NULL值

NULL值代表未知数据。无法比较NULL和0，因为它们是不等价的。

创建表时，可以指定列可以存放或者不能存放NULL值，详情请参见NOT NULL约束。本节介绍IS NULL和IS NOT NULL操作符。

创建表customer_t1，数据如下：

  1. openGauss=# SELECT * FROM customer_t1;
  2.  c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
  3. ---------------+---------------+--------------+-------------+--------
  4.           3869 | hello         | Grace        |             |   1000
  5.           3869 |               | Grace        |             |
  6.           3869 | hello         |              |             |
  7.           6985 | maps          | Joes         |             |   2200
  8.           9976 | world         | James        |             |   5000
  9.           4421 | Admin         | Local        |             |   3000
  10. (6 rows)

IS NOT NULL

在WHERE子句中查找为NULL值的字段。

例如，列出表customer_t1中c_customer_id不为空值的字段。

  1. openGauss=# SELECT * FROM customer_t1 WHERE c_customer_id IS NOT NULL;
  2.  c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
  3. ---------------+---------------+--------------+-------------+--------
  4.           3869 | hello         | Grace        |             |   1000
  5.           3869 | hello         |              |             |
  6.           6985 | maps          | Joes         |             |   2200
  7.           9976 | world         | James        |             |   5000
  8.           4421 | Admin         | Local        |             |   3000
  9. (5 rows)

IS NULL

在WHERE子句中查找为NULL值的字段。

例如，列出表customer_t1中c_customer_id为空值的字段。

  1. openGauss=# SELECT * FROM customer_t1 WHERE c_customer_id IS NULL;
  2.  c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
  3. ---------------+---------------+--------------+-------------+--------
  4.           3869 |               | Grace        |             |
  5. (1 row)