如何使用 BigQuery 显示自连接树层次结构 table？

Question

我正在处理主管及其下属员工的树状层次结构。难的是有些主管是其他主管下属的员工，而且数量很多。

对于我从class获得的SQL查询，只有简单的自连接，可能只有两个级别：A被B监督，仅此而已。

但现实世界的问题要复杂得多。有多个级别，我不确定确切的数量。比如A被B监管，B被C监管，C被D监管等等

我假设只有 4 个级别的监督。原始数据可能是这样的：

         Employee     Supervisor
            A             B
            C             B
            D             B
            B             V
            E             V
            F             E
            G             V
            V          (Blank which indicates no boss)

因此我生成了如下代码：

SELECT T1.Supervisor,ifnull(T2.EmployeeName, 'No Boss') as sup_emp1, 
ifnull(T3.EmployeeName, 'No Boss') as sup_emp2, ifnull(T4.EmployeeName, 
'No boss') as sup_emp3, ifnull(T5.EmployeeName, 'No Boss') as EMPLOYEE
FROM  T1
LEFT JOIN T2
ON T1.Supervisor=T2.EmployeeName
LEFT JOIN  T3
ON T2.Supervisor = T3.EmployeeName
LEFT JOIN T4
ON T3.Supervisor = T4.EmployeeName
LEFT JOIN T5
ON T4.Supervisor = T5.EmployeeName
WHERE T5. Emp_Status = 'Active'
ORDER BY T1.Supervisor ASC

然而，结果却是错误的。我只是对 table 的关系感到困惑，没有想到递归连接。

预期的结果应该是这样的：

Supervisor     # of Employees     
   V              7(include indirect supervised employees)
   B              3
   E              1

组织 table 应该是：

Supervisor   Employee1    Employee2
   V             B             A
   V             B             C
   V             B             D
   V             E             F
   V             G            Null

但是我的结果竟然是

Supervisor   Employee1    Employee2
   V             B             A
   V             B             C
   V             B             D
   V             E             F
   V             V             G

<- 我需要 G 在第二层。

那么，有人可以帮忙吗？谢谢！

Answer 1

我想你想要：

SELECT T1.Supervisor as BigBoss,
       T2.EmployeeName as level1, 
       T3.EmployeeName as level2,
       T4.EmployeeName as level3,
       T5.EmployeeName as level4
FROM T T1 LEFT JOIN
     T T2
     ON T1.Supervisor=T2.EmployeeName AND T2.Emp_Status = 'Active' LEFT JOIN 
     T T3
     ON T2.Supervisor = T3.EmployeeName AND T3.Emp_Status = 'Active' LEFT JOIN
     T T4
     ON T3.Supervisor = T4.EmployeeName AND T4.Emp_Status = 'Active' LEFT JOIN
     T T5
     ON T4.Supervisor = T5.EmployeeName AND T5.Emp_Status = 'Active'
WHERE T1.Emp_Status = 'Active' AND
      NOT EXISTS (SELECT 1 
                  FROM T ts
                  WHERE ts.EmployeeName = T1.SuperVisor
                 )
ORDER BY 1, 2, 3, 4, 5 ASC;

这从大老板开始，然后逐步下降。

Answer 2

... assume there are only like 4 levels for supervision ...

以下适用于 BigQuery 标准 SQL 并且只需将更多 LEFT JOIN 和相应字段添加到 COALESCE

即可轻松扩展到更多（合理更多）级别

#standardSQL
SELECT t.Supervisor,
  IF(t.Supervisor = t5.Supervisor, 
    STRUCT(Employee2 AS Employee1, NULL AS Employee2),
    STRUCT(t5.Supervisor AS Employee1, Employee2 AS Employee2)
  ).*
FROM (
  SELECT t1.Employee Supervisor,
    COALESCE(t4.Employee, t3.Employee, t2.Employee) Employee2
  FROM `project.dataset.table` t1
  LEFT JOIN `project.dataset.table` t2 ON t2.Supervisor = t1.Employee
  LEFT JOIN `project.dataset.table` t3 ON t3.Supervisor = t2.Employee
  LEFT JOIN `project.dataset.table` t4 ON t4.Supervisor = t3.Employee
  WHERE t1.Supervisor IS NULL
) t
LEFT JOIN `project.dataset.table` t5 ON t5.Employee = t.Employee2

如果应用于您问题中的示例数据 - 结果是

Row Supervisor  Employee1   Employee2    
1   V           B           A    
2   V           B           C    
3   V           B           D    
4   V           E           F    
5   V           G           null

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