How to get result from parent child table - sql

Question

Work on SQL-Server. My table structure is below
CREATE TABLE [dbo].[AgentInfo](
[AgentID] [int] NOT NULL,
[ParentID] [int] NULL,
CONSTRAINT [PK_AgentInfo] PRIMARY KEY CLUSTERED
(
[AgentID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
INSERT [dbo].[AgentInfo] ([AgentID], [ParentID]) VALUES (1, -1)
INSERT [dbo].[AgentInfo] ([AgentID], [ParentID]) VALUES (2, -1)
INSERT [dbo].[AgentInfo] ([AgentID], [ParentID]) VALUES (3, 1)
INSERT [dbo].[AgentInfo] ([AgentID], [ParentID]) VALUES (4, 2)
Required output
Use my below syntax get required output but not satisfied. Is there any better way to get the required output
--get parent child list
---step--1
SELECT *
INTO #temp1
FROM ( SELECT a.AgentID ,
a.ParentID,
a.AgentID AS BaseAgent
FROM dbo.AgentInfo a WHERE ParentID=-1
UNION ALL
SELECT a.ParentID ,
0 as AgentID,
a.AgentID AS BaseAgent
FROM dbo.AgentInfo a WHERE ParentID!=-1
UNION ALL
SELECT a.AgentID ,
a.ParentID,
a.AgentID AS BaseAgent
FROM dbo.AgentInfo a
WHERE ParentID!=-1
) AS d
SELECT * FROM #temp1
DROP TABLE #temp1
Help me to improve my syntax. If you have any questions please ask.

Answer 1

You could use a recursive SELECT, see the examples in the documentation for WITH, starting with example D.
The general idea within the recursive WITH is: You have a first select that is the starting point, and then a UNION ALL and a second SELECT which describes the step from on level to the next, where the previous level can either be the result of the first select or the result of the previous run of the second SELECT.

Answer 2

You can try this, to get a tree of the elements:
WITH CTE_AgentInfo(AgentID, ParentID, BaseAgent)
AS(
SELECT
AgentID,
ParentID,
AgentID AS BaseAgent
FROM AgentInfo
WHERE ParentID = -1
UNION ALL
SELECT
a.AgentID,
a.ParentID,
a.AgentID AS BaseAgent
FROM AgentInfo a
INNER JOIN CTE_AgentInfo c ON
c.AgentID = a.ParentID
)
SELECT * FROM CTE_AgentInfo
And here is an SQLFiddle demo to see it.

Answer 3

Try something like this:
WITH Merged (AgentId, ParentId) AS (
SELECT AgentId, ParentId FROM AgentInfo WHERE ParentId = -1
UNION ALL
SELECT AgentInfo.AgentId, AgentInfo.ParentId FROM AgentInfo INNER JOIN Merged ON AgentInfo.AgentId = Merged.ParentId
)
SELECT * FROM Merged

Answer 4

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?