SQL Self Join with null values - mysql

Question

I am having all employees(manager and employees) under one table called Employee. Table looks as follows,
Table
+-------+------------+---------+---------+------------+
|emp_id | name | dept_id | salary | manager_id |
+=======+============+=========+=========+============+
| 1 | Sally | 1 | 20000 | null |
| 2 | Ajit | 2 | 20000 | 1 |
| 3 | Rahul | 1 | 20000 | 1 |
| 4 | uday | 1 | 20000 | null |
| 5 | john | 1 | 20000 | null |
| 6 | netaji | 2 | 20000 | 2 |
| 7 | prakriti | 3 | 1111 | 3 |
| 8 | sachin | 3 | 1111 | 3 |
| 9 | santosh | 1 | 1111 | 2 |
| 10 | Ravi | 1 | 1111 | 2 |
+-------+------------+---------+---------+------------+
Both managers and employees belong to same table. manager_id refers = emp_id who is manager.
I want to write query to count number of employees belonging to each manager. So even if certain manager doesn't have any employee under her or him the count will show as 0
Result should be as follows,
Expected Output
+------+----------+
|Count | Manager |
+======+==========+
| 2 | Sally |
| 3 | Ajit |
| 2 | Rahul |
| 0 | Uday |
| 0 | John |
+------+----------+

Answer 1

You need to do left self-join on the table. The left join will ensure that there is a row for every manager even if there are no employees under them. You need to use the COUNT() aggregate on a field from the employee side of the join that will be NULL if the manager has no employees. COUNT() doesn't actually count NULLs so this should give you zeroes where you want them.
The WHERE clause in this query defines managers by looking if their manager_id is NULL or if there are any matches in the joined table which means there are people that have them set as their manager.
SELECT mgr.name, COUNT(emp.emp_id) AS employee_count
FROM Employee AS mgr
LEFT JOIN Employee AS emp ON emp.manager_id=mgr.emp_id
WHERE mgr.manager_id IS NULL OR emp.emp_id IS NOT NULL
GROUP BY mgr.name

Answer 2

The correct solution likely involves fixing the scheme as any approach will fail for a "sub-manager" (who is managed and thus has a manager_id) but does not currently manage anybody.
Anyway, if the above limitation is acceptable, then people are managers if either
They have a NULL manager_id (as stated in a comment), or
They currently manage people other employees
Then this query (example sqlfiddle) can be used:
SELECT m.name as Manager, COUNT(e.id) as `Count`
FROM employee m
LEFT JOIN employee e
ON m.id = e.manager_id
GROUP BY m.id, m.name, m.manager_id
HAVING `Count` > 0 OR m.manager_id IS NULL
Notes/explanation:
The LEFT [OUTER] join is important here; otherwise managers who did not manage anybody would not be found. The filtering is then applied via the HAVING clause on the grouped result.
The COUNT is applied to a particular column, instead of *; when done so, NULL values in that column are not counted. In this case that means that employees (m) without a match (e) are not automatically selected by the COUNT condition in the HAVING. (The LEFT JOIN leaves in the left-side records, even when there is no join-match - all the right-side columns are NULL in this case.)
The GROUP BY contains all the grouping fields, even if they appear redundant. This allows the manager_id field to be used in the HAVING, for instance. (The group on ID was done in case two managers ever have the same name, or it is to be selected in the output clause.)

Answer 3

here is the solution; you are to make self join on employee table.
SELECT e1.manager_id, e2.name, COUNT (1) AS COUNT
FROM Employee e1 JOIN Employee e2 ON e1.manager_id = e2.id
GROUP BY e1.manager_id, e2.name
UNION ALL
SELECT e3.id, e3.name, 0 AS COUNT
FROM Employee e3
WHERE manager_id IS NULL
AND e3.id NOT IN ( SELECT e1.manager_id
FROM Employee e1
JOIN
Employee e2
ON e1.manager_id = e2.id
GROUP BY e1.manager_id, e2.name)

Answer 4

Maybe that helps:
select t1.name, count(*) -- all managers with emps
from t t1
join t t2
on t1.emp_id = t2.manager_id
group
by t1.name
union
all
select t1.name, 0 -- all managers without emps
from t t1
left
join t t2
on t1.emp_id = t2.manager_id
where t1.manager_id is null
and t2.emp_id is null

Answer 5

try below:
select (select count(*) from employees b where b.manager_id = a.emp_id)) as Count, a.Name as manager from employees a where a.emp_id in (select distict c.manager_id from employees c)

Answer 6

Query
CREATE TABLE employee(emp_id varchar(5) NOT NULL,
emp_name varchar(20) NULL,
dt_of_join date NULL,
emp_supv varchar(5) NULL,
CONSTRAINT emp_id PRIMARY KEY(emp_id) ,
CONSTRAINT emp_supv FOREIGN KEY(emp_supv)
REFERENCESemployee(emp_id));

Answer 7

you need to do
LEFT OUTER JOIN like this:
SELECT movies.title,sequelies.title AS sequel_title
FROM movies
LEFT OUTER JOIN movies sequelies
ON movies.sequel_id = sequelies.id ;