SQL ALL 操作员什么都不做。 它是干什么用的？

Question

I don't understand the use of ALL operator in SQL Server. In the following example, ALL doesn't seem to do much since when I remove it, I get the same results. Do I really have to use it?

 SELECT *
    FROM EMPLOYEE
    WHERE Salary > ALL (
                    SELECT AVG(Salary) 
                    FROM EMPLOYEE); 

So my question is, what does ALL do, when should I use it and is it necessary? Thanks in advance.

Answer 1

From W3Schools :

The ALL operator:

• returns a boolean value as a result
• returns TRUE if ALL of the subquery values meet the condition
• is used with SELECT, WHERE and HAVING statements

ALL means that the condition will be true only if the operation is true for all values in the range.

In your query, SEELCT AVG(Salary) FROM Employee returns a single result, so you're comparing each individual employee's salary to "all" of the averages (for which you have one average). This is why your query doesn't need the ALL .

If you had a query like this:

SELECT *
FROM EMPLOYEE
WHERE Salary > ALL (SELECT SomeValue FROM SomeTable); 

And the SomeValue values of SomeTable were:

5000
2000
800

Then you would only get results from Employee where Salary > 5000 AND Salary > 2000 AND Salary > 800 (effectively Salary > 5000).

Borrowing an example from sqltutorial.org , you might want to find all employees whose salaries are higher than the salary of employees working in a specific department. Obviously the employees that this query finds will not be in that department:

SELECT 
    first_name, last_name, salary
FROM
    employees
WHERE
    salary > ALL (SELECT 
            salary
        FROM
            employees
        WHERE
            department_id = 2)
ORDER BY salary;