SQL Isnull is not working and returns a value even if it is a space

问题

I am using SQL Server 2016 and came across an issue with the isnull function. The value in that field is a space but keeps getting selected, below is my code:

SELECT * FROM table
WHERE  isnull (field1,'')<>''
       AND field1<>' '

If I copy and paste from field1, it is one space ' '.

Thanks

回答1:

Your space is not normal space (hex 0x20, decimal 32)

select ASCII(LEFT(field1, 1), * From table
where  isnull (field1,'')<>''

Most likely it is one of

• Linefeed decimal 10
• Carriage return decimal 13
• Null decimal 0
• hard space decimal 160
• tab decimal 9 (as per comments on question from @SeanLange)

回答2:

I have tried to put every possible combination in insert.. try this

Create table #t(Field1 varchar(100))
Insert into #t values 
(NULL), (' '), ('  '), ('a'),('')


select  field1 From #t
where LTRIM(RTRIM(field1)) <> ''

output :

field1
---------
a

回答3:

As we found out through comments and other answers (gbn's especially) there could be other invisible characters except space.

But that doesn't give you a solution how to write your query which could be something like:

where field1 NOT IN (CHAR(0), CHAR(9), CHAR(10), CHAR(11), CHAR(12), CHAR(13), CHAR(14), CHAR(32), CHAR(160))

回答4:

I found the solution for this issue as:

SELECT ISNULL(NULLIF(field1,''), field2)

来源：https://stackoverflow.com/questions/45461543/sql-isnull-is-not-working-and-returns-a-value-even-if-it-is-a-space