foreign-key.html

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            <h1>SQL FOREIGN KEY Constraint</h1>

            <p>The <strong>FOREIGN KEY</strong> constraint is used to prevent actions that would destroy links between
                tables.</p>
            <p>A FOREIGN KEY is a field (or collection of fields) in one table that refers to the <strong>PRIMARY
                    KEY</strong> in another table.</p>
            <p>The table with the foreign key is called the <strong>child table</strong>, and the table with the primary
                key is called the <strong>referenced</strong> or <strong>parent table</strong>.</p>
            <p>The FOREIGN KEY constraint prevents invalid data from being inserted into the foreign key column because
                it must exist in the parent table.</p>

            <h2>FOREIGN KEY on CREATE TABLE</h2>
            <p>The following SQL creates a FOREIGN KEY on the "PersonID" column when the "Orders" table is created:</p>
            <pre><code class="language-sql">-- MySQL
CREATE TABLE Orders (
    OrderID int NOT NULL,
    OrderNumber int NOT NULL,
    PersonID int,
    PRIMARY KEY (OrderID),
    FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);</code></pre>

            <pre><code class="language-sql">-- SQL Server / Oracle / MS Access
CREATE TABLE Orders (
    OrderID int NOT NULL PRIMARY KEY,
    OrderNumber int NOT NULL,
    PersonID int FOREIGN KEY REFERENCES Persons(PersonID)
);</code></pre>

            <p>To name a FOREIGN KEY constraint or define it on multiple columns:</p>
            <pre><code class="language-sql">-- MySQL / SQL Server / Oracle / MS Access
CREATE TABLE Orders (
    OrderID int NOT NULL,
    OrderNumber int NOT NULL,
    PersonID int,
    PRIMARY KEY (OrderID),
    CONSTRAINT FK_PersonOrder FOREIGN KEY (PersonID)
    REFERENCES Persons(PersonID)
);</code></pre>

            <h2>FOREIGN KEY on ALTER TABLE</h2>
            <p>To create a FOREIGN KEY on an existing table:</p>
            <pre><code class="language-sql">-- MySQL / SQL Server / Oracle / MS Access
ALTER TABLE Orders
ADD FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);

-- Naming or multiple columns
ALTER TABLE Orders
ADD CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);</code></pre>

            <h2>Drop a FOREIGN KEY Constraint</h2>
            <pre><code class="language-sql">-- MySQL
ALTER TABLE Orders
DROP FOREIGN KEY FK_PersonOrder;

-- SQL Server / Oracle / MS Access
ALTER TABLE Orders
DROP CONSTRAINT FK_PersonOrder;</code></pre>
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