Pandas DataFrame.join to SQL JOIN

Pandas join is merge on the index. SQL has no index in that sense. Name the key.

SELECT o.*, c.email
FROM Orders o
LEFT JOIN Customers c ON c.customer_id = o.customer_id;

how="inner" (the default for join is left, for merge is inner). Read the call.

Canonical translation

SELECT o.*, c.email
FROM Orders o
LEFT JOIN Customers c ON c.customer_id = o.customer_id;