mysql null 值相关规则

3.3.4.6 Working with NULL Values

http://dev.mysql.com/doc/refman/5.5/en/working-with-null.html

The NULL value can be surprising until you get used to it. Conceptually, NULL means “a missing unknown value” and it is treated somewhat differently from other values. In MySQL, 0 or NULL means false and anything else means true. This special ...

B.5.4.3 Problems with NULL Values

http://dev.mysql.com/doc/refman/5.5/en/problems-with-null.html

The concept of the NULL value is a common source of confusion for newcomers to SQL, who often think that NULL is the same thing as an empty string ''. For example, the following statements are completely different: mysql> INSERT INTO my_table ...

8.2.1.6 IS NULL Optimization

http://dev.mysql.com/doc/refman/5.5/en/is-null-optimization.html

MySQL can perform the same optimization on col_name IS NULL that it can use for col_name = constant_value. For example, MySQL can use indexes and ranges to search for NULL with IS NULL. This optimization does not occur in cases when the column ...

9.1.7 NULL Values

http://dev.mysql.com/doc/refman/5.5/en/null-values.html

The NULL value means “no data.” NULL can be written in any lettercase. Be aware that the NULL value is different from values such as 0 for numeric types or the empty string for string types. For text file import or export operations performed ...