To add a NOT NULL column to a table,we first add a NULL column,then we insert values into every row,and finally we change the NULL constraint to NOT NULL.

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This is a common approach when needing to add a NOT NULL column to an existing table. Initially adding the column as NULL allows you to populate the column for all existing rows. Once every row has a value for this column, altering the column to NOT NULL is safe because there are no NULL values that would violate the constraint.

To Add a NOT NULL Column to a Table,we First