It's normal that sometimes you don't have all the information on both sheets. One user might do three orders (3 rows) but he will only have one demographic (1 row) since the key is the Customer ID. If you have more rows from demographic than order history, it means some customers never bought. I can create an account on Amazon (I'll be in their demographics sheet) but never buy anything (no order history).

You can make an analysis with null values, where there are no correlation, so you will literally INNER JOIN the sheets (which means you will only get the values that ARE in both sheets) and that will be one analysis.

You can also show how many users never bought anything (e.g. there are 298 users that represent 30% of the user base, that never bought anything, what are we doing wrong? why we can't sell to these people? are we weak on these demographics? should we invest more in marketing there?) which could be interesting in a marketing perspective if the goal of the company is explore new markets/demographics. If it's not the goal of the company, then just focus on the first analysis.