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String Split Function in PostgreSQL 17.01, SQL Server 2019, or MySQL 9.0

Introduction

String splitting is a common operation in SQL databases, often used when dealing with data that is stored in delimited formats. It is especially useful in scenarios like processing CSV files, parsing user input, or handling lists stored as strings in database fields. In this article, we will explore how string splitting is performed across three major database systems: PostgreSQL 17.01, SQL Server 2019, and MySQL 9.0. We'll look at the functions and techniques available in each database to perform this operation efficiently, with real-world use cases and examples.

Table of Contents

1. Overview of String Splitting
2. String Splitting in PostgreSQL 17.01
   • 2.1. Using string_to_array
   • 2.2. Using unnest for Splitting
   • 2.3. Example Scenarios in PostgreSQL
3. String Splitting in SQL Server 2019
   • 3.1. Using STRING_SPLIT
   • 3.2. Creating a Custom Split Function
   • 3.3. Example Scenarios in SQL Server
4. String Splitting in MySQL 9.0
   • 4.1. Using SUBSTRING_INDEX
   • 4.2. Creating a Custom Split Function with Recursive Queries
   • 4.3. Example Scenarios in MySQL
5. Comparison of String Split Functions Across Databases
6. Use Cases and Performance Considerations
7. Conclusion

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Overview of String Splitting

String splitting refers to the process of breaking a single string into multiple components based on a delimiter, such as a comma, space, or another character. This is particularly useful in scenarios where data is stored in a single field but represents multiple values (e.g., a list of tags or CSV data). The ability to split strings efficiently is crucial for effective data manipulation, especially in databases.

Common Use Cases

• Parsing CSV data: Breaking down a CSV record into individual columns.
• Tagging systems: When tags are stored as a single string with delimiters.
• User input processing: When users provide multiple items in a single input field.
• Log data analysis: Parsing log entries where data is delimited by certain characters.

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String Splitting in PostgreSQL 17.01

PostgreSQL offers powerful string manipulation functions. In version 17.01, string splitting can be done using the string_to_array function, which splits a string into an array of substrings, and unnest for working with arrays.

2.1. Using string_to_array

The string_to_array function splits a string based on a delimiter and returns an array of substrings.

Syntax:

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string_to_array(string text, delimiter text)

Example 1: Splitting a Comma-Separated List

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SELECT string_to_array('apple,banana,orange', ',');

Result:

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{apple,banana,orange}

2.2. Using unnest for Splitting

To work with the results from string_to_array, you can use the unnest function, which expands an array into a set of rows.

Example 2: Expanding a Comma-Separated List into Multiple Rows

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SELECT unnest(string_to_array('apple,banana,orange', ',')) AS fruit;

Result:

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fruit
------
apple
banana
orange

2.3. Example Scenarios in PostgreSQL

Scenario 1: Processing Tags in a Post

Imagine a scenario where a blog post contains tags stored in a comma-separated string. You want to split these tags into individual rows to analyze them.

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SELECT post_id, unnest(string_to_array(tags, ',')) AS tag
FROM blog_posts;

Scenario 2: Parsing CSV Data

Consider a table with a column that holds CSV strings. You can use string_to_array to split and analyze each CSV entry.

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SELECT string_to_array(csv_column, ',')
FROM csv_data_table;

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String Splitting in SQL Server 2019

SQL Server 2016 introduced the STRING_SPLIT function, which is very useful for splitting strings. However, it's important to note that this function may not work as expected in older versions of SQL Server.

3.1. Using STRING_SPLIT

The STRING_SPLIT function splits a string into rows based on a specified delimiter.

Syntax:

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STRING_SPLIT ( string , separator )

Example 1: Splitting a Comma-Separated List

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SELECT value
FROM STRING_SPLIT('apple,banana,orange', ',');

Result:

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value
------
apple
banana
orange

3.2. Creating a Custom Split Function

If you are working with an older version of SQL Server, or if you need more flexibility, you can create a custom split function using a recursive CTE (Common Table Expression).

Example 2: Custom Split Function

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CREATE FUNCTION dbo.fn_SplitString (@String NVARCHAR(MAX), @Delimiter CHAR(1))
RETURNS @Output TABLE (Value NVARCHAR(MAX))
AS
BEGIN
    DECLARE @Start INT, @End INT
    SET @Start = 1
    SET @End = CHARINDEX(@Delimiter, @String)
    
    WHILE @Start < LEN(@String) + 1
    BEGIN
        IF @End = 0 
            SET @End = LEN(@String) + 1
        INSERT INTO @Output (Value)
        VALUES (SUBSTRING(@String, @Start, @End - @Start))
        SET @Start = @End + 1
        SET @End = CHARINDEX(@Delimiter, @String, @Start)
    END
    RETURN
END

3.3. Example Scenarios in SQL Server

Scenario 1: Parsing Tags in a Post

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SELECT post_id, value AS tag
FROM blog_posts
CROSS APPLY STRING_SPLIT(tags, ',');

Scenario 2: Querying Data from a CSV Column

If you store data as CSV, you can split and query it using STRING_SPLIT.

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SELECT value
FROM STRING_SPLIT(csv_column, ',')
WHERE value LIKE 'apple%';

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String Splitting in MySQL 9.0

MySQL lacks a built-in STRING_SPLIT function, but you can use SUBSTRING_INDEX and recursive queries to achieve similar results.

4.1. Using SUBSTRING_INDEX

The SUBSTRING_INDEX function extracts a substring from a string, up to a specified number of occurrences of a delimiter.

Syntax:

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SUBSTRING_INDEX(string, delimiter, count)

Example 1: Splitting a Comma-Separated List

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SELECT SUBSTRING_INDEX('apple,banana,orange', ',', 1) AS fruit1,
       SUBSTRING_INDEX(SUBSTRING_INDEX('apple,banana,orange', ',', 2), ',', -1) AS fruit2,
       SUBSTRING_INDEX('apple,banana,orange', ',', -1) AS fruit3;

Result:

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fruit1  | fruit2  | fruit3
--------|---------|--------
apple   | banana  | orange

4.2. Creating a Custom Split Function with Recursive Queries

To split strings into rows, you can create a recursive query.

Example 2: Recursive Split Function

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WITH RECURSIVE Split AS (
    SELECT
        SUBSTRING_INDEX('apple,banana,orange', ',', 1) AS value,
        SUBSTRING('apple,banana,orange', LENGTH(SUBSTRING_INDEX('apple,banana,orange', ',', 1)) + 2) AS rest
    UNION ALL
    SELECT
        SUBSTRING_INDEX(rest, ',', 1),
        SUBSTRING(rest, LENGTH(SUBSTRING_INDEX(rest, ',', 1)) + 2)
    FROM Split
    WHERE rest != ''
)
SELECT value FROM Split;

4.3. Example Scenarios in MySQL

Scenario 1: Parsing Tags in a Post

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SELECT post_id, SUBSTRING_INDEX(SUBSTRING_INDEX(tags, ',', n.n), ',', -1) AS tag
FROM blog_posts
JOIN (SELECT