How to Correctly Extract a Substring Between Two Delimiters in SQL?

Retrieving a String Between Known Delimiters Using SQL

In database scenarios, it's often necessary to extract a specific substring from a larger string based on known delimiters. For instance, you may need to isolate a portion of text between two given strings.

One common approach is to utilize the SUBSTRING function. However, a common error when using this function is including the first known string's index in the length calculation for the substring.

To correctly retrieve a substring between two delimiters, the following calculation is required:

CHARINDEX(second_delimiter, @Text) - CHARINDEX(first_delimiter, @Text) + Length(second_delimiter)

This formula ensures that the retrieved substring starts with the first delimiter and ends with the second delimiter.

For example, consider the following query:

SELECT SUBSTRING(@Text, CHARINDEX('the dog', @Text),
CHARINDEX('immediately', @Text) - CHARINDEX('the dog', @Text) + LEN('immediately'))

In this query, @Text represents the input string. By specifying the known strings "the dog" and "immediately", the query accurately retrieves the substring "the dog had been very bad and required harsh punishment immediately."

By correcting the substring length calculation, you can effectively extract specific portions of text within larger strings using SQL.

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