sql - Creating Queries in SQL

Question

I'm trying to create a query that will display the name of the staff who are not mentoring any other staff. It should also be ordered by the surname.

So far, I've got this:

SELECT a.name, m.mentor
FROM accountant AS a 
LEFT OUTER JOIN accountant AS m ON a.mentor = m.staff_id
WHERE m.mentor = NULL
ORDER BY m.surname;

When I run the query it doesn't return any results.

Any help would be nice.

Answer 1

尝试使用 IS Null Not = Null

SELECT a.name, m.mentor
FROM accountant AS a LEFT OUTER JOIN accountant AS m
ON a.mentor = m.staff_id
WHERE m.mentor is NULL ///  here
ORDER BY m.surname;

Answer 2

SELECT a.name, m.mentor
FROM accountant AS a LEFT OUTER JOIN accountant AS m
ON a.mentor = m.staff_id
WHERE m.mentor IS NULL
ORDER BY m.surname;

您需要使用IS NULL，您无法检查该值是否equals(=)为NULL