Pandas merge to SQL Server JOIN

merge is a join. The how= argument is the only word that changes.

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

suffixes=_x/_y are table aliases plus explicit column lists. SELECT * after a merge is how you get two customer_id columns.

Canonical translation

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