错误：找不到从未知到文本的转换功能 [英] ERROR: failed to find conversion function from unknown to text

问题描述

在PostgreSQL上有一个错误，它在我的select语句之一中给出。我在网上搜索了一个答案，然后空手而归。另一个问题给出的答案不适合我的问题。

There is an error on PostgreSQL that it gives on one of my select statements. I searched the web for an answer and came out empty handed. The answer given in another question did not suit my problem.

ERROR:  failed to find conversion function from unknown to text
********** Error **********
ERROR: failed to find conversion function from unknown to text
SQL state: XX000

我的查询看起来像这样：

My query looks something like this:

Select * 
from (select 'string'  as Rowname, Data
      From table)
Union all
     (select 'string2' as Rowname, Data
      From table)

这样做的目的是指定该行在哪一行点。字符串是行的名称。这是我想要的输出：

The point of doing this is to specify what the row is at one point. The string being the name of the row. Here is my desired output:

Rowname Data 
string  53
string2 87

有什么方法可以解决此错误？

Any possible way to fix this error?

推荐答案

您的语句有两个问题。但是错误消息表明您需要 显式强制转换 以声明字符串文字'string'的（至今未知）数据类型：

Your statement has a couple of problems. But the error message implies that you need an explicit cast to declare the (yet unknown) data type of the string literal 'string':

SELECT text 'string' AS rowname, data FROM tbl1
UNION ALL
SELECT 'string2', data FROM tbl2

足以投射一个<$ c $的 SELECT 个c> UNION 查询。通常是第一个，其中还要确定列名称。随后的具有未知类型的 SELECT 列表将排在一行。

It's enough to cast in one SELECT of a UNION query. Typically the first one, where column names are also decided. Subsequent SELECT lists with unknown types will fall in line.

在其他情况下（例如 INSERT 的> VALUES 子句）Postgres从目标列导出数据类型，并尝试自动强制转换为正确的类型。

In other contexts (like the VALUES clause attached to an INSERT) Postgres derives data types from target columns and tries to coerce to the right type automatically.

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