May I retrieve a hierarchical data structure from a SQL Server stored procedure?
I have a Web Service one of whose methods returns a list of items, each of which possesses another list of items:
<TopLevelItems>
<TopLevelItem field1="a" field2="b" ...>
<LowLevelItem fieldA="1" fieldB="2" .../>
<LowLevelItem fieldA="3" fieldB="4" .../>
</TopLevelItem>
</TopLevelItems>
These lists are retrieved from a SQL Server database using simple queries (both TopLevelItem and LowLevelItem correspond to related tables in the database).
Until now, to retrieve all this data, I needed two queries: one to retrieve top level items, which was executed once; and another to retrieve low level items, which was executed once per top level item.
However, this seems to be highly inefficient. I would like to define a single stored procedure that performs all the necessary queries and retrieves the result as a hierarchical data structure. Is it possible? If so, how?
Hierarchical data in SQL server can be obtained using FOR XML. In this case, you would just need to write a query to join tables, then parent-child relationships will manifest as nested XML elements:
DECLARE @sites TABLE ( ID INT, Name VARCHAR(50) )
INSERT INTO @sites
VALUES ( 1, 'abc' ),
( 2, 'def' )
DECLARE @siteEnergy TABLE
(
SiteFK INT,
Month INT,
Energy INT
)
INSERT INTO @siteEnergy
VALUES ( 1, 1, 50 ),
( 1, 2, 49 ),
( 1, 3, 50 ),
( 2, 1, 33 ),
( 2, 2, 34 ),
( 2, 3, 50 )
SELECT *
FROM @sites site
JOIN @siteEnergy siteEnergy ON site.id = siteEnergy.sitefk
FOR XML AUTO, ROOT('SiteInformation')
Results:
<SiteInformation>
<site ID="1" Name="abc">
<siteEnergy SiteFK="1" Month="1" Energy="50" />
<siteEnergy SiteFK="1" Month="2" Energy="49" />
<siteEnergy SiteFK="1" Month="3" Energy="50" />
</site>
<site ID="2" Name="def">
<siteEnergy SiteFK="2" Month="1" Energy="33" />
<siteEnergy SiteFK="2" Month="2" Energy="34" />
<siteEnergy SiteFK="2" Month="3" Energy="50" />
</site>
</SiteInformation>
Use can create the XML in the SP directly
Example
declare @TopLevelItem table (TopID int, field1 varchar(50), field2 varchar(50))
declare @LowLevelItem table (TopID int, fieldA int, fieldB int)
insert into @TopLevelItem values (1, 'a', 'b')
insert into @LowLevelItem values (1, 1, 2)
insert into @LowLevelItem values (1, 3, 4)
select
T.field1 as '@field1',
T.field2 as '@field2',
((select
L.fieldA as '@fieldA',
L.fieldB as '@fieldB'
from @LowLevelItem as L
where T.TopID = L.TopID
for xml path('LowLevelItem'), type))
from @TopLevelItem as T
for xml path('TopLevelItem'), root('TopLevelItems')
Result
<TopLevelItems>
<TopLevelItem field1="a" field2="b">
<LowLevelItem fieldA="1" fieldB="2" />
<LowLevelItem fieldA="3" fieldB="4" />
</TopLevelItem>
</TopLevelItems>
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