I am having trouble wrapping my head around the on statement when doing a self-join. Let's say we have the following table:
employeeid
name
managerid
salary
1
Mike
3
35000
2
Rob
1
45000
3
Todd
NULL
25000
4
Ben
1
55000
5
Sam
1
65000
I want to perform a self join to return the employee name and their manager's name.
When I perform the following self join I get an incorrect result:
SELECT E.name as Employee,M.name as Manager
FROM tblEmployees E
LEFT JOIN tblEmployees M
ON E.Employeeid=M.managerid
However, when I reverse the columns on the on statement using the query below:
SELECT E.name as Employee,M.name as Manager
FROM tblEmployees E
LEFT JOIN tblEmployees M
ON E.managerid=M.Employeeid
I get the correct answer.
Why? How do I know which columns to select in an on statement?
Here's my explanation:
The table you have is structured with each row representing an employee in the company.
You are interested in determining who is each employee's manager.
You are able to find that by joining the table on itself where the lookup values are the manager ids (managerid) and the reference column are the employee ids (employeeid).
The first query is wrong because the employeeid column is being used for the lookup values and the managerid column is being used for reference.
To get the manager of each employee you need to look use the managerid column as the lookup column and the employeeid column as the reference column.
Hope that's not too confusing!
Related
I have an application filter which can generate duplicate SQL queries to the result SQL like:
select * from articles
inner join users on articles.users_id = users.id
inner join users on articles.users_id = users.id
where users.name like %xxx%
The question is if a database is able to handle these duplicates or not. What happened in the database if this query comes inside? If I should remove it from the result SQL or if I can leave it as is.
This is a self join.
A self join is a regular join, but the table is joined with itself.
Example
SELECT
A.Id,
A.FullName,
A.ManagerId,
B.FullName as ManagerName
FROM Employees A
JOIN Employees B
ON A.ManagerId = B.Id
A and B are different table aliases for the same table.
The self join, as its name implies, joins a table to itself. To use a self join, the table must contain a column (call it X) that acts as the primary key and a different column (call it Y) that stores values that can be matched up with the values in Column X. The values of Columns X and Y do not have to be the same for any given row, and the value in Column Y may even be null.
Let’s take a look at an example. Consider the table Employees:
Id
FullName
Salary
ManagerId
1
John Smith
10000
3
2
Jane Anderson
12000
3
3
Tom Lanon
15000
4
4
Anne Connor
20000
5
Jeremy York
9000
1
Each employee has his/her own Id, which is our “Column X.” For a given employee (i.e., row), the column ManagerId contains the Id of his or her manager; this is our “Column Y.” If we trace the employee-manager pairs in this table using these columns:
The manager of the employee John Smith is the employee with Id 3,
i.e., Tom Lanon.
The manager of the employee Jane Anderson is the employee with Id 3,
i.e., Tom Lanon.
The manager of the employee Tom Lanon is the employee with Id 4,
i.e., Anne Connor.
The employee Anne Connor does not have a manager; her ManagerId is
null.
The manager of the employee Jeremy York is the employee with Id 1,
i.e., John Smith.
This type of table structure is very common in hierarchies. Now, to show the name of the manager for each employee in the same row, we can run the following query:
SELECT
employee.Id,
employee.FullName,
employee.ManagerId,
manager.FullName as ManagerName
FROM Employees employee
JOIN Employees manager
ON employee.ManagerId = manager.Id
which returns the following result:
Id
FullName
ManagerId
ManagerName
1
John Smith
3
Tom Lanon
2
Jane Anderson
3
Tom Lanon
3
Tom Lanon
4
Anne Connor
5
Jeremy York
1
John Smith
The query selects the columns Id, FullName, and ManagerId from the table aliased employee. It also selects the FullName column of the table aliased manager and designates this column as ManagerName. As a result, every employee who has a manager is output along with his/her manager’s ID and name.
In this query, the Employees table is joined with itself and has two different roles:
Role 1: It stores the employee data (alias employee).
Role 2: It stores the manager data (alias manager).
By doing so, we are essentially considering the two copies of the Employees table as if they are two distinct tables, one for the employees and another for the managers.
You can find more about the concept of the self join in our article "An illustrated guide to the SQL self join".
I am new to HIVE and HADOOP , here i am trying to join two sample tables in hive where the tables do not have any primary foreign key relationship just for practicing :-
the tables are as follows
Employees table:-
id name gender salary departmentid
1 mark male 3333 1
2 Steve male 5464 3
3 Ben male 3873 2
4 bender male 9298 1
5 fender male 654 2
departments table:-
id name location
1 IT NEW YORK
2 HR LONDON
3 PAYROLL SYDNEY
hive> select employees.name as employee_name, departments.name as department_name
> from employees
> join departments on departments.id = employees.departmentid;
RESULT :-
Query ID =
cloudera_20170911030505_93378edb-f8b8-45d0-9141-3fe065211f3d
Total jobs = 1
FAILED: Execution Error, return code 1 from org.apache.hadoop.hive.ql.exec.mr.MapredLocalTask
since i am new to hive from SQL suggest me how to solve this error ? any help would be appreciated.
set hive.auto.convert.join=false; Fixed the issue in my case..
Query:
select employees.name as employee_name, departments.name as department_name
from employees T1
join departments T2 on T1.departmentid=T2.id;
Hive up to version 0.13 does not support Primary key concepts. This has been introduced in the later releases on hive.
So, we just need to take care is that the columns are present, in case of duplicates in the columns, it will create multiple records.
If you want to have some other use cases, you can try using Left Outer Join, Right Outer join, Full join. Be careful while using cross joins.
If the error still persists, please send more details about the table schema that you have used. You can use show create table db_name.table_name to view your complete schema.
I'm supposed to answer this for class, and it's tricky (for me)
Write a SELECT query to output the name of all employees with the name of their supervisor. If the employee has no supervisor, the supervisor name column should contain the text 'No Supervisor'.
The primary key field in my db is the employeeid and they are provided with names, and each student also has a supervisorid
The table for this is shown below (sorry for the layout):
employeeid lastname firstname salary supervisorid
1 Stolz Ted 25000 NULL
2 Boswell Nancy 23000 1
3 Hargett Vincent 22000 1
4 Weekley Kevin 22000 3
5 Metts Geraldine 22000 2
6 McBride Jeffrey 21000 2
7 Xiong Jay 20000 3
I was wondering how I could go about this statement without using the case statement to apply each of the 7 students with:
when concat(firstname,' ',lastname)='Nancy Boswell' then 'Ted Stolz'
In larger tables this would simply be a HUGE statement, is there a better way to do it?
Thanks!
EDIT:
I've now tried this:
SELECT
EMP1.employeeid as 'employee',
EMP2.supervisorid as 'manager'
FROM
employee EMP1
LEFT OUTER JOIN
employee EMP2
ON
emp1.employeeid = emp2.supervisorid;
However, I am seeing duplicate fields, for some reason employee 2 and 3 are appearing twice, meaning there are 9 fields showing instead of 7.
Also, I need to display their names, not their id's does that mean I need to join the join that i've already done to the employee name ? How would I do this?
Thanks for the feedback guys!
You need to link the table with itself based on the supervisorId. This might be strange if you are new to SQL but it is very common to do. You tell with SQL to add the row of the supervisor to the row of the employee via its primary key.
SELECT
*
FROM
EMPLOYEES EMP1
LEFT OUTER JOIN
EMPLOYEES EMP2
ON
-- make link between tables here
Note that the above query is not 100% correct / complete, its an indication. The LEFT OUTER JOIN statement makes the employees without supervisor have null values for the supervisor, otherwise the whole record would be left out.
I have a table in SQL Server 2008 that for explanation purposes contains, ID, Employee and ManagerID.
eg:
ID Employee ManagerID
1 A NULL
2 B 2
3 C 2
I want to write a query that returns all non related ManagerID's and ID's where ManagerID is equal to the ID.
The result should look like this,
ID Employee ManagerID
1 A NULL
2 B 2
In essence no managers can be managers of managers.
At first I thought that it would be simple using a SELF Join and an EXCLUDE SQL statement however I cannot get this to work. I would prefer not to USE the EXCLUDE statement as my actual table has more columns and related data that I would like to return.
If you could help, I would be grateful.
select employee, managerid
from your_table
where managerid is null
or managerid = id
I have an Access database that has two tables that are related by PK/FK. Unfortunately, the database tables have allowed for duplicate/redundant records and has made the database a bit screwy. I am trying to figure out a SQL statement that will fix the problem.
To better explain the problem and goal, I have created example tables to use as reference:
alt text http://img38.imageshack.us/img38/9243/514201074110am.png
You'll notice there are two tables, a Student table and a TestScore table where StudentID is the PK/FK.
The Student table contains duplicate records for students John, Sally, Tommy, and Suzy. In other words the John's with StudentID's 1 and 5 are the same person, Sally 2 and 6 are the same person, and so on.
The TestScore table relates test scores with a student.
Ignoring how/why the Student table allowed duplicates, etc - The goal I'm trying to accomplish is to update the TestScore table so that it replaces the StudentID's that have been disabled with the corresponding enabled StudentID. So, all StudentID's = 1 (John) will be updated to 5; all StudentID's = 2 (Sally) will be updated to 6, and so on. Here's the resultant TestScore table that I'm shooting for (Notice there is no longer any reference to the disabled StudentID's 1-4):
alt text http://img163.imageshack.us/img163/1954/514201091121am.png
Can you think of a query (compatible with MS Access's JET Engine) that can accomplish this goal? Or, maybe, you can offer some tips/perspectives that will point me in the right direction.
Thanks.
The only way to do this is through a series of queries and temporary tables.
First, I would create the following Make Table query that you would use to create a mapping of the bad StudentID to correct StudentID.
Select S1.StudentId As NewStudentId, S2.StudentId As OldStudentId
Into zzStudentMap
From Student As S1
Inner Join Student As S2
On S2.Name = S1.Name
Where S1.Disabled = False
And S2.StudentId <> S1.StudentId
And S2.Disabled = True
Next, you would use that temporary table to update the TestScore table with the correct StudentID.
Update TestScore
Inner Join zzStudentMap
On zzStudentMap.OldStudentId = TestScore.StudentId
Set StudentId = zzStudentMap.NewStudentId
The most common technique to identify duplicates in a table is to group by the fields that represent duplicate records:
ID FIRST_NAME LAST_NAME
1 Brian Smith
3 George Smith
25 Brian Smith
In this case we want to remove one of the Brian Smith Records, or in your case, update the ID field so they both have the value of 25 or 1 (completely arbitrary which one to use).
SELECT min(id)
FROM example
GROUP BY first_name, last_name
Using min on ID will return:
ID FIRST_NAME LAST_NAME
1 Brian Smith
3 George Smith
If you use max you would get
ID FIRST_NAME LAST_NAME
25 Brian Smith
3 George Smith
I usually use this technique to delete the duplicates, not update them:
DELETE FROM example
WHERE ID NOT IN (SELECT MAX (ID)
FROM example
GROUP BY first_name, last_name)