What happens when we use the ALTER TABLE statement to apply a NOT NULL constraint on a column that contains NULL values?

In this case, MySQL will return an error message that the data for the column is truncated. Here is an example to demonstrate it -

Example

Suppose we have a table "test2" where row 2 contains NULL value in the "ID" column. Now, if we try to declare the column ID as NOT NULL, then MySQL will return an error as shown below -

mysql> Select * from test2;
+------+--------+
| ID   | Name   |
+------+--------+
| 1    | Gaurav |
| NULL | Rahul  |
+------+--------+
2 rows in set (0.00 sec)
mysql> ALTER TABLE TEST2 MODIFY ID INT NOT NULL;
ERROR 1265 (01000): Data truncated for column 'ID' at row 2

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