SQL Left Join

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A LEFT JOIN returns all rows from the left (first) Table and any matching rows from the right (second) table. If there is no match, the left-side row still appears, but with NULL for the right-side columns.

Here is a classic Venn-style diagram showing a LEFT JOIN in the abstract (i.e., not tied to any specific tables yet):

• Left Table: all rows appear in the result.
• Right Table: rows only appear if they match the Left Table on the join condition.
• Result: unmatched left rows appear with NULL for right-table columns.

Example with Countries and Cities

Below is a simplified scenario with Countries (left Table) and Cities (right Table).

    Countries:
        (1, 'France')
        (2, 'Italy')
        (3, 'Poland')
    Cities:
        (1, 'Paris') with country_id=1
        (2, 'Rome') with country_id=2
        (3, 'Milano') with country_id=2
        (4…