Click the "+" button. Repeat the same step for Opportunity.

The Resulting preview will sample and return 100 rows.

Columns will Display a list of available columns and "Profile" will allow you to quickly view stats for each column. We will explore that later.

Click on the "+" button to the right of the Opportunity. 

Click "Join" to connect the Opportunity to the Account.

Click "Select dataset…". This will allow you to select the "Account"

Click "Select Column…" on the Opportunity Object

Click "AccountId". The account ID is a foreign key on the Opportunity object.

Click "Select Column…" on the Account Object.

Click "Id". Now we have connected the AccountID from the opportunity to the "Id" (primary key) on the Account.

Click to enable "Include join key columns from both sources".

Click "Unmatched Rows". This view will locate records that are not joined.

This view tells us that there were no Opportunities without Accounts (left) BUT there are 4 Accounts without Opportunities. 

The view highlights visually the values you selected to join.

Click "Apply" to implement the join and return to the main canvas.

The Join Keys view provides a count how many matches occur. This is a great view to validate the uniqueness of your keys, uncover dupes, cartesian product problems, or simple validate the outcome of your join makes sense.

This view tells us we had 10 opportunities for a single account.

Additionally this view tells us that the Account ID is distinct.

Lets explore this account further. Select and copy the first record.  "0015e000003EhgIAAS"