How to create a unique index on a NULL column?

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Aryan Kumar · Answer

Sure. You can create a unique index on a null column in SQL, but it will not prevent you from inserting duplicate null values into the column. This is because NULL values are considered unique in SQL.
To create a unique index on a null column, you use the CREATE UNIQUE INDEX statement. The syntax for the
CREATE UNIQUE INDEX statement is as follows:
SQL
CREATE UNIQUE INDEX indename
ON table_name (column_name);
The indename is the name of the index. The table_name is the name of the table that the index is associated with. The
column_name is the name of the column that the index is created on.
Here is an example of how to create a unique index on a null column:
SQL
CREATE UNIQUE INDEX unique_null_column
ON customers (customer_id);
This will create a unique index on the customer_id column in the 
customers table. However, it will not prevent you from inserting duplicate null values into the
customer_id column.
If you want to prevent duplicate null values in a column, you can use a 
filtered unique index. A filtered unique index is a unique index that only applies to rows where the specified column is not NULL.
To create a filtered unique index, you use the CREATE UNIQUE INDEX statement with the
WHERE clause. The syntax for the CREATE UNIQUE INDEX statement with the
WHERE clause is as follows:
SQL
CREATE UNIQUE INDEX indename
ON table_name (column_name)
WHERE column_name IS NOT NULL;
Here is an example of how to create a filtered unique index on a null column:
SQL
CREATE UNIQUE INDEX unique_null_column
ON customers (customer_id)
WHERE customer_id IS NOT NULL;
This will create a unique index on the customer_id column in the 
customers table. However, it will only apply to rows where the customer_id column is not NULL. This will prevent you from inserting duplicate null values into the
customer_id column.

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How to create a unique index on a NULL column?

Revati S Misra

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