Sure, here's how you can create a unique constraint that also allows null:

SQL

CREATE TABLE my_table (
  id INT NOT NULL,
  name VARCHAR(255) NULL,
  CONSTRAINT unique_name UNIQUE (name)
);

In this example, the name column is allowed to contain null values. However, the unique_name constraint will still prevent duplicate rows from being inserted into the table, even if one or both of the name values are null.

Here's an explanation of the code:

• The CREATE TABLE statement creates a new table called my_table.
• The id column is an integer column that cannot be null.
• The name column is a string column that can be null.
• The CONSTRAINT unique_name UNIQUE (name) clause creates a unique constraint on the name column. This means that no two rows in the table can have the same value in the name column, even if one or both of the values are null.

To insert a row into the table, you would use the following syntax:

SQL

INSERT INTO my_table (id, name) VALUES (1, 'John Doe');

You could also insert a row with a null value in the name column:

SQL

INSERT INTO my_table (id, name) VALUES (2, NULL);

In either case, the unique_name constraint would prevent you from inserting a second row with the same value in the name column, even if the value is null.