在MSSQL中使用ISNULL（）的奇怪行为

Question

I have a select statement that is combining multiple segments of a persons name. This isn't anything new.

SELECT FirstName + ' ' + LastName AS FullName FROM MyTable

I then tried to add the middle initial to this and I came up with the following

SELECT FirstName + ' ' + ISNULL(MiddingInitial + ' ', '') + LastName AS FullName FROM MyTable

This appears to work, but during my testing of ISNULL() , I came across an odd behavior. I'm aware that NULL + 'any string' resolves to NULL . However this was just plain odd...

Here's my code, and what I get out as a result...

print '''' + isnull(null + 'Any String','Results in null') + ''''
print '''' + isnull(null + 'Any','Results in null') + ''''
print '''' + isnull(null + 'A','Results in null') + ''''
print '''' + isnull(null + '','Results in null') + ''''
/*
'Results in '
'Resu'
'Re'
'Re'
*/

Any idea of why this behavior occurs? Does it do the same for you?

Answer 1

It comes down to the datatypes you're working with and the behavior of the ISNULL function. Let's look at one example:

null + 'Any String'

The above fits perfectly into a varchar(11) datatype. The NULL (which is really just the result of char(0) and has length 1) and a regular 10-character string concatenated together makes for 11 characters total. The replacement string -- the second parameter to your ISNULL function -- is going to be forced to fit into a varchar(11) , so it is truncated to 11 characters.

The pattern repeats for the remaining items, with a special case for the empty string.

If you don't want this to happen, use COALESCE , which instead of taking the datatype of the first item in the list, it uses data type precedence. A varchar(15) takes precedence over a varchar(11) , so you will get the full replacement string:

print '''' + coalesce(null + 'Any String','Results in null') + ''''
print '''' + coalesce(null + 'Any','Results in null') + ''''
print '''' + coalesce(null + 'A','Results in null') + ''''
print '''' + coalesce(null + '','Results in null') + ''''
/*
'Results in null'
'Results in null'
'Results in null'
'Results in null'
*/