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            <h1>SQL DEFAULT Constraint</h1>

            <p>The <strong>DEFAULT</strong> constraint is used to set a default value for a column.</p>
            <p>The default value will be added to all new records if no other value is specified.</p>

            <h2>DEFAULT on CREATE TABLE</h2>
            <p>Set a default value for the "City" column when creating the "Persons" table:</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,
    City varchar(255) DEFAULT 'Sandnes'
);</code></pre>

            <p>DEFAULT can also use system values like <code>current_timestamp</code>:</p>
            <pre><code class="language-sql">-- MySQL / SQL Server / Oracle / MS Access
CREATE TABLE Orders (
    ID int NOT NULL,
    OrderNumber int NOT NULL,
    OrderDate date DEFAULT current_timestamp
);

-- Example insert
INSERT INTO Orders (ID, OrderNumber) VALUES (1,1);

SELECT * FROM Orders;</code></pre>

            <h2>DEFAULT on ALTER TABLE</h2>
            <p>Create a DEFAULT constraint on an existing column:</p>
            <pre><code class="language-sql">-- MySQL
ALTER TABLE Persons
ALTER City SET DEFAULT 'Sandnes';

-- SQL Server
ALTER TABLE Persons
ADD CONSTRAINT df_City
DEFAULT 'Sandnes' FOR City;

-- MS Access
ALTER TABLE Persons
ALTER COLUMN City SET DEFAULT 'Sandnes';

-- Oracle
ALTER TABLE Persons
MODIFY City DEFAULT 'Sandnes';</code></pre>

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

-- SQL Server / Oracle / MS Access
ALTER TABLE Persons
ALTER COLUMN City DROP DEFAULT;</code></pre>
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