存在的情况-列（SQL）

Question

如果没有特定的列（pv_an4），则尝试用'0'填充XML文件的字段；如果存在，则用列的值填充。

到目前为止，这是我的代码：

XMLELEMENT( Name "Telephone",
case
when not exists (SELECT 1 FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME = 'phonebook' AND COLUMN_NAME = 'pv_an4') then ''
when exists (SELECT 1 FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME = 'phonebook' AND COLUMN_NAME = 'pv_an4') then pv_an4
end ),

执行此操作时，出现以下错误：

FEHLER：第»pv_an4«列不存在ZEILE 25：... E_NAME ='电话簿'和COLUMN_NAME ='pv_an4'），然后是pv_an4

但这对我来说毫无意义，因为例如使用此代码，它将可以工作：

....

when exists (SELECT 1 FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME = 'phonebook' AND COLUMN_NAME = 'pv_an4') then 'pv_an4 exists'
end ),

....

我花了整整一天的时间来解决这个问题，请问有人可以向我提示正确的方向吗？

/ edit：完整查询，位于下面：

    SELECT
XMLFOREST( xmlpb.entry as "DivinusIPPhoneDirectory" )
FROM (
SELECT
XMLCONCAT (
XMLELEMENT ( NAME "Title", 'Phonelist' ),
XMLELEMENT ( NAME "Prompt", 'Prompt' ),
XMLAGG (
XMLELEMENT (
NAME "DirectoryEntry",
XMLELEMENT( Name "Name",
case
when (pb.company = '') IS FALSE AND pb.lastname != '' and pb.firstname != '' then pb.company || ' - ' || pb.lastname || ', ' || pb.firstname
when (pb.company = '') IS FALSE AND pb.lastname != '' and pb.firstname = '' then pb.company || ' - ' || pb.lastname
when (pb.company = '') IS FALSE AND pb.lastname = '' and pb.firstname = '' then pb.company
when (pb.company = '') IS FALSE AND pb.lastname = '' and pb.firstname != '' then pb.company || ' - ' || pb.firstname
when (pb.company = '') IS NOT FALSE AND pb.lastname != '' and pb.firstname != '' then pb.lastname || ', ' || pb.firstname
when (pb.company = '') IS NOT FALSE AND  pb.lastname != '' and pb.firstname = '' then pb.lastname
end ),
XMLELEMENT( Name "Telephone", pb.pv_an3 ),
XMLELEMENT( Name "Telephone",
case
when not exists (SELECT 1 FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME = 'phonebook' AND COLUMN_NAME = 'pv_an4') then ''
else pb.pv_an4
end ),
XMLELEMENT( Name "Telephone", pb.phonenumber ),
XMLELEMENT( Name "Telephone", pb.pv_an1 ),
XMLELEMENT( Name "Telephone", pb.pv_an2 )
)
)
) as entry
FROM
phonebook pb
WHERE fkidtenant = 1

Answer 1

整个sql语句在运行之前都会被解析和编译，因此postgresql会抱怨缺少字段。 没有捷径。

如果要处理此类情况，则需要使用动态生成的sql（检查列是否存在并创建适当的sql语句）。 尽管我无法真正想象为什么您不应该知道表中是否存在某个列。

Answer 2

尝试：

XMLELEMENT( Name "Telephone",
case
when not exists (SELECT 1 FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME   = 'phonebook' AND COLUMN_NAME = 'pv_an4') then ''
when exists (SELECT 1 FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME = 'phonebook' AND COLUMN_NAME = 'pv_an4') then 'pv_an4'
end ),

在第4行中，您将pv_an4用作第一个查询的列，但将其用作第二个查询的列值。