如何使用SQL Cross Apply和MERGE

Question

Is there a way to use a Cross Apply with a MERGE statement? I'm new to Cross Apply and still trying to figure all of it's tricks out. Here's a mock demonstration of my INSERT and UPDATE that I'm doing on a cell that contains XML nodes:

UPDATE NewTable 
SET
    NewTable.ID = ID,
    NewTable.Title =  Parent.Elm.value('(Title)[1]', 'varchar(100)'),
    NewTable.[Description] = Parent.Elm.value('(Description)[1]', 'varchar(100)'),
    NewTable.ChildTitle = Child.Elm.value('(ChildTitle)[1]', 'varchar(100)'),
    NewTable.StartDate = Child.Elm.value('(StartDate)[1]', 'DATETIME'), 
    NewTable.EndDate = Child.Elm.value('(EndDate)[1]', 'DATETIME')

FROM OldTable
           cross apply
      xmlFieldData.nodes('/ParentElement') xp(parent)
           cross apply
      xp.parent.nodes('ChildElement') xc(child)
WHERE OldTable.ID = NewTable.ID

INSERT INTO NewTable(ID, Title, [Description], ChildTitle, StartDate, EndDate )
SELECT

    Parent.Elm.value('(Title)[1]', 'varchar(100)') AS 'Title',
    Parent.Elm.value('(Description)[1]', 'varchar(100)') AS 'Description',
    Child.Elm.value('(ChildTitle)[1]', 'varchar(100)') AS 'ChildTitle',
    Child.Elm.value('(StartDate)[1]', 'DATETIME') AS 'StartDate',
    Child.Elm.value('(EndDate)[1]', 'DATETIME') AS 'EndDate'

FROM OldTable
           cross apply
      xmlFieldData.nodes('/ParentElement') Parent(Elm)
           cross apply
      Parent.Elm.nodes('ChildElement') Child(Elm)

This is the content from my XML File:

<ParentElement>
  <Title>parent1</Title>
  <Description />
  <ChildElement>
    <Title>Child 4</Title>
    <Description />
    <StartDate>2010-01-25T00:00:00</StartDate>
    <EndDate>2010-01-25T00:00:00</EndDate>
  </ChildElement>
  <ChildElement>
    <Title>Child1</Title>
    <Description />
    <StartDate>2010-01-25T00:00:00</StartDate>
    <EndDate>2010-01-25T00:00:00</EndDate>
  </ChildElement>
  <ChildElement>
    <Title>Child6</Title>
    <Description />
    <StartDate>2010-01-25T00:00:00</StartDate>
    <EndDate>2010-01-25T00:00:00</EndDate>
  </ChildElement>
</ParentElement>

Where I get stuck on the MERGE statement is where to put the cross apply in the 'USING' section:

MERGE INTO NewTable AS NewTable
USING OldTable
    cross apply
         xmlFieldData.nodes('/ParentElement') xp(parent)
    cross apply
     xp.parent.nodes('ChildElement') xc(child)
    WHERE OldTable.ID = NewTable.ID AND Child.Elm.value('(ChildTitle)[1]', (100)') = ChildTitle

Thanks for your help!

Answer 1

Try wrapping the OldTable as a Select statement in your Using . For example:

Merge Into NewTable As Target
Using
(   
    Select      Fields
    From        OldTable
    Cross Apply xmlFieldData.nodes('/ParentElement') xp(parent)
    Cross Apply xp.parent.nodes('ChildElement') xc(child)
    Where       Child.Elm.value('(ChildTitle)[1]', (100)) = ChildTitle
) As Source On Source.ID = Target.ID

The XML bit seems weird to me, but I really don't know as much about XML in SQL as I should. However, this should point you in the right direction.

All that you would need to add to the Merge statement are what you're doing When Matched or When Not Matched .

Using the fields in your original query, it would look like this:

Merge Into NewTable As Target
Using
(   
    Select      ID,
                Parent.Elm.value('(Title)[1]', 'varchar(100)')          [Title],
                Parent.Elm.value('(Description)[1]', 'varchar(100)')    [Description],
                Child.Elm.value('(ChildTitle)[1]', 'varchar(100)')      [ChildTitle],
                Child.Elm.value('(StartDate)[1]', 'DATETIME')           [StartDate], 
                Child.Elm.value('(EndDate)[1]', 'DATETIME')             [EndDate]
    From        OldTable
    Cross Apply xmlFieldData.nodes('/ParentElement') xp(parent)
    Cross Apply xp.parent.nodes('ChildElement') xc(child)
) As Source On Source.ID = Target.ID And Source.ChildTitle = Target.ChildTitle
When Matched Then
    Update
    Set     Title       = Source.Title,
            Description = Source.Description,
            StartDate   = Source.StartDate,
            EndDate     = Source.EndDate
When Not Matched Then
    Insert  (ID, Title, Description, ChildTitle, StartDate, EndDate)
    Values  (Source.ID, Source.Title, Source.Description, Source.ChildTitle, Source.StartDate, Source.EndDate)
;

Answer 2

Are you certain the syntax is correct on the cross apply? I think it should be written as:

CROSS APPLY (SELECT FieldName = xmlFieldData.nodes('/ParentElement') xp(parent)) p

I have never used cross apply with merge or XML like that before but I would wonder if this fixes it.