PostgreSQL中NULL的意义

#include <stddef.h>#include <stdio.h>void main(){    if ('0' == NULL)        printf("NULL is '0' \n");    if ("" == NULL)        printf("NULL is empty string \n");    if (' ' == NULL)        printf("NULL is space \n");    if (0 == NULL)        printf("NULL is 0 \n");}

public class Test
{ 
    public static void main (String[] args) throws java.lang.Exception
    {
        System.out.println("Null is: " + null);
    }
}
Null is: null

postgres=# SELECT 1 = 1 result;
 result 
--------
 t
(1 row)
postgres=# SELECT 'foo' = 'foo' result;
 result 
--------
 t
(1 row)

postgres=# SELECT NULL = NULL result;
result
--------
(1 row)

postgres=# SELECT NULL != NULL result;
result
--------
(1 row)

postgres=# SELECT NULL * 10 is NULL result;
result
--------
t
(1 row)

postgres=# SELECT NULL is NULL result;
result
--------
t
(1 row)
postgres=# SELECT NULL is NOT NULL result;
result
--------
f
(1 row)

COALESCE (NULL, 2 , 1);

postgres=# SELECT NULLIF (10, 10);
nullif
--------


(1 row)
postgres=# SELECT NULLIF (10, 100);
nullif
--------
10
(1 row)

postgres=# CREATE TABLE student(id INTEGER, fname TEXT, sname TEXT, lname TEXT, age INTEGER);
postgres=# SELECT * FROM STUDENT;
id | fname | sname | lname | age
----+-------+-------+-------+-----
1 | Adams | Baker | Clark |  21
2 | Davis |       | Evans | 22
3 | Ghosh | Hills |       | 24
(3 rows)

postgres=# SELECT * FROM STUDENT WHERE sname = '';
id | fname | sname | lname | age
----+-------+-------+-------+-----
(0 rows)

postgres=# SELECT * FROM STUDENT WHERE sname IS NULL;
id | fname | sname | lname | age
----+-------+-------+-------+-----
2 | Davis |       | Evans | 22
(1 row)

postgres=# CREATE TABLE person(id INTEGER, name TEXT, type TEXT, divorced bool);
postgres=# SELECT * FROM person;
 id | name  | type | divorced 
----+-------+-------+---------
  1 | Alice | WOMAN | f
  3 | Davis | KID   | 
  2 | Bob   | MAN | t
(3 rows)

postgres=# SELECT * FROM students_mark;
 id | name  | marks 
----+-------+-------
  1 | Alex  | 90
  2 | Bob   | 0
  2 | Davis |      
(3 rows)


postgres=# SELECT * FROM students_mark WHERE marks IS NULL;
 id | name  | marks 
----+-------+-------
  2 | Davis |      
(1 row)


postgres=# SELECT * FROM students_mark WHERE marks = 0;
 id | name | marks 
----+------+-------
  2 | Bob  | 0
(1 row)