Foreign keys in SQL

A foreign key ties a child row to the parent it belongs to.

CREATE TABLE order_items (
    order_id INTEGER NOT NULL REFERENCES orders(id) ON DELETE CASCADE,
    sku TEXT NOT NULL,
    quantity INTEGER NOT NULL CHECK (quantity > 0),
    UNIQUE (order_id, sku)
);

How it works

1. REFERENCES names the parent table and column.
2. ON DELETE CASCADE removes the children with the parent.
3. UNIQUE on a pair of columns forbids duplicate rows.

Keywords and builtins used here

• CASCADE
• CHECK
• CREATE
• DELETE
• INTEGER
• NOT
• NULL
• ON
• REFERENCES
• TABLE
• TEXT
• UNIQUE

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Step 2 of 8 in Tables & rows, step 15 of 23 in Language basics.

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