how does mysql user defined function know a selected row was found? - mysql

Question

a MYSQL user defined function selects a row from a table. How does the UDF code determine if the selected row was found in the table?
CREATE FUNCTION snippetFolder_folderPath(folder_id int)
RETURNS varchar(512)
BEGIN
declare vFolder_id int;
declare vParent_id int;
declare vPath varchar(512) default '';
declare vFolderName varchar(256) default '';
set vFolder_id = folder_id;
build_path:
while (vFolder_id > 0) do
/* -------- how to know this select statement returns a row?? ---------- */
select a.parent_id, a.folderName
into vParent_id, vFolderName
from SnippetFolder a
where a.folder_id = vFolder_id;
if vPath = ' ' then
set vPath = vFolderName;
else
set vPath = concat_ws( '/', vFolderName, vPath );
end if ;
set vFolder_id = vParent_id;
end while ;
return vPath;
END

Answer 1

https://dev.mysql.com/doc/refman/8.0/en/select-into.html says:
If the query returns no rows, a warning with error code 1329 occurs (No data), and the variable values remain unchanged.
So you could declare a continue handler on warnings, something like the example from the documentation:
BEGIN
DECLARE i INT DEFAULT 3;
DECLARE done INT DEFAULT FALSE;
retry:
REPEAT
BEGIN
DECLARE CONTINUE HANDLER FOR SQLWARNING
BEGIN
SET done = TRUE;
END;
IF done OR i < 0 THEN
LEAVE retry;
END IF;
SET i = i - 1;
END;
UNTIL FALSE END REPEAT;
END
I'll leave it to you to read the documentation and adapt that example to your table and your loop.
Alternatively, if you're using MySQL 8.0 you can use recursive common table expression:
CREATE FUNCTION snippetFolder_folderPath(vFolder_id int)
RETURNS varchar(512)
BEGIN
DECLARE vPath varchar(512) DEFAULT '';
WITH RECURSIVE cte AS (
SELECT folderName, parent_id, 0 AS height
FROM SnippetFolder WHERE folder_id = vFolder_id
UNION
SELECT f.folderName, f.parent_id, cte.height+1
FROM SnippetFolder AS f JOIN cte ON cte.parent_id = f.folder_id
)
SELECT GROUP_CONCAT(folderName ORDER BY height DESC SEPARATOR '/')
INTO vPath
FROM cte;
RETURN vPath;
END
The recursive CTE result is all the ancestors of the row matching vFolder_id, and then one can use GROUP_CONCAT() to concatenate them together as one string.