The best way to extract data from xml with xquery

问题

Consider the following xml:

<Persons num="3">
  <Person age="5" />
  <Person age="19" />
</Persons>

There is a need to extract this xml into a relational table:

Persons table (Age1 int, Age2 int, Age3 int , Age4 int)

Parsing has to satisfy the following constraints:

all persons with age >=18 must be assigned to columns with smallest column number and the value has to be 18
if the age of the person is not given it is equal to 18
all persons with age <18 must follow
if there are less than 4 persons, those which are not provided must have age=-1

In a given example, there are 3 persons, ages of 2 of them are provided: 5 and 19 respectively. The content of the table Persons has to be the following:

18 18 5 -1

Is there the best way to do so with xpath?

Till now I can parse the xml and assign ages but what is not clear is to how make ordering:

declare @XmlData xml = 
'<Persons num="3">
    <Person age="5" />
    <Person age="19" />
</Persons>'

declare @Persons table (Age1 int, Age2 int, Age3 int , Age4 int)
insert into @Persons (Age1, Age2, Age3, Age4)
select ISNULL(Age1, case when Num>= 1 then 18 else -1 end) Age1
    , ISNULL(Age2, case when Num>= 2 then 18 else -1 end) Age2
    , ISNULL(Age3, case when Num>= 3 then 18 else -1 end) Age3
    , ISNULL(Age4, case when Num>= 4 then 18 else -1 end) Age4
from (
    select Persons.Person.value('@num','smallint') as Num
          ,Persons.Person.value('Person[@age<18][1]/@age','smallint') as Age1
          ,Persons.Person.value('Person[@age<18][2]/@age','smallint') as Age2
          ,Persons.Person.value('Person[@age<18][3]/@age','smallint') as Age3
          ,Persons.Person.value('Person[@age<18][4]/@age','smallint') as Age4
    from @XmlData.nodes('/Persons') Persons(Person)
 ) Persons  

select *
from @Persons

Result is

5 18 18 -1

回答1:

I have found a bit dirty solution:

select ISNULL(Age1, case when Num>= 1 then 18 else -1 end) Age1
    , ISNULL(Age2, case when Num>= 2 then 18 else -1 end) Age2
    , ISNULL(Age3, case when Num>= 3 then 18 else -1 end) Age3
    , ISNULL(Age4, case when Num>= 4 then 18 else -1 end) Age4
from (
    select Persons.Person.value('@num','smallint') as Num
          ,Persons.Person.value('xs:integer(fn:number(@num))+1','int') as Num1
          ,Persons.Person.value('Person[@age<18][xs:integer(fn:number(../@num))][1]/@age','smallint') as Age1
          ,Persons.Person.value('Person[@age<18][xs:integer(fn:number(../@num))-1][1]/@age','smallint') as Age2
          ,Persons.Person.value('Person[@age<18][xs:integer(fn:number(../@num))-2][1]/@age','smallint') as Age3
          ,Persons.Person.value('Person[@age<18][xs:integer(fn:number(../@num))-3][1]/@age','smallint') as Age4 
    from @XmlData.nodes('/Persons') Persons(Person)
 ) Persons

The idea of a solution is to first extract those contacts that are >=18, then extract those that are 0 < age < 18 and finally set those that are not provided to -1

UPD: despite the fact that solution provided correct results, its cost is high: ~1000 in estimated execution plan

回答2:

Another solution requires a bit more sql code but costs only ~80 in estimated execution plan.

There is one constraint wrt the problem statement: Persons/@num has to be equal to a number of Person tags

Limitations are:

limited number of persons per room

Here is sql code:

--//initial xml data
declare @XmlData xml = 
'<Persons roomid="1" num="3">
    <Person age="19" />
    <Person age="10" /> 
    <Person age="5" />
</Persons>
<Persons roomid="4" num="4">
    <Person age="17" />
    <Person age="10" /> 
    <Person age="5" />
    <Person age="1" />
</Persons>'

--//shade xml into temporal table: rank is applied to an age in descreasing order
declare @tmp table (age int, roomid int, orderid int)
insert into @tmp(age,roomid,orderid)
select Persons.age
      ,Persons.roomid
      ,ROW_NUMBER () over (partition by Persons.roomid order by Persons.age desc)
from(
    select Ps.P.value('(@age)[1]','smallint') age       
          ,Ps.P.value('(../@roomid)[1]','smallint') roomid
    from @XmlData.nodes('/Persons/Person') Ps(P)
)Persons
order by Persons.roomid,Persons.age desc    

--//provide ordering for roomid: since roomid may be different (the only thing that is required that roomid is unique)
declare @roomidmapping table (roomid int, roomorderid int)
insert into @roomidmapping(roomid, roomorderid)
select roomid, ROW_NUMBER () over  (order by roomid asc)
from @tmp
group by roomid
declare @roomnumber int = @@ROWCOUNT
--//final result
;WITH ConsequtiveNums AS
(
    SELECT 1 AS Number
    UNION ALL
    SELECT Number+1
    FROM ConsequtiveNums
    WHERE Number<@roomnumber
)
select (select case when age>18 then 18 else age end from @tmp T inner join @roomidmapping M on T.roomid = M.roomid where T.orderid = 1 and M.roomorderid = CN.Number)
      ,(select case when age>18 then 18 else age end from @tmp T inner join @roomidmapping M on T.roomid = M.roomid where T.orderid = 2 and M.roomorderid = CN.Number)
      ,(select case when age>18 then 18 else age end from @tmp T inner join @roomidmapping M on T.roomid = M.roomid where T.orderid = 3 and M.roomorderid = CN.Number)
      ,(select case when age>18 then 18 else age end from @tmp T inner join @roomidmapping M on T.roomid = M.roomid where T.orderid = 4 and M.roomorderid = CN.Number)
from ConsequtiveNums CN

来源：https://stackoverflow.com/questions/6070256/the-best-way-to-extract-data-from-xml-with-xquery

标签

xml

sql-server-2008

xquery-sql