This is one of those WTF moments. After more than a decade of working in software development I learn something this basic about T-SQL (or rather, any flavour based on SQL-92). What would you think happens when running this script?
IF ''='                 ' SELECT 'WTF?!' -- empty string compared to a bunch of spaces
IF ' '=' ' SELECT 'WTF?!' -- bunch or spaces compared to another bunch of spaces of different length
IF 'x'='x ' SELECT 'WTF?!' -- 'x' compared to 'x' followed by a bunch of spaces
IF 'x'=' x' SELECT 'WTF?!' -- 'x' compared to 'x' preceded by a bunch of spaces

There will be three WTF rows returned, for the first three queries. You don't believe me? Try it yourself. The motive is explained here: INF: How SQL Server Compares Strings with Trailing Spaces. Short story shorter: in order for SQL to compare two strings of different lengths, it first right-pads the shorter one with spaces.

So what can you do to fix it? Easy enough, use LEN ,right? Nope. Read the definition carefully: Returns the number of characters of the specified string expression, excluding trailing blanks. A possible but weird solution is to use DATALENGTH. A string is empty only is it has a datalength of 0. In the case of NVARCHAR you could even divide the resulting number to 2 in order to get the true length of the string. WTF, right?


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