primary-key.html

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            <h1>SQL PRIMARY KEY Constraint</h1>

            <p>The <strong>PRIMARY KEY</strong> constraint uniquely identifies each record in a table.</p>
            <p>Primary keys must contain <strong>UNIQUE</strong> values, and cannot contain <strong>NULL</strong>
                values.</p>
            <p>A table can have only <strong>ONE</strong> primary key; this primary key can consist of single or
                multiple columns (fields).</p>

            <h2>PRIMARY KEY on CREATE TABLE</h2>
            <p>The following SQL creates a PRIMARY KEY on the "ID" column when the "Persons" table is created:</p>
            <pre><code class="language-sql">-- MySQL
CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    PRIMARY KEY (ID)
);</code></pre>

            <pre><code class="language-sql">-- SQL Server / Oracle / MS Access
CREATE TABLE Persons (
    ID int NOT NULL PRIMARY KEY,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int
);</code></pre>

            <p>To name a PRIMARY KEY constraint or define it on multiple columns:</p>
            <pre><code class="language-sql">-- MySQL / SQL Server / Oracle / MS Access
CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    CONSTRAINT PK_Person PRIMARY KEY (ID, LastName)
);</code></pre>

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

-- Naming or multiple columns
ALTER TABLE Persons
ADD CONSTRAINT PK_Person PRIMARY KEY (ID, LastName);</code></pre>

            <p><em>Note:</em> If you use ALTER TABLE to add a primary key, the primary key column(s) must have been
                declared NOT NULL when the table was first created.</p>

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

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