TSQL CASE，当id.id为null时，则为” else'@'结束

Question

In my select, I'm using a statement like this to add a second cell, if it is not null, and to zero out the cell if it is null.

Child.Tag + '@' + ISNULL(CONVERT(varchar(50),parent.Tag),'') as 'Location'

This returns a location of F123@A123 if there is a parent location. However if there isn't a parent location, it will return F123@, where I would like it to only return F123.

I have adjusted the select to this:

Child.Tag + CASE Parent.Tag when null then '' else '@' end

However this still returns F123@ if the parent location is null.

Does anyone know of a way to have it stop returning the '@', and if anyone has an explanation as to why my CASE statement isn't working that would be greatly appreciated as well.

Thank you.

(Other info: I am on SSMS 2008)

Answer 1

I think you want this:

Child.Tag + COALECE('@' + Parent.Tag, '')

This adds the '@' only when there is a value for Parent.Tag .

The documentation for the simple case is:

• Evaluates input_expression, and then in the order specified, evaluates input_expression = when_expression for each WHEN clause.

NULL fails the equality test. This is subtle, but the COALESCE() form is easier to read and write anyway.

Answer 2

As Gordon notes, CASE Parent.Tag when null never evaluates as true, even when Parent.Tag is NULL - however, CASE when Parent.Tag is null will evaluate as true when Parent.Tag is NULL.

So you could have rewritten your existing expression from the CASE expression WHEN value ... form, to the CASE WHEN expression-is-true ... form, like so:

Child.Tag + CASE when Parent.Tag is null then '' else '@' end + ...