问题
This is my table tblEmployee ,when I'm trying to use a self join query it's result is varying while shifting columns, can someone please explain me the difference between those two queries.Thanks
EmployeeID Name ManagerID
1 Mike 3
2 Rob 1
3 Todd NULL
5 Ben 1
6 Sam 1
Query_1
SELECT E.Name AS Employee, M.Name AS Manager
from tblEmployee E
LEFT JOIN tblEmployee M
on E.ManagerID = M.EmployeeID
OUTPUT
Employee Manager
Mike Todd
Rob Mike
Todd NULL
Ben Mike
Sam Mike
QUERY_2
SELECT E.Name AS Employee, M.Name AS Manager
from tblEmployee E
LEFT JOIN tblEmployee M
on E.EmployeeID = M.ManagerID
OUTPUT
Employee Manager
Mike Rob
Mike Ben
Mike Sam
Rob NULL
Todd Mike
Ben NULL
Sam NULL
回答1:
First a simple explanation: the left join takes every row in the first table that has at least a match in the second table on the column you selected.
In the first query you are considering the Manager ID on every row and looking for a match on the Employee ID in the second table: since value in the second table (Employee ID) is a key (at least in the example definition), you can find only one row that matches that ID.
Eg: First row Mike has Manager ID = 3, when you are looking at the table you have only one row that has Employee ID = 3, and that is Todd
In the second query you are considering the Employee ID on every row and looking for a match on the Manager ID in the second table: since there could be more than one row with the same Manager ID, you will receive more value for every row.
Eg: First row Mike has Employee ID = 1, when you are looking at the table you have three rows with anager ID = 1 and those are Rob, Ben and Sam.
回答2:
With Query1 you are selecting each employee (E) and their manager (M) if there is one (left join).
With Query2 you reversed that. You are selecting each employee (E) and their subordinates (M). The alias for M.Name is incorrect now, because it's not the manager but a subordinate. As some employees have more than one subordinate, they are listed once for each of them (Mike).
回答3:
Basically the second query is little out of order in the sense that since you have swapped the join criteria columns, you get
all combination of manager name followed by the employee name they are manager of
unlike the first query which gave you
all combination of employee names followed by their manager name
So your corrected 2 query should be
SELECT E.Name AS Manager, M.Name AS Employee -- note changes here
from tblEmployee E
LEFT JOIN tblEmployee M
on E.EmployeeID = M.ManagerID
OUTPUT
Manager Employee
Mike Rob
Mike Ben
Mike Sam
Rob NULL
Todd Mike
Ben NULL
Sam NULL
Or maybe you wanted the same result the other way round
then your query should be using RIGHT JOIN instead of LEFT JOIN
SELECT E.Name AS Employee, M.Name AS Manager
from tblEmployee E
RIGHT JOIN tblEmployee M -- note the change in JOIN
on E.EmployeeID = M.ManagerID
OUTPUT
Employee Manager
Todd Mike
Mike Rob
NULL Todd
Mike Ben
Mike Sam
来源：https://stackoverflow.com/questions/36615531/not-quite-understanding-the-query-after-just-shifting-column-names