Hi @rhovath94 That’s an interesting find - can you share your workflow where this is happening with some sample data that can be reproduced? I’d like to look closer at this one Can you also share what version of n8n you’re using?
I have just ran your workflow and the first item doesn’t have a value for RenamedKey so it is correctly undefined, The second item does have the key set but it doesn’t have the filter field set which you are using for the If condition.
If you change to the Table view it will give you an idea of what has happened.
There are 115 items there, As a test can you add an If node before that and do something like If building name is not empty and see if that shows anything.
it does work that and shows some up.
Might be the merge node not functioning properly or i am using it wrongly.
at one branch i have 37element other branch 78 → merge and append becomes 115 as shown on the previous picture. and than i would like to compare this data with an IF but the "building name"shows up as undefined
When you added the if did it remove any items or did all 115 move forward? The append doesn’t actually merge the data as such so if one of the inputs doesn’t have that field it won’t add it for that you would need to use one of the combine options.
it did removes/devides the items correctly.
I would exactly want to compare that 2 data set. like the “‘building name’” key comapered to and other key called “'ëntity name”’ with a IF/contains method.
I guess it would depend on what it is you are tyring to do in the flow, Are you trying to compare one item against all of the items in the other dataset… Are you planning to merge the 2 records or ignore anything doesn’t match?
That’s an interesting find - can you share your workflow where this is happening with some sample data that can be reproduced? I’d like to look closer at this one 
Can you also share what version of n8n you’re using?