Error when using union and putting column into variable

Question

I am using Mariadb (10.5.12-MariaDB-1build1) on Ubuntu.

I had created an insert trigger on a table; that uses union statement to check for previous records. It work fine in previous version (10.3.31-MariaDB-0ubuntu.20.04.1) but throw error in this version

I had simplified the statement

 SET @ID = '';

union is on several table but I had reduced it to two

SELECT id FROM table1 
union 
SELECT id 
 INTO @ID 
FROM table2;

Give me this error

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'FROM table2 at line 'line number'

Obviously if i comment the INTO clause it is fine.

So i guess problem is in INTO. i guess more specifically the placement of into. Because single select statement doesn't give any error.

I will check into documentation for what i am doing wrong.

---

@Barmar here is the actual code

DELIMITER ;;
/*!50003 CREATE*/ /*!50017 DEFINER=`root`@`localhost`*/ /*!50003 TRIGGER CRPInsertTrigger
BEFORE INSERT ON CRP
FOR EACH ROW
BEGIN
    SET @msg = '';
    SET @tire = @ID = @Name = '';

    SELECT 'coMem' `tire`, coMemID `ID`, coMemName `Name` FROM coMem 
    WHERE institute = NEW.institute and registration = NEW.registration and bookNo = NEW.bookNo
    union
    SELECT 'commercialMem' `tire`, commercialMemID `ID`, commercialMemName `Name` FROM commercialMem 
    WHERE institute = NEW.institute and registration = NEW.registration and bookNo = NEW.bookNo
    union
    SELECT 'communityGeneralMem' `tire`, communityGeneralMemID `ID`, communityGeneralMemName `Name` FROM communityGeneralMem 
    WHERE institute = NEW.institute and registration = NEW.registration and bookNo = NEW.bookNo
    union
    SELECT 'CRP' `tire`, CRPID `ID`, CRPName `Name` FROM CRP 
    WHERE institute = NEW.institute and registration = NEW.registration and bookNo = NEW.bookNo
    union
    SELECT 'lsoMem' `tire`, lsoMemID `ID`, lsoMemName `Name` FROM lsoMem 
    WHERE institute = NEW.institute and registration = NEW.registration and bookNo = NEW.bookNo
    union
    SELECT 'voMem' `tire`, voMemID `ID`, voMemName `Name` 
    INTO @tire, @ID, @Name FROM voMem 
    WHERE institute = NEW.institute and registration = NEW.registration and bookNo = NEW.bookNo;

    IF @Name != '' THEN
    SET @msg = CONCAT('Error: Duplicate in registration found: ', CAST(NEW.registration AS CHAR CHARACTER SET utf8) COLLATE utf8_bin);
    SET @msg = CONCAT(@msg, '(', @tire, 'ID:',CAST(@ID AS CHAR CHARACTER SET utf8) COLLATE utf8_bin);
    SET @msg = CONCAT(@msg, ', Name:', CAST(@Name AS CHAR CHARACTER SET utf8) COLLATE utf8_bin, ')');
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = @msg;
    END IF;
END */;;
DELIMITER ;