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Data & Analytics · Foundational

LEFT OUTER JOIN

A join that returns every row from the left table plus matching rows from the right, with NULLs where no match exists.

Definition

LEFT JOIN preserves the left (first) table. Placing a condition on a right-table column in WHERE (other than IS NULL) effectively turns it back into an inner join; put such conditions in the ON clause instead.

Example

SELECT c.Name, o.OrderID FROM Customers c LEFT JOIN Orders o ON c.CustomerID = o.CustomerID;

Exam tip

Filtering the right table in WHERE silently converts a LEFT JOIN into an INNER JOIN, a frequent trick question.

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