Pattern Matching to Create a New Column in SQL

In this article, we will explore how to create a new column in an SQL table based on pattern matching. We’ll dive into the specifics of the problem presented and provide detailed solutions using various SQL techniques.

Understanding the Problem

The problem at hand involves creating a new column called “Parent Property Name” in a given SQL table. The values in this column should match the parent property name for each unique value in the “PropertyID” column before the hyphen. If there is no hyphen, the value should be the same as the original “Property Name”.

Breaking Down the Problem

To approach this problem, we need to understand the following concepts:

Pattern matching: This refers to the process of searching for specific patterns in strings.
Left join: A left join is used to retrieve records from one table and match them with records from another table based on a common column.
Partition by: The PARTITION BY clause is used to divide the result set produced by an aggregate function into partitions to which the function is applied.

Solution 1: Using Left Join

One way to solve this problem is by using a left join. We’ll join the original table with itself on the condition that the value before the hyphen in the “PropertyID” column matches the entire value in the “PropertyId” column of the other instance.

SELECT t.*, tparent.PropertyId AS parent_propertyid
FROM t
JOIN t tparent ON LEFT(t.PropertyId, CHARINDEX('-', t.PropertyId + '-') - 1) = tparent.PropertyId;

In this query:

We’re using a left join to combine the original table with itself on the specified condition.
The LEFT function returns the specified number of characters from the start of a string.
The CHARINDEX function is used to find the position of the hyphen in the “PropertyId” column.
We’re assigning the value of parent_propertyid from the joined instance as an alias.

Solution 2: Using First Value Function

Another approach is by using the FIRST_VALUE function, which allows us to retrieve the first occurrence of a pattern within a string.

SELECT t.*, 
       FIRST_VALUE(t.PropertyName) OVER (PARTITION BY LEFT(t.PropertyId, CHARINDEX('-', t.PropertyId + '-') - 1)
                                          ORDER BY PropertyId) AS parent_propertyname
FROM t;

In this query:

We’re using the FIRST_VALUE function to retrieve the first occurrence of the “PropertyName” within a specified pattern.
The PARTITION BY clause is used to divide the result set into partitions based on the pattern.
The ORDER BY clause specifies that we want to order the results by the “PropertyId”.

Conclusion

In this article, we explored how to create a new column in an SQL table based on pattern matching. We delved into two different solutions using left join and first value function, providing detailed explanations of the underlying concepts.

These techniques can be applied to various real-world scenarios where you need to perform string manipulation or pattern recognition in SQL queries.

Example Use Case

Suppose we have a database containing information about properties, including their names and IDs. We want to create a new column that shows the parent property name for each unique value in the “PropertyID” column before the hyphen.

CREATE TABLE Properties (
    PropertyId VARCHAR(255),
    PropertyName VARCHAR(255)
);

INSERT INTO Properties (PropertyId, PropertyName) 
VALUES ('A001', 'Jefferson'), ('A002', 'Madison'), ('A002-01', 'Madison Outhouse');

We can use one of the solutions provided in this article to create the new column:

SELECT t.*, 
       LEFT(t.PropertyId, CHARINDEX('-', t.PropertyId + '-') - 1) AS parent_propertyname
FROM Properties;

This query will return a table with the original “PropertyId” and “PropertyName”, as well as the “parent_propertyname” column containing the parent property name for each unique value in the “PropertyId” column before the hyphen.

Last modified on 2023-12-05