sql - PostgreSQL - 不为 null 且不为空字符串的空值

Question

我在 PostgreSQL 表上运行了此查询:

select * wkt from table  where column <>'' and  column is not null

..意外地收到了几行，该列中没有可见的值。为什么是这样？这些行的该列中是否存在一些“隐藏”值，或者损坏的表，或者其他什么？

Answer 1

t=# select ascii(chr(9));
 ascii
-------
     9
(1 row)

因此

select ascii(column) from table  where column <>'' and  column is not null

应该给出想法

https://www.postgresql.org/docs/current/static/functions-string.html

ASCII code of the first character of the argument. For UTF8 returns the Unicode code point of the character. For other multibyte encodings, the argument must be an ASCII character.