How to add a unique constraint column in SQL

Adding a unique constraint column with SQL: Using the ALTER TABLE statement, followed by the ADD UNIQUE clause, you can add a unique constraint column to an existing table. Steps: 1. Connect to the database. 2. Identify the table. 3. Write an ALTER TABLE statement (ALTER TABLE table_name ADD UNIQUE (column_name)). 4. Execute the statement. Note: After adding a unique constraint, the constraint is enforced to avoid inserting duplicate values ​​in the unique_column column.

Adding unique constraint columns using SQL

Question: How to add a unique constraint column using SQL?

Answer: Using the ALTER TABLE statement, followed by the ADD UNIQUE clause, you can add unique constraint columns to an existing table.

step:

1. Connect to a database: Use the database client or command line to connect to the database containing the tables you want to modify.

2. Identify tables: Use the USE statement to select the table to which a unique constraint column is to be added. For example:

    <code class="sql">USE my_database;</code>

3. Writing an ALTER TABLE statement: Write an ALTER TABLE statement using the following syntax:

    <code class="sql">ALTER TABLE table_name ADD UNIQUE (column_name);</code>

   in:

   • table_name is the table name to be modified.
   • column_name is the column name to add a unique constraint.

4. Execution statement: Use the following command to execute the ALTER TABLE statement:

    <code class="sql">ALTER TABLE table_name ADD UNIQUE (column_name);</code>

Example:

The following example demonstrates how to add a unique constraint column named unique_column to a table named my_table :

 <code class="sql">USE my_database; ALTER TABLE my_table ADD UNIQUE (unique_column);</code>

Notes:

After adding a unique constraint, the database enforces the constraint, which means that no duplicate values ​​are inserted in unique_column column. If you try to insert a duplicate value, the database returns an error.

Before adding a unique constraint, make sure that the values ​​in unique_column column are unique. Otherwise, adding constraints may result in data loss.

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