN
            SELECT INTO unique_record_id urid FROM (
                SELECT urid
                FROM #unique_record_table
                WHERE simularity_value = max_simularity_value
            EXECUTE 'select COALESCE(MAX(urid)+1,1) FROM ' || table_name INTO unique_record_id;
        END IF;
        /* Update table with new URID value */
        EXECUTE 'UPDATE ' || table_name || ' SET urid = ' || quote_literal(unique_record_id) || ' WHERE id = ' || quote_literal(;

        /* Drop temporary table and repeat process */
        EXECUTE 'DROP TABLE #unique_record_table';

        max_simularity_value = 0.0;

$$ LANGUAGE plpgsql;

Now that the stored procedure has been created, create the unique record IDs for the customer table by running the following in the Query Editor V2. This will update the urid column of the customer table.

CALL find_unique_id('customer'); 
select * from customer;

Screenshot showing the customer table now has values inserted in the URID column

When the procedure has completed its run, you can identify what duplicate customers were given unique IDs by running the following query:

select * 
from customer
where urid in (
    select urid 
    from customer 
    group by urid 
    having count(*) > 1
order by urid asc

Screenshot showing the records that have been identified as duplicate records

From this you can see that IDs 1, 101, and 121 have all been given the same URID, as have IDs 7 and 107.

Screenshot showing the result for IDs 1, 101, and 121

Screenshot showing the result for IDs 7, and 107

The procedure has also correctly identified that IDs 6 and 106 are different customers, and they therefore don’t have the same URID.

Screenshot showing the result for IDs 6, and 106

Clean up

To avoid incurring future reoccurring charges, go to the AWS CloudFormation console and delete the stack deployed in this post. This will delete all created resources.


In this post, we showed one approach to identifying imperfect duplicate records by applying a fuzzy matching algorithm in Amazon Redshift. This solution allows you to identify data quality issues and apply more accurate analytics to your dataset residing in Amazon Redshift.

We showed how you can use open-source Python libraries to create Lambda UDFs, and how to create a generic stored procedure to identify imperfect matches. This solution is extendable to provide any functionality required, including adding as a regular process in your ELT (extract, load, and transform) workloads.

Test the created procedure on your datasets to investigate the presence of any imperfect duplicates, and use the knowledge learned throughout this post to create stored procedures and UDFs to implement further functionality.

If you’re new to Amazon Redshift, refer to Getting started with Amazon Redshift for more information and tutorials on Amazon Redshift. You can also refer to the video Get started with Amazon Redshift Serverless for information on starting with Redshift Serverless.

About the Author

Sean Beath is an Analytics Solutions Architect at Amazon Web Services. He has experience in the full delivery lifecycle of data platform modernisation using AWS services and works with customers to help drive analytics value on AWS.