I want to ask how can I join the table with itself, using the same column twice, and giving each a different WHERE to sort out. I have an Employees table that contain Employee_name, job_id and manager_id. I want it to get the name of all the managers through the job_id on one column, and then all the employees that they manage on the other column. This is what I came up with :
SELECT A.LAST_NAME AS MANAGER_NAME, B.LAST_NAME AS EMPLOYEES_NAME
FROM EMPLOYEES A
RIGHT JOIN EMPLOYEES B
ON A.EMPLOYEE_ID = B.EMPLOYEE_ID
WHERE A.JOB_ID LIKE '%MAN%'
AND A.MANAGER_ID = B.MANAGER_ID
This simply put the manager name on both columns, and I don't have another idea on how to do this. Any help would be greatly appreciated, thank you so much.
You join condition needs to change, the WHERE criteria should be moved to the ON clause, and also I would use a left join here:
SELECT emp1.LAST_NAME AS EMPLOYEES_NAME, COALESCE(emp2.LAST_NAME, 'NA') AS MANAGER_NAME
FROM EMPLOYEES emp1
LEFT JOIN EMPLOYEES emp2
ON emp2.EMPLOYEE_ID = emp1.MANAGER_ID AND emp2.JOB_ID LIKE '%MAN%';
It is unclear whether you even need the check on the JOB_ID, but in any case I use COALESCE above to display NA as the manager name for any employee not having a manager.
Related
I try to display the number of reviews which an employee provided and the number of reviews which he received. I try to do something like this, but in my output table (3) I receive incorrect data. I believe that this happens due to the wrong reference:
pr.reviewer_id = e.employee_id
however, if I reference it like this:
pr.employee_id = e.employee_id
then nr_of_reviews_receieved is correct and nr_of_reviews_posted is not, and if I change it back it's vice versa. So, I need in one case to use one reference and in another a different one, but all in one query.
SELECT
e.employee_id,
CONCAT_WS(' ',employee_first_name, employee_last_name) AS full_name,
COUNT(pr.reviewer_id) AS nr_of_reviews_posted,
COUNT(pr.employee_id) AS nr_of_reviews_received
FROM
employee AS e
LEFT JOIN
performance_review AS pr ON pr.reviewer_id = e.employee_id
GROUP BY
employee_first_name, employee_last_name, e.employee_id
You have to sample the tables twice and join to the table each time
for example:
SELECT
e.employee_id,
CONCAT_WS(' ',employee_first_name, employee_last_name) AS full_name,
COUNT(pr.reviewer_id) AS nr_of_reviews_posted,
COUNT(pr2.employee_id) AS nr_of_reviews_received
FROM
employee AS e
LEFT JOIN
performance_review AS pr ON pr.reviewer_id = e.employee_id
LEFT JOIN
performance_review AS pr2 ON pr2.employee_id = e.employee_id
GROUP BY
Necessary columns ...
I have this task:
Count the number of employees for every country. Show only those countries, when works more than 20 employees
employee_id is dedicated for Employees table
country belongs to different table - Countries table and we need country_name from this table
I have no idea how to solve this task. Below what I was able to create. I think we should use Inner Join.
SELECT a.employee_id
, b.country_name
, COUNT(a.employee_id) AS count
FROM employees a
INNER JOIN countries b ON a.employee_id = b.country_name
GROUP BY b.country_name
WHERE employee_id >20;
I think I need help from the beginning.
Thanks
Your join doesn't seem correct but as I don't know the table structure, I can't say what the right column is (I'm going to assume that it should be country_name. Even so, try this:
SELECT b.country_name
, COUNT(a.employee_id) AS count
FROM employees a
INNER JOIN countries b ON a.country_name = b.country_name
GROUP BY b.country_name
HAVING COUNT(employee_id) >20;
When grouping you need to use the HAVING statement to filter.
I have two tables:
Employees (columns: ID, Name)
and
employee partners (EmployeeID1, EmployeeID2, Time)
I want to output EmployeName1, EmployeeName2, Time instead of imployee ids.
(In other words, replace the ids with names, but in two columns at a time)
How would I do this? Would JOIN be the appropriate command?
you need to join the employee table 2 times as the employee partners table acts as many to many connection.
The select should be:
SELECT emp1.name, emp2.name, em.time
FROM Employees emp1
JOIN employee_partners em ON emp1.id = EmployeeID1
JOIN Employees emp2 on emp2.id = EmployeeID2
Often in these situations, you want to use LEFT JOIN:
SELECT e1.name as name1, e2.name as name2, em.time
FROM employee_partners ep LEFT JOIN
Employees e1
ON e1.id = ep.EmployeeID1 LEFT JOIN
Employees e2
ON e2.id = ep.EmployeeID2;
Notes:
The LEFT JOINs ensure that you do not lose rows if either of the employee columns is NULL.
Use tables aliases; they make the query easier to write and to read.
Qualify all columns names; that is, include the table name so you know where the column is coming from.
I also added column aliases so you can distinguish between the names.
Table DEPARTMEnT
TABLE EMPLOYEE
There is the Operations Department which has not any employee. So, i believed that the query would retrieved also the row(image 1):
Department_ID=10 , Department_Name =Operations, Employee=0
Why doesnt happen???
SELECT EMPLOYEE.Department_ID, DEPARTMENT.Department_Name, Count(*) AS Employees
FROM EMPLOYEE right JOIN DEPARTMENT ON DEPARTMENT.Department_ID = EMPLOYEE.Department_ID
GROUP BY DEPARTMENT.Department_Name,.EMPLOYEE.Department_ID
Since the principal data you care about for this query is coming from the DEPARTMENT table, you may want to consider rewriting your query to be:
SELECT DEPARTMENT.Department_ID, DEPARTMENT.Department_Name, Count(EMPLOYEE.Employee_ID) As Employees
FROM DEPARTMENT
LEFT JOIN EMPLOYEE ON EMPLOYEE.Department_ID = DEPARTMENT.Department_ID
GROUP BY DEPARTMENT.Department_ID, DEPARTMENT.Department_Name
The default join is an inner join, which only returns rows for which at least one row is found on both sides. Replace join with left join to retrieve departments without employees.
Example code:
SELECT e.Department_ID
, d.Department_Name
, count(e.Employee_ID) AS Employees
FROM Department d
LEFT JOIN
Employee e
ON d.Department_ID = e.Department_ID
GROUP BY
d.Department_ID
, d.Department_Name
This should do the trick. You could put in a RIGHT JOIN if you have the EMPLOYEE table first, but the reason this is not good is because soon your queries will start being a mix of LEFT and RIGHT joins, which becomes very hard to read, even for seasoned SQL professionals. By sticking with LEFT JOIN you keep the query maintainable and understandable. (In very rare circumstances RIGHT JOIN may simplify a query that has a complex order of precedence but I have only done it something like twice to avoid having to add parentheses around groups of joins).
SELECT
D.Department_ID,
D.Department_Name,
Employees = Count(*)
FROM
dbo.DEPARTMENT D
LEFT JOIN dbo.EMPLOYEE E
ON D.Department_ID = E.Department_ID
GROUP BY
D.Department_ID,
D.Department_Name
Also, I recommend that you use aliases for your tables instead of full table names. The query becomes much easier to scan and understand when there is consistent use of aliases. Spelling out the entire table name all too often obscures other parts of the query.
This may seems the most dumbest question ever on stackoverflow but I am just wondering why would you write such a query:
Select e1.Emploee_ID, e1.Departement_ID From Employee e
Inner join Employee E1 on e.employee_id= e1.Employee_ID
Inner join Departement d on d.dep_id= e1.departement_id
Why do we have to join on employee? my obvious query would be
select e.employee_ID, e.Departement_id from employee e
inner join Departement d on d.dep_id= e1.departement_id
Referencing the PK with an inner join is redundant.
You would normally join on the same table to link with another record, for example if you have a FK-column that reference the boss of an employee.
Assuming you would have a nullable foreign-key column Boss_ID in table Employee
Select e.Employee_ID, boss.Employee_id, d.Departement_ID
From Employee e
LEFT OUTER JOIN Employee boss on boss.Employee_ID=e.Boss_ID
INNER JOIN Departement d on d.dep_id= e.departement_id
Note that i've used a LEFT OUTER JOIN to get also the employee that have no bosses.
What I can see. You do not need this join.
Inner join Employee E1 on e.employee_id= e1.Employee_ID
The two queries will give the same result. I can not see the point of JOINing twice on the Employee table.
I can't see any reason to join Employee to Employee in this query. In the past I've occasionally used two subsets of the same table in the same query, but there's nothing like that going on here. To me it looks like this was done by mistake.
If employee_id is a PK then it doesn't make sense, but if it is not the two queries will return different results.
The first query will not return NULL employee_id and will return N^2 results for multiple entries with N occurrences.
Even simpler would be this:
select e.employee_ID, d.dep_name from employee e,Departement d where d.dep_id= e.departement_id