MySQL replace only first occurence in text field/column - mysql

Question

I wrote a function to replace first occurence in MySQL Text colum, but it's a little bit complicated...
UPDATE
table_name
SET
column=CONCAT(
LEFT(column,LOCATE('some string', column)-1),
CONCAT(substring(column, LOCATE('some string', column) + $length),
'new string'))
Where $length is length of string, that we want to replace. If we use php it is strlen() function but in MySQL it would be CHAR_LENGTH() function.
Do you know better way to replace only first match in text columns ?

Answer 1

You could use TRIM:
UPDATE table_name SET column = TRIM(LEADING 'some string' FROM column);
assuming 'some string' does not have more than 1 consecutive occurrence at the start of the contents of 'column'.
So, it would work if column contained:
"some string foo some string"
but not for:
"some string some string foo some string"
Edit - Added MySQL function to simplify process
I can't see an alternative to the mechanism you are using, but executing it could be simplified by creating a function in MySQL (if you have the privilege):
delimiter $$
create function replace_first(
p_text_to_search varchar(255),
p_text_to_replace varchar(255)
)
returns varchar(255)
begin
declare v_found_pos int(11);
declare v_found_len int(11);
declare v_text_with_replacement varchar(255);
select locate(p_text_to_replace, p_text_to_search)
into v_found_pos;
select char_length(p_text_to_replace)
into v_found_len;
select concat(
left(p_text_to_search, v_found_pos-1),
mid(p_text_to_search, (v_found_pos + v_found_len))
)
into v_text_with_replacement;
return v_text_with_replacement;
end$$
delimiter ;
then you can call it using:
select replace_first('bar foo foo baz foo', 'foo');
result:
'bar foo baz'

Answer 2

I have created function can replace any index of text:
/************** REPLACE_TEXT_OF_INDEX ***************/
DROP FUNCTION IF EXISTS REPLACE_TEXT_OF_INDEX;
DELIMITER $$
CREATE FUNCTION REPLACE_TEXT_OF_INDEX(_text VARCHAR(3072), _subText VARCHAR(1024), _toReplaceText VARCHAR(1024), _index INT UNSIGNED)
RETURNS VARCHAR(3072)
BEGIN
DECLARE _prefixText, _sufixText VARCHAR(3072);
DECLARE _starIndex INT;
DECLARE _loopIndex, _textIndex INT UNSIGNED DEFAULT 0;
IF _text IS NOT NULL AND LENGTH(_text) > 0 AND
_subText IS NOT NULL AND LENGTH(_subText) > 0 AND
_toReplaceText IS NOT NULL AND _index > 0 THEN
WHILE _loopIndex < LENGTH(_text) AND _textIndex < _index DO
SELECT LOCATE(_subText, _text, _loopIndex + 1) INTO _loopIndex;
IF _loopIndex > 0 THEN
SET _textIndex = _textIndex + 1;
ELSE
SET _loopIndex = LENGTH(_text) + 1;
END IF;
END WHILE;
IF _textIndex = _index THEN
SELECT LOCATE(_subText, _text, _loopIndex) INTO _starIndex;
SELECT SUBSTRING(_text, 1, _starIndex -1) INTO _prefixText;
SELECT SUBSTRING(_text, _starIndex + LENGTH(_subText), LENGTH(_text)) INTO _sufixText;
RETURN CONCAT(_prefixText, _toReplaceText, _sufixText);
END IF;
END IF;
RETURN _text;
END;
$$
DELIMITER ;
SELECT REPLACE_TEXT_OF_INDEX('WORD1 WORD2 WORD3 WORD4 WORD5', 'WORD', '*',1);