在t-sql中使用XPath检索元素的值

Question

我在六列的表格中有一行。其中一列称为MessageContent，是包含XML文档的nvarchar（max）。我想从表中检索所有六列：

SELECT col1
      ,col2
      ,col3
      ,MessageContent (element: ErrorMessage)
      ,col5
      ,col6
from tablea;

这甚至可能吗？我看过的所有示例都涉及返回XML，而我只想要元素的值：ErrorMessage。

Answer 1

这里是一个解决方案。需要注意两点：（1）命名空间处理（2）TRY_CAST()处理数据库表中的“不良” XML。

SQL

-- DDL and sample data population, start
DECLARE @tbl TABLE (ID INT IDENTITY(1,1) PRIMARY KEY, MessageContent NVARCHAR(MAX));
INSERT INTO @tbl
VALUES (N'<?xml version="1.0"?>
<Acknowledgement xmlns="http://localhost/MeetingBrokerServices">
    <DocumentId>60051951-8f28-47d3-8fd8-0ba89b537c87</DocumentId>
    <TransactionId>62820a70-97f5-42b0-922e-a5f0908b9c8f</TransactionId>
    <DocumentDate>2019-10-10T04:00:00.7475266Z</DocumentDate>
    <StatusCode>NPE</StatusCode>
    <Success>false</Success>
    <Errors>
        <Error>
            <Code>301</Code>
            <ErrorText>Invalid LocationIdentifier was received in the request. Valid LocationIdentifier must be populated and sent for this request to succeed. Request Details: ExternalRfpId: SecondaryExternalRfpId: RfpId: 12499772 SiteId: LocationIdentifierType: MeetingBroker LocationId: ExternalBookingId: 111.11 MbUserGuid: 625bb5f9-0bc7-4c7f-900a-a6436555ea19 RequestMetaDataGuid: BizTalk MessageId: c6e05156-4a35-4be4-b9fe-209173049539 Please see WebServiceTransactionLog and RequestMetaData for details.</ErrorText>
            <ErrorType>Critical</ErrorType>
        </Error>
    </Errors>
</Acknowledgement>');
-- DDL and sample data population, end

;WITH XMLNAMESPACES(DEFAULT 'http://localhost/MeetingBrokerServices'), rs AS 
(
    SELECT *
        , TRY_CAST(MessageContent AS XML) AS [config]
    FROM @tbl
)
SELECT ID
    , col.value('(ErrorText/text())[1]','VARCHAR(4000)') AS ErrorText
FROM rs AS tbl
    CROSS APPLY tbl.[config].nodes('/Acknowledgement/Errors/Error') AS tab(col);