Difference between NULL and empty value in MySQL

Usually when we use MySQL, we cannot understand the difference between NULL values ​​and null values ​​in MySQL. Note that the NULL value is unknown and takes up space without indexing. DBA suggests that it is best to set the field to NOT NULL when building a table to avoid such inefficiencies.
   Question 1: First of all, we need to understand the concept of "null value" and "NULL":
    1: Null value ('') does not occupy space
    2: NULL in MySQL actually occupies space. The official documentation states:
"NULL columns require additional space in the row to record whether their values ​​are NULL. For MyISAM tables, each NULL column takes one bit extra, rounded up to the nearest byte."
Length verification: pay attention to the '' of null values There are no spaces in between.
mysql> select length(''),length(null),length(' ');
+------------+--------------+- -------------+
| length('') | length(null) | length(' '




   Question 2:
When judging that the field is not empty, whether the query statement uses select * from tablename where columnname <> '' or
select * from tablename where column is not null, what is the difference between the two query statements.
eg:
mysql> show create table testaa;
+---------+------------------------------- -------------------------------------------------- -------------------------------------------------- -----------------------------------+
| Table | Create Table |
+------- -+------------------------------------------------ -------------------------------------------------- -------------------------------------------------- ------------------+
| testaa | CREATE TABLE `testaa` (
  `a` int(11) NOT NULL,
  `b` varchar(20) DEFAULT NULL,
  `c` varchar(20) NOT NULL,
  PRIMARY KEY (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 ;

插入测试数据:
mysql> insert testaa  values (1,'aa','');
Query OK, 1 row affected (0.00 sec)
mysql> insert testaa  values (2,'','');
Query OK, 1 row affected (0.00 sec)
mysql> insert testaa  values (3,null,'');
Query OK, 1 row affected (0.00 sec)
mysql> insert testaa  values (4,NULL,'');
Query OK, 1 row affected (0.00 sec)
mysql> insert testaa  values (5,'aafa','fa');
Query OK, 1 row affected (0.00 sec)

mysql> insert testaa values (6,'',NULL);
ERROR 1048 (23000): Column 'c' cannot be null

mysql> select * from testaa;
+---+------+----+
| a | b    | c  |
+---+------+----+
| 1 | aa   |    |
| 2 |      |    |
| 3 | NULL |    |
| 4 | NULL |    |
| 5 | aafa | fa |
+---+------+----+

查询验证过程:

mysql> select * from testaa where c is not null;
+---+------+----+
| a | b    | c  |
+---+------+----+
| 1 | aa   |    |
| 2 |      |    |
| 3 | NULL |    |
| 4 | NULL |    |
| 5 | aafa | fa |
+---+------+----+
5 rows in set (0.00 sec)

mysql> select * from testaa where c <> '';
+---+------+----+
| a | b    | c  |
+---+------+----+
| 5 | aafa | fa |
+---+------+----+
1 row in set (0.00 sec)
mysql> select * from testaa  where c = '';
+---+------+---+
| a | b    | c |
+---+------+---+
| 1 | aa   |   |
| 2 |      |   |
| 3 | NULL |   |
| 4 | NULL |   |
+---+------+---+
4 rows in set (0.00 sec)

mysql> select * from testaa where  c is null;
Empty set (0.00 sec)


mysql> select * from testaa where b is not null;
+---+------+----+
| a | b    | c  |
+---+------+----+
| 1 | aa   |    |
| 2 |      |    |
| 5 | aafa | fa |
+---+------+----+
3 rows in set (0.00 sec)

mysql> select * from testaa where b <> '';
+---+------+----+
| a | b    | c  |
+---+------+----+
| 1 | aa   |    |
| 5 | aafa | fa |
+---+------+----+
2 rows in set (0.00 sec)

mysql> select * from testaa where b ='';
+---+------+---+
| a | b    | c |
+---+------+---+
| 2 |      |   |
+---+------+---+
1 row in set (0.00 sec)
mysql> select * from testaa where  b is null;
+---+------+---+
| a | b    | c |
+---+------+---+
| 3 | NULL |   |
| 4 | NULL |   |
+---+------+---+

mysql> select length(b),length(c) from testaa;
+-----------+-----------+
| length(b) | length(c) |
+-----------+-----------+
| 2 | 0 |
| 0 | 0 |
| NULL | 0 |
| NULL | 0 |
| 4 | 2 |
+-----------+-----------+
5 rows in set (0.00 sec)


mysql> select count(b),count(c) from testaa;
+----------+----------+
| count(b) | count(c) |
+----------+----------+
| 3 | 5 |
+----------+----------+
1 row in set (0.00 sec)

mysql> create table testbb ( a int primary key , b timestamp);
Query OK, 0 rows affected (0.07 sec)
mysql> show create table testbb;
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| testbb | CREATE TABLE `testbb` (
`a` int(11) NOT NULL,
`b` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
PRIMARY KEY (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+--------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

mysql> insert into testbb vales (1,null) ;
mysql> insert into testbb values (2,'');
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> show warnings;
+---------+------+----------------------------------------+
| Level | Code | Message |
+---------+------+----------------------------------------+
| Warning | 1265 | Data truncated for column 'b' at row 1 |
+---------+------+----------------------------------------+
mysql> select * from testbb;
+---+---------------------+
| a | b |
+---+---------------------+
| 1 | 2014-08-15 14:32:10 |
| 2 | 0000 -00-00 00:00:00 |
+---+---------------------+
2 rows in set (0.00 sec)


Notes:
1: When count() counts the number of records in a column, if the NULL value is used, the system will automatically ignore it, but the NULL value will be counted into it.
2: Use IS NULL or is not null to judge NULL, you can use the ifnull() function in the SQL statement function to process it, and use ='' or <>'' to process the null character
3: For MySQL special precautions, For the timestamp data type, if a NULL value is inserted into a column of this data type, the value that appears is the current system time. Insert a null value, and '0000-00-00 00:00:00' will appear.
4: Whether to use is null or ='' for the judgment of null value should be distinguished according to the actual business.

From: http://blog.sina.com.cn/s/blog_3f2a82610102v4dn.html