MySQL: SELECT to local variable in stored procedure I have the following mysql stored procedure:
DELIMITER //
CREATE PROCEDURE INSERT_INTO_WORKDAY(IN deviceName VARCHAR(16),
                                     IN cardUID VARCHAR(14)) 
BEGIN
    SET @projectId = (SELECT MIN(ID) FROM PROJECT WHERE DEVICEID =
          (SELECT MIN(ID) FROM DEVICE WHERE NAME = deviceName)
             ORDER BY ID DESC LIMIT 1);
    SET @workerId = (SELECT MIN(ID) FROM WORKER WHERE CARDNUMBER = cardUID);
    SELECT @workerId;
    
    SET @id = (SELECT MIN(ID) FROM WORKDAY
                   WHERE WORKERID = @workerId
                     AND PROJECTID = @projectId
                     AND STOPTIME IS NULL
                     AND STARTTIME IS NOT NULL);
    IF (@id &gt;= 0)
        THEN
            UPDATE WORKDAY SET STOPTIME = now() WHERE ID = @id;
    ELSE
        INSERT INTO WORKDAY (WORKERID, PROJECTID, STARTTIME)
            VALUES (@workerId, @projectId, now());
    END IF;
END; //
DELIMITER ;

When I call the procedure once, I get the result:
mysql&gt; CALL INSERT_INTO_WORKDAY(&quot;Pepper_C1-1A6318&quot;, &quot;045D91B22C5E80&quot;);
+-----------+
| @workerId |
+-----------+
|         1 |
+-----------+
1 row in set (0.01 sec)

Query OK, 1 row affected (0.09 sec)

From second run, is not working anymore:
mysql&gt; CALL INSERT_INTO_WORKDAY(&quot;Pepper_C1-1A6318&quot;, &quot;045D91B22C5E80&quot;);
+-----------+
| @workerId |
+-----------+
|      NULL |
+-----------+
1 row in set (0.00 sec)

ERROR 1048 (23000): Column 'WORKERID' cannot be null

I did try to isolate and run the procedure with only this two lines of code:
    SET @workerId = (SELECT MIN(ID) FROM WORKER WHERE CARDNUMBER = cardUID);
    SELECT @workerId;

And it behaves just like above, but if I try to run SELECT MIN(ID) FROM WORKER WHERE CARDNUMBER = &quot;045D91B22C5E80&quot;; alone, works without problems.
The @projectId variable behaves normal everytime I call the stored procedure.
Can someone help me to find a solution?
Thank you!
My setup:
mihai:~$ mysql -V
mysql  Ver 8.0.22-0ubuntu0.20.04.3 for Linux on x86_64 ((Ubuntu))

mysql&gt; describe WORKER;
+------------+-------------+------+-----+---------+----------------+
| Field      | Type        | Null | Key | Default | Extra          |
+------------+-------------+------+-----+---------+----------------+
| ID         | int         | NO   | PRI | NULL    | auto_increment |
| FIRSTNAME  | varchar(15) | NO   |     | NULL    |                |
| LASTNAME   | varchar(20) | NO   |     | NULL    |                |
| CARDNUMBER | varchar(14) | NO   |     | NULL    |                |
| POSITIONID | int         | NO   | MUL | NULL    |                |
+------------+-------------+------+-----+---------+----------------+
5 rows in set (0.01 sec)

mysql&gt; SELECT * FROM WORKER;
+----+-------------+-----------+----------------+------------+
| ID | FIRSTNAME   | LASTNAME  | CARDNUMBER     | POSITIONID |
+----+-------------+-----------+----------------+------------+
|  1 | Ionut Mihai | Popescu   | 045D91B22C5E80 |          1 |
|  2 | Ilie        | Zbagan    | 040D7FB22C5E81 |          2 |
|  3 | Ilie        | Ungureanu | 043FA8B22C5E80 |          1 |
|  4 | Adrian      | Tehanciuc | 04D894B22C5E80 |          2 |
|  5 | Ioan        | Bitoanca  | 0405C6B22C5E81 |          2 |
+----+-------------+-----------+----------------+------------+
5 rows in set (0.00 sec)

DROP TABLE IF EXISTS `WORKER`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!50503 SET character_set_client = utf8mb4 */;
CREATE TABLE `WORKER` (
  `ID` int NOT NULL AUTO_INCREMENT,
  `FIRSTNAME` varchar(15) NOT NULL,
  `LASTNAME` varchar(20) NOT NULL,
  `CARDNUMBER` varchar(14) NOT NULL,
  `POSITIONID` int NOT NULL,
  PRIMARY KEY (`ID`),
  KEY `POSITIONID` (`POSITIONID`),
  CONSTRAINT `WORKER_ibfk_1` FOREIGN KEY (`POSITIONID`) REFERENCES `POSITION` (`ID`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;
The problem was my declared local variable id.
After I changed her name, everything worked just fine.
My previous answer was:
I don't know what the problem was, but I did found a solution.
I wrote:
    SELECT MIN(ID) FROM WORKER WHERE CARDNUMBER = cardUID INTO @workerId;
    SELECT @workerId;

instead of
    SET @workerId = (SELECT MIN(ID) FROM WORKER WHERE CARDNUMBER = cardUID);
    SELECT @workerId;