Null in Oracle

Null if no assignment is given and no default value

Oracle allows fields of any one data type to be empty except for the following two cases: 1, primary key field (primary key), 2, field defined with a NOT NULL restriction condition

Description: 1, equivalent to no value, is unknown (unassigned, and there is no default value). 2, null and 0, empty string, space are different. 3, add, subtract, multiply and divide the null value, and the result is still empty. 4, NULL processing uses the NVL function. 5. Use the keyword "is null" and "is not NULL" when comparing. 6, the null value cannot be indexed , so the query when some eligible data may not be found, COUNT (*), with NVL (column name, 0) processing and then check. 7, the sorting is larger than other data (the index by default is descending, small → large),   so the null value is always ranked at the end.  Use the method:sql> select 1 from dual where null=null; no records sql> select 1 from dual where null= '; No records found sql> select 1 from Dual where ' = '; no record sql> select 1 from dual where null is null;        1---------        1sql> Select 1 from dual where NVL (null,0) =NVL (null,0);        1---------        1 The null value is added, minus, multiply, divide, and so on, the result is still empty. Sql> select 1+null from dual; Sql> select 1-null from dual; Sql> select 1*null from dual; Sql> select 1/null from dual; query to a record. Note: This record is the null in the SQL statement
Set some column null values for UPDATE table1 set 1=null where column 1 is not NULL;
 an existing commodity sales table sale, the table structure is: Month char (6)--month Sellnumber (10,2)-Monthly Sales Amount CREATE TABLE sale (month char (6), sell number); insert into sale values (' 200001 ', +), insert into sale values (' 200002 ', 1100), insert into sale values (' 200003 ', "), insert into sale values (' 200004 ', 1300), insert into sale values (' 200005 ', 1400), insert into sale values (' 200006 ', "), insert into sale values (' 200007 ', "), insert into sale values (' 200101 ', 1100), insert into sale values (' 200202 ',"), insert         into sale values (' 200301 ', 1300), insert into sale values (' 200008 ', +), insert into sale (month) VALUES (' 200009 '); (Note: The sell value of this record is null) commit; Enter 12 records in total 
Sql> select * FROM sale where sell like '% '; MONTH       SELL---------------200001      1000200002      1100200003      1200200004      1300200005      1400200006      1500200007      1600200101      1100200202      1200200301      1300200008       Search to 11 records.       Result Description: The query results indicate that this SQL statement query does not have a column value of NULL for fields that need to be treated differently when the field is null. Sql> select * FROM sale where sell like '% ' or sell is null; Sql> select * FROM sale where NVL (sell,0) like '% ';       MONTH       SELL---------------200001      1000200002      1100200003      1200200004      1300200005      1400200006      1500200007      1600200101      1100200202      1200200301      1300200008      1000200009       query to 12 records.

Null in Oracle