key - the name of the "key" field - this is just a field to be used as

the first column of the output table i.e. it identifies the record from

which each output row came.

which each output row came (see note below about multiple values).

document - the name of the field containing the XML document

as a more complicated example. Of course, you could wrap all

of this in a view for convenience.

Multivalued results

The xpath_table function assumes that the results of each XPath query

might be multi-valued, so the number of rows returned by the function

may not be the same as the number of input documents. The first row

returned contains the first result from each query, the second row the

second result from each query. If one of the queries has fewer values

than the others, NULLs will be returned instead.

In some cases, a user will know that a given XPath query will return

only a single result (perhaps a unique document identifier) - if used

alongside an XPath query returning multiple results, the single-valued

result will appear only on the first row of the result. The solution

to this is to use the key field as part of a join against a simpler

XPath query. As an example:

CREATE TABLE test

(

id int4 NOT NULL,

xml text,

CONSTRAINT pk PRIMARY KEY (id)

)

WITHOUT OIDS;

INSERT INTO test VALUES (1, '<doc num="C1">

<line num="L1"><a>1</a><b>2</b><c>3</c></line>

<line num="L2"><a>11</a><b>22</b><c>33</c></line>

</doc>');

INSERT INTO test VALUES (2, '<doc num="C2">

<line num="L1"><a>111</a><b>222</b><c>333</c></line>

<line num="L2"><a>111</a><b>222</b><c>333</c></line>

</doc>');

The query:

SELECT * FROM xpath_table('id','xml','test',

'/doc/@num|/doc/line/@num|/doc/line/a|/doc/line/b|/doc/line/c','1=1')

AS t(id int4, doc_num varchar(10), line_num varchar(10), val1 int4,

val2 int4, val3 int4)

WHERE id = 1 ORDER BY doc_num, line_num

Gives the result:

id | doc_num | line_num | val1 | val2 | val3

----+---------+----------+------+------+------

1 | C1 | L1 | 1 | 2 | 3

1 | | L2 | 11 | 22 | 33

To get doc_num on every line, the solution is to use two invocations

of xpath_table and join the results:

SELECT t.*,i.doc_num FROM

xpath_table('id','xml','test',

'/doc/line/@num|/doc/line/a|/doc/line/b|/doc/line/c','1=1')

AS t(id int4, line_num varchar(10), val1 int4, val2 int4, val3 int4),

xpath_table('id','xml','test','/doc/@num','1=1')

AS i(id int4, doc_num varchar(10))

WHERE i.id=t.id AND i.id=1

ORDER BY doc_num, line_num;

which gives the desired result:

id | line_num | val1 | val2 | val3 | doc_num

----+----------+------+------+------+---------

1 | L1 | 1 | 2 | 3 | C1

1 | L2 | 11 | 22 | 33 | C1

(2 rows)

XSLT functions

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