How to get result from parent child table

Question

Work on SQL-Server. My table structure is below

CREATE TABLE [dbo].[AgentInfo](
    [AgentID] [int] NOT NULL,
    [ParentID] [int] NULL,
 CONST         


        

Answer 1

You can use a Common Table Expression to do this.

The sql statement will then look like this:

WITH [Parents]([AgentID], [ParentID], [BaseAgent])
AS
(
    SELECT 
      [AgentID],
      [ParentID],
      [AgentID] AS [BaseAgent]
    FROM [AgentInfo]
    WHERE [ParentID] = -1
    UNION ALL
    SELECT 
      [ai].[AgentID],
      [ai].[ParentID],
      [p].[BaseAgent]
    FROM [AgentInfo] [ai]
    INNER JOIN [Parents] [p] 
      ON [ai].[ParentID] = [p].[AgentID]
)

SELECT *
FROM [Parents]
ORDER BY 
  [BaseAgent] ASC,
  [AgentID] ASC

But, the results are different from your desired output, since every Agent is only listed once. The output is:

AGENTID     PARENTID    BASEAGENT
1           -1          1
3            1          1
2           -1          2
4            2          2

The Fiddle is over here.

And here is a nice post on working with hierarchies: What are the options for storing hierarchical data in a relational database?