存在时的情况 - 列 (SQL)

Question

我正在尝试使用“0”填充 XML 文件的字段，如果特定列 (pv_an4) 不存在，或者使用该列的值，如果它存在。

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 ),

当我执行这个时，我得到这个错误：

FEHLER: Column »pv_an4« does not exist ZEILE 25: ...E_NAME = 'phonebook' AND COLUMN_NAME = 'pv_an4') then pv_an4

但这对我来说毫无意义，因为例如使用这段代码它会起作用：

....

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

....

我花了一整天的时间试图解决这个问题，请问有人能给我指明正确的方向吗？

/edit：下面是完整查询：

    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

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，您使用 pv_an4 作为第一个查询中的列，但在第二个查询中用作列值。

Answer 2

整个sql语句在之前被解析和编译它是运行，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（检查该列是否存在并创建适当的sql 语句）。虽然我真的无法想象为什么你不应该知道某个列是否存在于 table 中。

存在时的情况 - 列 (SQL)

Case when exists - column (SQL)

postgresql

psql