Unearthing the Last Substring Position in SQLite Queries

3. Recursive Logic (within the query): You can employ a recursive approach within the SQL query itself. Here's how it works:

   • Use INSTR to find the first occurrence of the substring.
   • If INSTR doesn't find the substring (returns 0), then the substring isn't present, and the last index is -1 (not found).
   • If INSTR finds the substring, use SUBSTR to extract the remaining string after the first occurrence.
   • Apply the logic again to this remaining string (recursive call) to search for the substring within it. This effectively searches for occurrences further down the original string.
   • The final position returned by INSTR (after all the recursive calls) will be the last occurrence's starting index.

SELECT myString,
       CASE 
           WHEN instr(myString, mySubString) = 0 THEN -1  -- Substring not found
           ELSE length(myString) - length(
               substr(myString, instr(myString, mySubString) + length(mySubString))
           )
       END AS last_index
FROM yourTable;

Explanation:

1. We define two variables:

   • myString: This represents the column containing the string where you want to find the substring.
   • mySubString: This represents the substring you're searching for.

2. The CASE statement checks for two conditions:

3. Otherwise, a more complex expression is used:

   • length(myString) gets the total length of the original string.
   • instr(myString, mySubString) finds the first occurrence of the substring and returns its starting position.
   • We add the starting position (instr(myString, mySubString)) and the substring length to get the index right after the first occurrence.
   • substr(myString, ..., ) extracts the remaining string after that index (effectively removing the first occurrence).
   • length(...) on the result of substr gets the length of the remaining string (which doesn't include the first occurrence).
   • Finally, we subtract the remaining string length from the original string length. This effectively calculates the starting position of the last occurrence (since everything after that position has been removed).

1. Regular Expressions (LIKE Operator with Wildcards):

   • SQLite's LIKE operator allows pattern matching using wildcards. You can construct a pattern that essentially matches the entire string followed by the substring.

   Example:

   SELECT myString,
          CASE WHEN myString LIKE '%mySubString' THEN instr(myString, 'mySubString') - 1
               ELSE -1
          END AS last_index
   FROM yourTable;
   

   • myString LIKE '%mySubString' checks if the string ends with the substring (% represents any character sequence).
   • If the pattern matches, instr(myString, 'mySubString') finds the starting position of the substring, and we subtract 1 to get the last character's index (since the pattern matches the entire string followed by the substring).
   • If there's no match, the last_index is set to -1.

   Note: This method might be less performant for complex patterns, and it won't work if the substring can appear anywhere in the string (it only checks for the end).

2. User-Defined Functions (UDFs):

   • SQLite allows you to create custom functions written in languages like C or C++. You can develop a UDF specifically designed to find the last occurrence of a substring within a string.

   Advantage: This approach offers greater flexibility and potentially better performance for complex scenarios.

   Disadvantage: Creating UDFs requires more programming knowledge and can be less portable compared to using built-in functions.

The best method for your specific situation depends on factors like:

• Complexity of the substring search pattern
• Performance requirements
• Your comfort level with UDF development