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What happens when we apply NOT NULL constraint, with ALTER TABLE statement, to a column contains NULL values?
In this case, MySQL will return an error message regarding data truncated for the column. Following is an example of demonstrating it −
Example
Suppose we have a table ‘test2’ which contains a NULL value in column ‘ID’ at 2nd row. Now, if we will try to declare the column ID to NOT NULL then MySQL will return the error as follows −
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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