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while doing select in MS SQL very often NULL and '' can be treated identical. There is good description to Combine These: how to use null or empty string

However I wonder if it make sense to put this into a user defined function to simplify queries for this. Does anybody uses such function? Are there strong pro and cons on this one?

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    Putting this is a UDF (as simple as it is) could be a performance killer Commented Jan 21, 2018 at 0:04
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    It depends on your data as well. In some cases '' and NULL may not represent the same thing. As a very simple idea, you could have a Yes/No question on your website/application. Values of '' could represent that the customer has not answered the question, when it was presented to them, however, NULL may represent that the customer hasn't answered and it isn't applicable, or they haven't been given the opportunity (perhaps they registered before the question was added). I would never say yes '' and NULL represent the same thing without knowing the data they represent. Commented Jan 21, 2018 at 11:20
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    @JohnCappelletti also raises a good point. I no doubt imagine the "easiest" way to add a UDF for this would be to use a Scaler, as it goes in the WHERE. Scalar functions, however, are notoriously slower compared to Table-Value Function, but then it could look messy in your FROM clause. Personally, if you know that for that column NULL also means '' use an OR clause: AND (YourColumn = '' OR YourColumn IS NULL). Or, otherwise, if you don't wants NULLs, then use an UPDATE statement to change the NULL values to '' and then set the column to be non-NULLable. Commented Jan 21, 2018 at 11:25
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