Postgresql 简明教程

PostgreSQL - NULL Values

PostgreSQL NULL 是用于表示缺失值的一项术语。表中的 NULL 值是字段中看起来为空的值。

The PostgreSQL NULL is the term used to represent a missing value. A NULL value in a table is a value in a field that appears to be blank.

带 NULL 值的字段是没有任何值的字段。理解到 NULL 值与零值或包含空格的字段不同，这一点非常重要。

A field with a NULL value is a field with no value. It is very important to understand that a NULL value is different from a zero value or a field that contains spaces.

Syntax

在创建表时使用 NULL 的基本语法如下 −

The basic syntax of using NULL while creating a table is as follows −

CREATE TABLE COMPANY(
   ID INT PRIMARY KEY     NOT NULL,
   NAME           TEXT    NOT NULL,
   AGE            INT     NOT NULL,
   ADDRESS        CHAR(50),
   SALARY         REAL
);

NOT NULL 在此表示该列应始终接受给定数据类型的显式值。有两列我们没有使用 NOT NULL。因此，这意味着这些列可能为 NULL。

Here, NOT NULL signifies that column should always accept an explicit value of the given data type. There are two columns where we did not use NOT NULL. Hence, this means these columns could be NULL.

具有 NULL 值的字段在创建记录时留空。

A field with a NULL value is one that has been left blank during record creation.

Example

当选择数据时，NULL 值会导致问题，因为在将未知值与其他任何值进行比较时，结果始终未知且不会包含在最终结果中。考虑具有以下记录的表 COMPANY -

The NULL value can cause problems when selecting data, because when comparing an unknown value to any other value, the result is always unknown and not included in the final results. Consider the following table, COMPANY having the following records −

ID          NAME        AGE         ADDRESS     SALARY
----------  ----------  ----------  ----------  ----------
1           Paul        32          California  20000.0
2           Allen       25          Texas       15000.0
3           Teddy       23          Norway      20000.0
4           Mark        25          Rich-Mond   65000.0
5           David       27          Texas       85000.0
6           Kim         22          South-Hall  45000.0
7           James       24          Houston     10000.0

让我们使用 UPDATE 语句将一些可为空值设置为 NULL，如下所示 -

Let us use the UPDATE statement to set few nullable values as NULL as follows −

testdb=# UPDATE COMPANY SET ADDRESS = NULL, SALARY = NULL where ID IN(6,7);

现在，COMPANY 表应具有以下记录 -

Now, COMPANY table should have the following records −

 id | name  | age | address     | salary
----+-------+-----+-------------+--------
  1 | Paul  |  32 | California  |  20000
  2 | Allen |  25 | Texas       |  15000
  3 | Teddy |  23 | Norway      |  20000
  4 | Mark  |  25 | Rich-Mond   |  65000
  5 | David |  27 | Texas       |  85000
  6 | Kim   |  22 |             |
  7 | James |  24 |             |
(7 rows)

接下来，让我们看看 IS NOT NULL 运算符的使用，以列出 SALARY 不为 NULL 的所有记录 -

Next, let us see the usage of IS NOT NULL operator to list down all the records where SALARY is not NULL −

testdb=#  SELECT  ID, NAME, AGE, ADDRESS, SALARY
   FROM COMPANY
   WHERE SALARY IS NOT NULL;

以上给出的 PostgreSQL 语句将产生以下结果 -

The above given PostgreSQL statement will produce the following result −

 id | name  | age | address    | salary
----+-------+-----+------------+--------
  1 | Paul  |  32 | California |  20000
  2 | Allen |  25 | Texas      |  15000
  3 | Teddy |  23 | Norway     |  20000
  4 | Mark  |  25 | Rich-Mond  |  65000
  5 | David |  27 | Texas      |  85000
(5 rows)

以下是 IS NULL 运算符的使用，它将列出 SALARY 为 NULL 的所有记录 -

The following is the usage of IS NULL operator which will list down all the records where SALARY is NULL −

testdb=#  SELECT  ID, NAME, AGE, ADDRESS, SALARY
        FROM COMPANY
        WHERE SALARY IS NULL;

以上给出的 PostgreSQL 语句将产生以下结果 -

The above given PostgreSQL statement will produce the following result −

 id | name  | age | address | salary
----+-------+-----+---------+--------
  6 | Kim   |  22 |         |
  7 | James |  24 |         |
(2 rows)