MySQL Inserts record with NULL value in NOT NULL column

Question

Why does the first INSERT go through for table2. Note that table2.col_1 is NOT NULL. It doesn't insert NULL for col_1, but mysteriously converts the NULL value to an empty string. I am using MySQL Version 5.5.28. Thanks

mysql> DROP TABLE IF EXISTS table1, table2;

Query OK, 0 rows affected (0.01 sec)   

mysql> CREATE  TABLE IF NOT EXISTS table1 (
    -> id INT UNSIGNED NOT NULL AUTO_INCREMENT ,
    -> col_1 VARCHAR(45) NOT NULL ,
    -> col_2 VARCHAR(45) NOT NULL ,
    -> PRIMARY KEY (`id`))
    -> ENGINE = InnoDB;

Query OK, 0 rows affected (0.01 sec)

mysql> CREATE TABLE table2 LIKE table1;
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO table1 (id, col_1, col_2) VALUES (NULL, "xxx","yyy");
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO table2 (id, col_1, col_2) SELECT NULL, NULL, col_2 FROM table1 WHERE id=1;
Query OK, 1 row affected, 1 warning (0.00 sec)
Records: 1  Duplicates: 0  Warnings: 1

mysql> SHOW WARNINGS;
+---------+------+-------------------------------+
| Level   | Code | Message                       |
+---------+------+-------------------------------+
| Warning | 1048 | Column 'col_1' cannot be null |
+---------+------+-------------------------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM table2;
+----+-------+-------+
| id | col_1 | col_2 |
+----+-------+-------+
|  1 |       | yyy   |
+----+-------+-------+
1 row in set (0.00 sec)

mysql> INSERT INTO table2 (id, col_1, col_2) VALUES( NULL, NULL, "zzz");
ERROR 1048 (23000): Column 'col_1' cannot be null

mysql> SELECT * FROM table2;
+----+-------+-------+
| id | col_1 | col_2 |
+----+-------+-------+
|  1 |       | yyy   |
+----+-------+-------+
1 row in set (0.00 sec)

Answer 1

This behavior is well documented in MySQL docs . MySQL doc

If you are not using strict mode, then whenever you insert an “incorrect” value into a column, such as a NULL into a NOT NULL column or a too-large numeric value into a numeric column, MySQL sets the column to the “best possible value” instead of producing an error:,but the warning count is incremented

Answer 2

You have MySQL's STRICT mode OFF. Turn it on and you'll get an error.

Otherwise you can test for those warnings with PDO via: http://php.net/manual/en/pdo.errorinfo.php