Endless Loop or No Output for Recursive SQL Query - sql

Question

I have the following table,
CREATE TABLE AGENT (
A_CODE int,
A_FNAME varchar(15),
S_CODE int /* supervisor */
);
INSERT INTO AGENT VALUES(1,'John',1);
INSERT INTO AGENT VALUES(2,'Nancy',1);
INSERT INTO AGENT VALUES(3,'Lottie',2);
INSERT INTO AGENT VALUES(4,'Jennie',1);
INSERT INTO AGENT VALUES(5,'Robert',3);
INSERT INTO AGENT VALUES(6,'Cary',1);
INSERT INTO AGENT VALUES(7,'Roberto',3);
INSERT INTO AGENT VALUES(8,'Elizabeth',1);
INSERT INTO AGENT VALUES(9,'Jack',2);
INSERT INTO AGENT VALUES(10,'Rose',4);
INSERT INTO AGENT VALUES(11,'Tom',3);
INSERT INTO AGENT VALUES(12,'Alan',2);
INSERT INTO AGENT VALUES(13,'Peter',3);
INSERT INTO AGENT VALUES(14,'Sherry',4);
INSERT INTO AGENT VALUES(15,'Howard',5);
INSERT INTO AGENT VALUES(16,'Barry',5);
INSERT INTO AGENT VALUES(17,'Jeanine',4);
INSERT INTO AGENT VALUES(18,'Andrew',4);
INSERT INTO AGENT VALUES(19,'Peter',4);
INSERT INTO AGENT VALUES(20,'Robert',1);
INSERT INTO AGENT VALUES(21,'Jennifer',3);
My goal is to create a supervisor view in which there is a 4th column that has the name of the supervisor that matches the S code.
My query returns to me either nothing except the column headers or a never ending recursion that loops into the 100,000's.
What I've written is
WITH SUPERVISOR AS
(
SELECT A_CODE, A_FNAME, S_CODE, CAST('' AS VARCHAR(50)) S_NAME FROM AGENT
UNION ALL
SELECT A.A_CODE,A.A_FNAME, A.S_CODE,CAST(S.A_FNAME AS VARCHAR(50)) S_NAME FROM AGENT AS A
INNER JOIN SUPERVISOR AS S ON S.A_CODE=A.S_CODE
)
SELECT * FROM SUPERVISOR
option (maxrecursion 0);
Returns endlessly,
or
WITH SUPERVISOR AS
(
SELECT A_CODE, A_FNAME, S_CODE, CAST('' AS VARCHAR(50)) S_NAME FROM AGENT WHERE S_CODE is NULL
UNION ALL
SELECT A.A_CODE,A.A_FNAME, A.S_CODE,CAST(S.A_FNAME AS VARCHAR(50)) S_NAME FROM AGENT AS A
INNER JOIN SUPERVISOR AS S ON S.A_CODE=A.S_CODE
)
SELECT * FROM SUPERVISOR
option (maxrecursion 0);
I've been trying to read about this and figure it out on my own but at this point I've struggled for long enough that I think I need it explained to me as to why it's not working or how wildly wrong I'm going about this.

Answer 1

Please try the following solution. It is using self-join approach with two aliases for the same table: e(mployee) and s(upervisor).
SQL
-- DDL and sample data population, start
DECLARE #AGENT TABLE (A_CODE INT PRIMARY KEY, A_FNAME varchar(15), S_CODE int /* supervisor */);
INSERT INTO #AGENT VALUES
(1,'John',1)
,(2,'Nancy',1)
,(3,'Lottie',2)
,(4,'Jennie',1)
,(5,'Robert',3)
,(6,'Cary',1)
,(7,'Roberto',3)
,(8,'Elizabeth',1)
,(9,'Jack',2)
,(10,'Rose',4)
,(11,'Tom',3)
,(12,'Alan',2)
,(13,'Peter',3)
,(14,'Sherry',4)
,(15,'Howard',5)
,(16,'Barry',5)
,(17,'Jeanine',4)
,(18,'Andrew',4)
,(19,'Peter',4)
,(20,'Robert',1)
,(21,'Jennifer',3);
-- DDL and sample data population, end
SELECT e.*, s.A_FNAME AS Supervisor
FROM #AGENT AS e INNER JOIN
#AGENT AS s ON s.A_CODE = e.S_CODE;
Output
+--------+-----------+--------+------------+
| A_CODE | A_FNAME | S_CODE | Supervisor |
+--------+-----------+--------+------------+
| 1 | John | 1 | John |
| 2 | Nancy | 1 | John |
| 3 | Lottie | 2 | Nancy |
| 4 | Jennie | 1 | John |
| 5 | Robert | 3 | Lottie |
| 6 | Cary | 1 | John |
| 7 | Roberto | 3 | Lottie |
| 8 | Elizabeth | 1 | John |
| 9 | Jack | 2 | Nancy |
| 10 | Rose | 4 | Jennie |
| 11 | Tom | 3 | Lottie |
| 12 | Alan | 2 | Nancy |
| 13 | Peter | 3 | Lottie |
| 14 | Sherry | 4 | Jennie |
| 15 | Howard | 5 | Robert |
| 16 | Barry | 5 | Robert |
| 17 | Jeanine | 4 | Jennie |
| 18 | Andrew | 4 | Jennie |
| 19 | Peter | 4 | Jennie |
| 20 | Robert | 1 | John |
| 21 | Jennifer | 3 | Lottie |
+--------+-----------+--------+------------+