Ciclo infinito in CTE durante l'analisi della tabella autoreferenziale

Question

Sto usando la seguente espressione di tabella comune per analizzare la tabella autoreferenziale. Ma il CTE non funziona, produce e ciclo infinito e genera un errore:

Msg 530, Level 16, State 1, Line 1 The statement terminated. The maximum recursion 100 has been exhausted before statement completion.

Come modificare questo CTE per farlo funzionare?

SET NOCOUNT ON; 
USE tempdb; 

IF OBJECT_ID('dbo.Employees', 'U') IS NOT NULL DROP TABLE dbo.Employees; 

CREATE TABLE dbo.Employees 
(
    empid INT   NOT NULL PRIMARY KEY, 
    mgrid INT   NULL  REFERENCES dbo.Employees, 
    empname VARCHAR(25) NOT NULL, 
    salary MONEY  NOT NULL, 
    CHECK (empid > 0) 
); 

INSERT INTO dbo.Employees(empid, mgrid, empname, salary) VALUES 
    (1, 1, 'David' , $10000.00), 
    (2, 1, 'Eitan' , $7000.00), 
    (3, 1, 'Ina' , $7500.00), 
    (4, 2, 'Seraph' , $5000.00), 
    (5, 2, 'Jiru' , $5500.00), 
    (6, 2, 'Steve' , $4500.00), 
    (7, 3, 'Aaron' , $5000.00), 
    (8, 5, 'Lilach' , $3500.00), 
    (9, 7, 'Rita' , $3000.00), 
    (10, 5, 'Sean' , $3000.00), 
    (11, 7, 'Gabriel', $3000.00), 
    (12, 9, 'Emilia' , $2000.00), 
    (13, 9, 'Michael', $2000.00), 
    (14, 9, 'Didi' , $1500.00); 

; with Tree as 
     (
     SELECT empid 
     ,  mgrid 
     ,  1 as lv 
     ,  1 as level1 
     ,  null as level2 
     ,  null as level3 
     ,  null as level4 
     ,  null as level5 
     FROM Employees 
     WHERE empid = 1 and mgrid = 1 
     UNION ALL 
     SELECT E.empid 
     ,  E.mgrid 
     ,  T.lv + 1 
     ,  T.level1 
     ,  case when T.lv = 1 then E.empid else t.level2 end 
     ,  case when T.lv = 2 then E.empid else t.level3 end 
     ,  case when T.lv = 3 then E.empid else t.level4 end 
     ,  case when T.lv = 4 then E.empid else t.level5 end 
     FROM Employees AS E 
     JOIN Tree T 
     ON  E.mgrid = T.empid 
     ) 
select * 
from Tree 
order by empid 

uscita preferita è

+-------+-------+----+--------+--------+--------+--------+--------+ 
| empid | mgrid | lv | level1 | level2 | level3 | level4 | level5 | 
+-------+-------+----+--------+--------+--------+--------+--------+ 
|  1 |  1 | 1 |  1 | NULL | NULL | NULL | NULL | 
|  2 |  1 | 2 |  1 | 2  | NULL | NULL | NULL | 
|  3 |  1 | 2 |  1 | 3  | NULL | NULL | NULL | 
|  4 |  2 | 3 |  1 | 2  | 4  | NULL | NULL | 
|  5 |  2 | 3 |  1 | 2  | 5  | NULL | NULL | 
|  6 |  2 | 3 |  1 | 2  | 6  | NULL | NULL | 
|  7 |  3 | 3 |  1 | 3  | 7  | NULL | NULL | 
|  8 |  5 | 4 |  1 | 2  | 5  | 8  | NULL | 
|  9 |  7 | 4 |  1 | 3  | 7  | 9  | NULL | 
| 10 |  5 | 4 |  1 | 2  | 5  | 10  | NULL | 
| 11 |  7 | 4 |  1 | 3  | 7  | 11  | NULL | 
| 12 |  9 | 5 |  1 | 3  | 7  | 9  | 12  | 
| 13 |  9 | 5 |  1 | 3  | 7  | 9  | 13  | 
| 14 |  9 | 5 |  1 | 3  | 7  | 9  | 14  | 
+-------+-------+----+--------+--------+--------+--------+--------+ 

Answer 1

La ragione di un ciclo infinito è il primo record dove empid=mgrid. Per gestire questo problema è necessario includere un campo cumulativo (levels in questo esempio) per memorizzare mgrid già elaborato e verificare se emid è già presente in questo elenco per evitare un loop.

Ecco una query:

with Tree as 
     (
     SELECT empid 
     ,  mgrid 
     ,  1 as lv 
     ,  1 as level1 
     ,  null as level2 
     ,  null as level3 
     ,  null as level4 
     ,  null as level5 
     ,  cast(mgrid as varchar(max)) levels 
     FROM Employees 
     WHERE empid = 1 and mgrid = 1 
     UNION ALL 
     SELECT E.empid 
     ,  E.mgrid 
     ,  T.lv + 1 
     ,  T.level1 
     ,  case when T.lv = 1 then E.empid else t.level2 end 
     ,  case when T.lv = 2 then E.empid else t.level3 end 
     ,  case when T.lv = 3 then E.empid else t.level4 end 
     ,  case when T.lv = 4 then E.empid else t.level5 end 
     ,  T.levels+','+cast(E.mgrid as varchar(max)) levels 

      FROM Employees AS E 
     JOIN Tree T 
     ON  E.mgrid = T.empid 
       and (','+T.levels+',' 
         not like 
        '%,'+cast(E.empid as varchar(max))+',%') 
     ) 
select * 
from Tree 
order by empid 

E qui è SQLFiddle demo