Joining Tables with Similar Values Using a Common Table Expression (CTE)

In this article, we will explore how to join two tables based on similar values in their respective columns. We will also discuss how to prevent multiple results for a single entry in the main table.

Introduction

When working with databases, it’s not uncommon to encounter situations where you need to join two tables together based on similar values in their columns. However, what if the similarity is not exact, but rather a prefix or suffix of one column value in another? In this article, we will explore how to achieve this using a Common Table Expression (CTE).

The Problem

Suppose we have two tables: tab1 and tab2. We want to join these tables based on the similarity between the first four characters of name in tab1 and the first four characters of name in tab2. However, we don’t want multiple results for a single entry in tab1, as it’s not allowed.

The Query

The query provided in the Stack Overflow post uses a LEFT OUTER JOIN to join the two tables. However, this approach doesn’t guarantee that we’ll only get one result per entry in tab1.

To achieve our desired outcome, we can use a Common Table Expression (CTE) to first filter out rows from tab2 where the similarity between the prefix of name is less than 4 characters.

WITH 
  TAB1 (NAME) AS 
(
VALUES
  'jackson'
, 'michael'
, 'Jack Daniels'
, 'Michelin'
)
, TAB2 (NAME, CODE) AS 
(
VALUES
  ('JACK',       '12345')
, ('JACK X',     '67890')
, ('Micha',      '12000')
, ('Michael T.', '90000')
)
SELECT A.NAME, B.CODE
FROM TAB1 A
LEFT JOIN TABLE
(
SELECT B.CODE
FROM TAB2 B
WHERE 
  LOWER (SUBSTR (A.NAME, 1, 4)) 
  LIKE '%' || LOWER (SUBSTR (B.NAME, 1, 4)) || '%'
FETCH FIRST 1 ROW ONLY
) B ON 1 = 1

The Solution

The key to this query is the WHERE clause inside the CTE. We use the LOWER function to convert both columns to lowercase and then apply the SUBSTR function to extract the first four characters of each column.

We then use the LIKE operator with a wildcard (%) to match any value that starts with the prefix of the other column. The FETCH FIRST 1 ROW ONLY clause ensures that we only retrieve one row for each prefix-matching combination, which prevents multiple results from appearing in our final result set.

Conclusion

In this article, we explored how to join two tables based on similar values using a Common Table Expression (CTE). By filtering out rows from tab2 where the similarity between prefixes is less than 4 characters, we ensured that only one result appears for each entry in tab1. This approach can be applied to various database queries and scenarios where you need to join tables based on non-exact matching values.

Example Use Cases

Joining customer information with order details based on the first four characters of the customer’s name.
Merging data from different sources into a single dataset, where similar values in one column are matched against another.
Creating a report that shows only the top N results for each category, where categories are defined by prefixes or suffixes.

Step-by-Step Solution

Define your two tables: tab1 and tab2.
Create a CTE to filter out rows from tab2 based on prefix matching.
Join the CTE with tab1 using the filtered results.
Apply any additional filtering or sorting as needed.

Advice

Use meaningful table and column names to improve query readability.
Experiment with different prefix lengths and similarity thresholds to customize your query.
Consider using indexes on columns used in the LIKE operator for improved performance.

Last modified on 2023-10-03