Easiest way to find IsManager in SQL - sql

Question

Simple structure table of employees
Employee Manager
Joe Smith Jon Smith
Jon Smith Pete Stevens
Pete Stevens NULL
Jared Scho Pete Stevens
....
Im just trying to return some results but I want an indicator on whether the person is a manager or not so the result should be:
Employee Manager IsAManager
Joe Smith Jon Smith 0
Jon Smith Pete Stevens 1
Pete Stevens NULL 1
Jared Scho Pete Stevens 0
The result set is showing that Joe Smith and Jared Scho are not managers...
So If I had a simple SQL Query
SELECT
Employee,
Manager,
As IsAManager --tried to do a case statement here....
FROM
Employee
I tried to do a case statement something to this effect:
SELECT CASE ISNULL(COUNT(*), 0) > 0 THEN 1 ELSE 0 END FROM Employee WHERE Manager = Employee
Not sure how to word it :)

Answer 1

Hopefully this is just a demo example not your real table structure.
SELECT Employee,
Manager,
CASE
WHEN EXISTS(SELECT *
FROM Employee e2
WHERE e2.Manager = e1.Employee) THEN 1
ELSE 0
END As IsAManager
FROM Employee e1
For details of how SQL Server processes EXISTS Subqueries in CASE Expressions see this article.

Answer 2

To determine whether an employee is a manager, you need to match the Employee's ID (in this case, the name) against the list of Manager IDs (in this case the Manager column). If you find a match, the employee is a manager. If you don't find a match, the employee is not a manager.
You can do this with a LEFT OUTER JOIN as shown here:
SELECT DISTINCT
E.Employee,
E.Manager,
CASE WHEN M.Employee IS NULL THEN 0 ELSE 1 END As IsAManager
FROM
Employee E LEFT OUTER JOIN Employee M
ON E.Employee = M.Manager
Please note the following:
You did not specify the SQL product you're using, so I tried to make the solution general. I'm guessing from your attempt to use ISNULL that it's SQL Server, but this solution should work in any product that supports CASE.
Your method of storing manager status has one problem, which is that you cannot represent a manager with no employees (you derive the manager status from the existence of the employee-manager relationship). If you want to be able to store manager status separately from the employee-manager relationship then CREATE TABLE Managers (Employee. . . PRIMARY KEY). This will make the code necessary to get back manager status a little easier to write as well.