A SQL Query to select a string between two known strings

Handling edge-cases

Datatypes are as unpredictable as cats, often containing non-existing or overlapping strings. To handle such instances, consider the following:

Taking care of ghost strings

If end_string is absent from your data, you want to ensure your SQL query doesn't pull a Houdini and disappear into the ether. Use a CASE expression in such instances:

SELECT 
    CASE 
        /* This CASE plays a better detective than Sherlock */
        WHEN CHARINDEX('end_string', text_column) > 0 
        THEN SUBSTRING(text_column, 
                       CHARINDEX('start_string', text_column) + LEN('start_string'), 
                       CHARINDEX('end_string', text_column) - CHARINDEX('start_string', text_column) - LEN('start_string'))
        ELSE NULL 
    END
FROM 
    your_table
WHERE 
    /* LIKE is on a mission this time */
    text_column LIKE '%start_string%'


Handling strings that overlap

Imagine a scenario where your start_string and end_string overlap, like abcabc. You need to ensure it captures the right match:

SELECT 
    /* This query does yoga to find the right match */
    SUBSTRING(text_column,
              CHARINDEX('start_string', text_column) + LEN('start_string'),
              CHARINDEX('end_string', text_column, CHARINDEX('start_string', text_column) + LEN('start_string')) - CHARINDEX('start_string', text_column) - LEN('start_string'))
FROM 
    your_table
WHERE 
    /* LIKE's got no chill */
    text_column LIKE '%start_string%%end_string%'


Cheat codes: Using variables

Variables in SQL are akin to cheat codes in video games. They make your life a lot easier:

DECLARE @StartIndex INT, @EndIndex INT, @StartLen INT;

SET @StartLen = LEN('start_string');

SELECT 
    /* Variables in action */
    @StartIndex = CHARINDEX('start_string', text_column),
    @EndIndex = CHARINDEX('end_string', text_column, @StartIndex + @StartLen)
FROM 
    your_table
WHERE 
    /* LIKE's back, back again */
    text_column LIKE '%start_string%%end_string%';

SELECT 
    /* The stage is set, the variables act */
    SUBSTRING(text_column, @StartIndex + @StartLen, @EndIndex - @StartIndex - @StartLen)
FROM 
    your_table
WHERE 
    /* LIKE still refusing to quit */
    text_column LIKE '%start_string%%end_string%';


Takeaways and tips: The deeper dive

Plunge into the depths of SQL querying by understanding the nitty-gritty.

Case sensitivity matters

For accurately matching patterns in case-sensitive databases, use the COLLATE clause:

SELECT 
    SUBSTRING(text_column COLLATE SQL_Latin1_General_CP1_CS_AS,
              CHARINDEX('start_string' COLLATE SQL_Latin1_General_CP1_CS_AS, text_column) + LEN('start_string'),
              CHARINDEX('end_string' COLLATE SQL_Latin1_General_CP1_CS_AS, text_column) - CHARINDEX('start_string' COLLATE SQL_Latin1_General_CP1_CS_AS, text_column) - LEN('start_string'))
FROM 
    your_table
WHERE 
    /* LIKE says, "Case Sensitive? Challenge Accepted!" */
    text_column LIKE '%start_string%end_string%' COLLATE SQL_Latin1_General_CP1_CS_AS


Reusability for the win

A reusable query is the gift that keeps on giving. How about wrapping your logic into a user-defined function?

CREATE FUNCTION dbo.ExtractString (@Text NVARCHAR(MAX), @StartString NVARCHAR(255), @EndString NVARCHAR(255))
RETURNS NVARCHAR(MAX)
AS
BEGIN
    RETURN (
        SELECT 
            /* Don't you wish everything in life was this reusable? */
            SUBSTRING(@Text, CHARINDEX(@StartString, @Text) + LEN(@StartString), CHARINDEX(@EndString, @Text) - CHARINDEX(@StartString, @Text) - LEN(@StartString))
    )
END


Optimization is key

When running on large datasets, consider indexes on patterns and even full-text search for performance enhancements.

Testing is saving

Don't forget to rigorously test your SQL query on diverse inputs. Think of it as saving your present self from your future self's wrath!