Null 處理

SQL has an unconventional way of handling NULL values, since it treats them as unknown values. As a result, in SQL:

• NULL is not a value indicating a missing entry, but a placeholder for anything possible,
• NULL = NULL evaluates to NULL, since one cannot know if one unknown is equal to another unknown,
• NULL <> NULL evaluates to NULL, using same logic,
• to check if a value is NULL, SQL introduces IS NULL and IS NOT NULL operators,
• DISTINCT column may return multiple NULL values.

For more information, check out the Postgres documentation.

PRQL, on the other hand, treats null as a value, which means that:

• null == null evaluates to true,
• null != null evaluates to false,
• distinct column cannot contain multiple null values.

from employees
filter first_name == null
filter null != last_name

SELECT
  *
FROM
  employees
WHERE
  first_name IS NULL
  AND last_name IS NOT NULL

Note that PRQL doesn’t change how NULL is compared between columns, for example in joins. (PRQL compiles to SQL and so can’t change the behavior of the database).

For more context or to provide feedback check out the discussion on issue #99.