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            <h1>SQL UNIQUE Constraint</h1>

            <p>The <strong>UNIQUE</strong> constraint ensures that all values in a column are different.</p>
            <p>Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a column or set of
                columns. A PRIMARY KEY constraint automatically has a UNIQUE constraint.</p>
            <p>You can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.</p>

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

-- display the empty table Persons
SELECT * FROM Persons;</code></pre>

            <p>MySQL alternative syntax:</p>
            <pre><code class="language-sql">CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    UNIQUE (ID)
);</code></pre>

            <p>To name a UNIQUE 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 UC_Person UNIQUE (ID, LastName)
);</code></pre>

            <h2>UNIQUE Constraint on ALTER TABLE</h2>
            <p>To create a UNIQUE constraint on the "ID" column after the table is already created:</p>
            <pre><code class="language-sql">-- MySQL / SQL Server / Oracle / MS Access
ALTER TABLE Persons
ADD UNIQUE (ID);

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

            <h2>Drop a UNIQUE Constraint</h2>
            <p>To drop a UNIQUE constraint:</p>
            <pre><code class="language-sql">-- MySQL
ALTER TABLE Persons
DROP INDEX UC_Person;

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