I'm not sure this is the right forum but I give it a try here.

I have two costumer databases where each customer have a unique customer number.

How can I compare the two databases and find if some customers are registered in both databases?

Create a calc field in which you will determine the exact criteria for what you consider to be the same customer. Possibly, Name&Address&City, etc for both files.

Then create a file reference to the second file and use those two calc fields as they keys in a new relationship. Next create a layout that shows the records from the Table occurence of the second file. Then you can use Go To Related Record [show only related records; Match found set]

I don't know if there was a missunderstanding. I have 2 databases / 2 files that I want to compare.

Could you help me with this (and get paid) plase? It will take to long time and to many questions to do it my self. ;)

Before we go any further, the unique customer number in the two databases are they the same for each customer in both databases?

Yes

Well if that is the case, then you wont need to create the calc fields. Just do this.

Create a file reference to the second file.

Put a copy of a Table Occurence from the second table (in the Relationships Graph) and create a relationship from the first file to the second file keying on customer number in each file.

Next create a layout that shows the records from the Table occurence of the second file.

Then you can use Go To Related Record [show only related records; Match found set]

I want to ask you again if you have the possibility to make it for me for a fee please?

Zip and Post your files. I dont think it is necessary to pay a developer for this; unless there is more to it that you have not conveyed.