区分MySQL中的空值（null）和空字符（''）

CREATE TABLE test_ab (id int,
	col_a varchar(128),
	col_b varchar(128) not null
);

insert test_ab(id,col_a,col_b) values(1,1,1);
insert test_ab(id,col_a,col_b) values(2,'','');
insert test_ab(id,col_a,col_b) values(3,null,'');
insert test_ab(id,col_a,col_b) values(4,null,1);

mysql> select * from test_ab;
+------+-------+-------+
| id  | col_a | col_b |
+------+-------+-------+
|  1 | 1   | 1   |
|  2 |    |    |
|  3 | NULL |    |
|  4 | NULL | 1   |
+------+-------+-------+
4 rows in set (0.00 sec)

mysql> select * from test_ab where col_a = '';
+------+-------+-------+
| id  | col_a | col_b |
+------+-------+-------+
|  2 |    |    |
+------+-------+-------+
1 row in set (0.00 sec)

mysql> select * from test_ab where col_a is null;
+------+-------+-------+
| id  | col_a | col_b |
+------+-------+-------+
|  3 | NULL |    |
|  4 | NULL | 1   |
+------+-------+-------+
2 rows in set (0.00 sec)

mysql> select col_a+1 from test_ab where id = 4;
+---------+
| col_a+1 |
+---------+
|  NULL |
+---------+
1 row in set (0.00 sec)

mysql> select col_b+1 from test_ab where id = 4;
+---------+
| col_b+1 |
+---------+
|    2 |
+---------+
1 row in set (0.00 sec)

mysql> select count(col_a) from test_ab;
+--------------+
| count(col_a) |
+--------------+
|      2 |
+--------------+
1 row in set (0.00 sec)

mysql> select count(col_b) from test_ab;
+--------------+
| count(col_b) |
+--------------+
|      4 |
+--------------+
1 row in set (0.00 sec)

mysql> alter table test_ab modify `col_b` varchar(128) NOT NULL DEFAULT '';
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> desc test_ab;
+-------+--------------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id  | int     | YES |   | NULL  |    |
| col_a | varchar(128) | YES |   | NULL  |    |
| col_b | varchar(128) | NO  |   |     |    |
+-------+--------------+------+-----+---------+-------+
3 rows in set (0.00 sec)