sql - Oracle の xml では、単一行のサブクエリが複数の行を返します

Question

これは、テーブルを作成し、xml 列を挿入するための私のコードです。

-- スキーマを削除

BEGIN
DBMS_XMLSCHEMA.DELETESCHEMA(
   'http://xmlns.oracle.com/xdb/schemas/HZHE18/handBook',
   dbms_xmlschema.DELETE_CASCADE_FORCE);
END;

-- バイナリ スキーマを登録する

BEGIN
DBMS_XMLSCHEMA.REGISTERSCHEMA(SCHEMAURL => 'handBook',
schemadoc => '<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">

<xs:annotation>
    <xs:documentation> Author: Andy(Haichuan) Zheng Schema for Web Handbook data
    </xs:documentation>
</xs:annotation>

<xs:simpleType name="code-style">
    <xs:restriction base="xs:string">
        <xs:pattern value="[A-Z][A-Z][A-Z][0-9][0-9][0-9][0-9]"/>
    </xs:restriction>
</xs:simpleType>

<xs:simpleType name="level-style">
    <xs:restriction base="xs:string">
        <xs:enumeration value="Undergraduate"/>
        <xs:enumeration value="Postgraduate"/>
    </xs:restriction>
</xs:simpleType>

<xs:simpleType name="email-style">
    <xs:restriction base="xs:string">
        <xs:pattern value="[^@]+@[^\.]+\..+"/>
    </xs:restriction>
</xs:simpleType>

<xs:simpleType name="point_lvl_type">
    <xs:restriction base="xs:positiveInteger">
        <xs:enumeration value="3"/>
        <xs:enumeration value="6"/>
        <xs:enumeration value="12"/>
        <xs:enumeration value="24"/>
    </xs:restriction>
</xs:simpleType>

<xs:simpleType name="offered_type">
    <xs:restriction base="xs:string">
        <xs:enumeration value="First semester"/>
        <xs:enumeration value="Second semester"/>
    </xs:restriction>
</xs:simpleType>

<xs:simpleType name="mode_type">
    <xs:restriction base="xs:string">
        <xs:enumeration value="Day"/>
        <xs:enumeration value="Evening"/>
        <xs:enumeration value="Off-campus"/>
    </xs:restriction>
</xs:simpleType>

<xs:simpleType name="campus_type">
    <xs:restriction base="xs:string">
        <xs:enumeration value="Caulfield"/>
        <xs:enumeration value="Gippsland"/>
        <xs:enumeration value="Sunway"/>
        <xs:enumeration value="Clayton"/>
        <xs:enumeration value="South Africa"/>
    </xs:restriction>
</xs:simpleType>

<xs:element name="handbook">
    <xs:complexType>
        <xs:sequence>
            <xs:element ref="page_info" minOccurs="1" maxOccurs="unbounded"/>
            <xs:element ref="unit" minOccurs="1" maxOccurs="unbounded"/>
        </xs:sequence>
    </xs:complexType>
</xs:element>

<xs:element name="page_info">
    <xs:complexType>
        <xs:sequence>
            <xs:element name="sub_date" type="xs:date"/>
        </xs:sequence>
        <xs:attribute name="author" type="xs:string"/>
    </xs:complexType>
</xs:element>

<xs:element name="unit">
    <xs:complexType>
        <xs:sequence>
            <xs:element name="u_name" type="xs:string"/>
            <xs:element name="level" type="level-style"/>
            <xs:element ref="chief_examiner"/>
            <xs:element name="point_lvl" type="point_lvl_type"/>
            <xs:element name="prerequisites" type="code-style" maxOccurs="unbounded"
                minOccurs="0"/>
            <xs:element name="prohibitions" type="code-style" maxOccurs="unbounded"
                minOccurs="0"/>
            <xs:element ref="availability"/>
            <xs:element name="synopsis" type="xs:string"/>
            <xs:element name="assessment" type="xs:string"/>
            <xs:element name="contact_hours" type="xs:string"/>
        </xs:sequence>
        <xs:attribute name="code" type="code-style"/>
    </xs:complexType>
</xs:element>

<xs:element name="chief_examiner">
    <xs:complexType>
        <xs:sequence>
            <xs:element name="name" type="xs:string"/>
            <xs:element name="email" type="email-style"/>
        </xs:sequence>
    </xs:complexType>
</xs:element>

<xs:element name="availability">
    <xs:complexType>
        <xs:sequence>
            <xs:element name="offered" type="offered_type" maxOccurs="unbounded"/>
            <xs:element name="mode" type="mode_type" maxOccurs="unbounded"/>
            <xs:element name="campus" type="campus_type" maxOccurs="unbounded"/>
        </xs:sequence>
    </xs:complexType>
</xs:element>

</xs:schema>',
LOCAL => TRUE,
GENTYPES => FALSE,
GENTABLES => FALSE,
FORCE => FALSE,
options => DBMS_XMLSCHEMA.REGISTER_BINARYXML);
end;

-- ごみ箱を落としてきれいにする

drop table units_handbook purge;

PURGE RECYCLEBIN;

--create tables CREATE TABLE units_handbook ( unit_code varchar2(7) PRIMARY KEY, XML_detail XMLTYPE NOT NULL) XMLTYPE COLUMN XML_detail STORE AS BINARY XML XMLSCHEMA " http://xmlns.oracle.com/xdb/schemas/HZHE18/handBook " ELEMENT "ハンドブック";

-- テーブルに入力します

insert into units_handbook values('FIT2077', 
'<handbook>
    <page_info author = "Haichuan Zheng">
        <sub_date>2013-05-28</sub_date>
    </page_info>
    <unit code="FIT2077">
        <u_name>Advanced Data Management</u_name>
        <level>Undergraduate</level>
        <chief_examiner>
             <name>Mr Lindsay Smith</name>
             <email>Lindsay.Smith@monash.edu</email>
        </chief_examiner>
        <point_lvl>6</point_lvl>
        <prerequisites>FIT1004</prerequisites>
       <prerequisites>CSE2132</prerequisites>
    <prohibitions>CSE3000</prohibitions>
        <prohibitions>FIT3118</prohibitions>
    <prohibitions>FIT4038</prohibitions>
    <availability>
        <offered>First semester</offered>
        <mode>Day</mode>
        <campus>Caulfield</campus>
    </availability>
    <synopsis><![CDATA[This unit extends the study from FIT1004 Data management. 
        FIT2077 will introduce more advanced concepts in the areas of database design, SQL, 
        query optimisation and the handling of unstructured data (XML) both externally
        and within a database. The issue of "Big Data" and the role played by BI technologies 
        and data warehouses will be explored.]]></synopsis>
    <assessment>Examination (3 hours): 60%; In-semester assessment: 40%</assessment>
    <contact_hours>2 hrs lectures/wk, 2 hrs laboratories/wk</contact_hours>
</unit>
</handbook>');

insert into units_handbook values('FIT4038', 
'<handbook>
<page_info author = "Haichuan Zheng">
    <sub_date>2013-05-28</sub_date>
</page_info>
<unit code="FIT4038">
    <u_name>Database management and implementation</u_name>
    <level>Postgraduate</level>
    <chief_examiner>
        <name>Associate Professor David Taniar</name>
        <email>David.Taniar@monash.edu</email>
    </chief_examiner>
    <point_lvl>6</point_lvl>
    <prerequisites>FIT9019</prerequisites>
    <prerequisites>FIT9003</prerequisites>
    <prohibitions>FIT3118</prohibitions>
    <prohibitions>CSE3000</prohibitions>
    <availability>
        <offered>First semester</offered>
        <mode>Day</mode>
        <campus>Caulfield</campus>
    </availability>
    <synopsis><![CDATA[This unit looks at the design and implementation issues of 
        database management systems. Advanced database design using the object-relational approach 
        and multi-dimensional database design are explored. Record, file 
        and index structures are dealt with at the basic level. 
        Higher level details of consistency, atomicity and durability are
        introduced along with modern trends in databases.]]></synopsis>
    <assessment>Examination (3 hours): 60%; In-semester assessment: 40%</assessment>
    <contact_hours>2 hrs lectures/wk, 2 hrs laboratories/wk</contact_hours>
</unit>
</handbook>');

私の質問は、xml 形式ですべての u_name を取得したいときに、ORA-01427: single-row subquery returns more than one row というエラーが表示されることです。

ここに私のコードがあります

  select xmlelement("unit",xmlelement("u_name",
  (
   SELECT
      EXTRACTVALUE(u.XML_detail, '/handbook/unit/u_name')
    FROM
       units_handbook u
 )
  ) )
  from units_handbook;