Relationship between "and null" in Oracle Database

In Oracle databases, ''is equivalent to null. Both indicate null values, rather than Null strings like ''in other databases. null indicates null values.

 

Oracle allows blank fields of any data type, except for the following two cases:

1. primary key field (Primary Key ),

2. fields with the not null restriction added during definition

 

Note:

1. null is equivalent to no value and is unknown.

2. null and 0, empty strings, and spaces are different.

3. add, subtract, multiply, and divide null values. The result is still null.

4. The nvl function is used for null processing.

5. Use the keyword "is null" and "is not null" for comparison ". It cannot be compared with other values equal to, not equal to, greater than, or less than. Of course, it also includes the null value itself (except in decode, the two null values are considered to be equivalent)

6. null values cannot be indexed. Therefore, some data that meets the query conditions may not be found. In count (*), nvl (column name, 0) is used for processing and then query.

7. Sorting is larger than other data (the index is sorted in descending order by default, small → large), so the null value is always at the end.

8. In the where condition, if Oracle considers the condition that the result is null to be false, select statements with such conditions do not return rows or error messages.