my problem is this:
I have a table named
Doctor(id, name, department)
and another table named
department(id, name).
a Doctor is associated with a department (only one department, not more)
I have to do a query returning the department with the maximum number of doctors associated with it.
I am not sure how to proceed, I feel like I need to use a nested query but I just started and I'm really bad at this.
I think it should be something like this, but again I'm not really sure and I can't figure out what to put in the nested query:
SELECT department.id
FROM (SELECT FROM WHERE) , department d, doctor doc
WHERE doc.id = d.id
A common approach to the "Find ABC with the maximum number of XYZs" problem in SQL is as follows:
Query for a list of ABCs that includes each ABC's count of XYZs
Order the list in descending order according to the count of XYZs
Limit the result to a single item; that would be the top item that you need.
In your case, you can do it like this (I am assuming MySQL syntax for taking the top row):
SELECT *
FROM department dp
ORDER BY (SELECT COUNT(*) FROM doctor d WHERE d.department_id=dp.id) DESC
LIMIT 1
You can use Group BY
Select top (1) department.id,count(Doctor.*) as numberofDocs
from department inner join Doctor on Doctor.id = department.id
Group by department.id
Order by count(Doctor.*) desc
I generally avoid using sub queries in MySQL due to a well known bug in MySQL. Due to the bug, MySQL executes the inner query for every single outer query result. Therefore, if you have 10 departments, then doctor query would be executed 10 times. The bug may have been fixed in MySQL 5.6. In this particular case the number of departments may not be large, therefore performance may not be your main concern. However, the following solution should work for MySQL and much more optimized. The answer by dasblinkenlight is almost the same, just got ahead of me :). But MySQL does not support the command top.
select dep.id, dep.name, count(doc.id) as dep_group_count from Doctor doc join department dep on doc.department = dep.id group by doc.department order by dep_group_count DESC LIMIT 1
Related
I have Two Tables, one with Employees Details and another with vacations taken by them in different years.Please check this image for the tables
Here as you'll find out in the vacation table, for the same employee with same employeeId and in sam year different vacation days are mentioned. Like John Smith in 2011 have two entries one with 10 vacation and one with 3 vacation. I want my query to return a single row with vacations mentioned as 13.
I tried the following query but no luck
SELECT Employee_Details.EmployeeId, Employee_Details.EmployeeName, Employees_Vacation.Year, Employees_Vacation.Vacation, Employee_Details.Department
FROM Employees_Vacation INNER JOIN Employee_Details ON Employees_Vacation.EmployeeId=Employee_Details.EmployeeId group by Employee_Details.EmployeeId ORDER BY Employee_Details.EmployeeName, Employees_Vacation.Year ;
if i understood you right, i think this may help you
select sum(vacation) as sum, ev.year, ed.EmployeeName from employee_Details as ed inner join employee_Vacation as ev
on ed.employeeID = ev.employeeID
group by ev.year, ed.EmployeeName
A lot here will depend on the sql engine you are using, however there are some things that will apply regardless of the engine to consider:
Your current GROUP BY clause is grouping only by employeeId - from the question text it seems like you are instead looking for results grouped by employee AND vacation year
Your projection (SELECT statement) currently isn't actually aggregating anything - it's just projecting a bunch of fields. On some db engines, this actually isn't even allowed (SQL Server for example will only allow grouped or aggregated columns in the projection). Again, from the question text it seems like you are looking for the SUM of vacation days per employee and year.
Taking these into account and assuming the assumptions made are accurate, something like the following should work in most/all modern RDBMS's:
SELECT Employee_Details.EmployeeId,
Employee_Details.EmployeeName,
Employees_Vacation.Year,
SUM(Employees_Vacation.Vacation) AS TotalVacationDays,
Employee_Details.Department
FROM Employees_Vacation
INNER JOIN Employee_Details
ON Employees_Vacation.EmployeeId = Employee_Details.EmployeeId
GROUP BY
Employee_Details.EmployeeId, Employee_Details.EmployeeName,
Employees_Vacation.Year, Employee_Details.Department
ORDER BY
Employee_Details.EmployeeName,
Employee_Details.EmployeeId,
Employees_Vacation.Year;
You may be able to get away with fewer grouping clauses in some engines (MySql for example). Additionally I added an EmployeeId to the order by clause to ensure records for the same employee remain together in the results (for employees with the same names for example).
I have the following relations:
teaches(ID,course_id,sec_id,semester,year)
instructor(ID,name,dept_name,salary)
I am trying to express the following as an SQL query:
Find the ID and name of the instructor who has taught the most courses(i.e has the most tuples in teaches).
My Query
select ID, name
from teaches
natural join instructor
group by ID
order by count(*) desc
I know this isn't correct, but I feel like I'm on the right track. In order to answer the question, you need to work with both relations, hence the natural join operation is required. Since the question asks for the instructor that has taught the most courses, that tells me that we are trying to count the number of times each instructor ID appears in the teaches relation. From what I understand, we are looking to count distinct instructor IDs, hence the group by command is needed.
Don't use natural joins: all they do is rely on column names to decide which columns relate across tables (they don't check for foreign keys constraints or the-like, as you would thought). This is unreliable by nature.
You can use a regular inner join:
select i.id, i.name
from teaches t
inner join instructor i on i.id = t.sec_id
group by i.id, i.name
order by count(*) desc
limit 1
Notes:
this assumes that column teaches.sec_id relates to instructor.id (I cannot see which other column could be used)
I added a limit clause to the query since you stated that you want the top instructor - the syntax may vary across databases
always prefix the column names with the table they belong to, to make the query unambiguous and easier to understand
it is a good practice (and a requirement in many databases) that in an aggregate query all non-aggregared columns listed in the select clause should appear in the group by clause; I added the instructur name to your group by clause
Before start, Sorry some of results and datas are written in Korean.
Here is a code that I currently am looking on.
SELECT S.*, D.DNAME
FROM STUDENT S, DEPARTMENT D
WHERE (SUBSTR(S.JUMIN,7,1),S.WEIGHT) IN (SELECT SUBSTR(JUMIN,7,1),MAX(WEIGHT)
FROM STUDENT GROUP BY SUBSTR(JUMIN,7,1))
AND S.DEPTNO1 = D.DEPTNO;
And here is the DEPARTMENT data.
And this is the STUDENT data.
I got a result as I want. But I have some questions when I change this part of the code
WHERE (SUBSTR(S.JUMIN,7,1),S.WEIGHT) IN (SELECT SUBSTR(JUMIN,7,1),MAX(WEIGHT)
into this one
WHERE (SUBSTR(S.JUMIN,7,1),S.WEIGHT) IN (SELECT
S.SUBSTR(JUMIN,7,1),MAX(S.WEIGHT)
What I do is simply put S infront of JUMIN and WEIGHT in line3.
But when I do this it shows me the whole data.
I thought JOIN ALIAS (which are S,D in this code) is used as way show that I have two tables to use that is labeled with S and D. S means this data is in STUDENT and D is in DEPARTMENT.
But I think I get it in a wrong way.
Anyway I have no idea how this result is come out.
This one is referring to your main table not the subquery table.
WHERE (SUBSTR(S.JUMIN,7,1),S.WEIGHT) IN (SELECT
S.SUBSTR(JUMIN,7,1),MAX(S.WEIGHT)
Your original query is already correct. In which you already have a new result set based on your aggregation.
WHERE (SUBSTR(S.JUMIN,7,1),S.WEIGHT) IN (SELECT SUBSTR(JUMIN,7,1),MAX(WEIGHT)
I have two tables in a Database
and
I need to retrieve the number of staff per manager in the following format
I've been trying to adapt an answer to another question
SELECT bankNo AS "Bank Number",
COUNT (*) AS "Total Branches"
FROM BankBranch
GROUP BY bankNo
As
SELECT COUNT (*) AS StaffCount ,
Employee.Name AS Name
FROM Employee, Stafflink
GROUP BY Name
As I look at the Group BY I'm thinking I should be grouping by The ManID in the Stafflink Table.
My output with this query looks like this
So it is counting correctly but as you can see it's far off the output I need to get.
Any advice would be appreciated.
You need to join the Employee and Stafflink tables. It appears that your FROM clause should look like this:
FROM Employee INNER JOIN StaffLink ON Employee.ID = StaffLink.ManID
You have to join the Eployee table twice to get the summary of employees under manager
select count(*) as StaffCount,Manager.Name
from Employee join Stafflink on employee.Id = StaffLink.EmpId
join Employee as Manager on StaffLink.ManId = Manager.Id
Group by Manager.Name
The answers that advise you on how to join are correct, assuming that you want to learn how to use SQL in MS Access. But there is a way to accomplish the same thing using the ACCESS GUI for designing queries, and this involves a shorter learning curve than learning SQL.
The key to using the GUI when more than one table is involved is to realize that you have to define the relationships between tables in the relationship manager. Once you do that, designing the query you are after is a piece of cake, just point and click.
The tricky thing in your case is that there are two relationships between the two tables. One relationship links EmpId to ID and the other links ManId to ID.
If, however, you want to learn SQL, then this shortcut will be a digression.
If you don't specify a join between the tables, a so called Cartesian product will be built, i.e., each record from one table will be paired with every record from the other table. If you have 7 records in one table and 10 in the other you will get 70 pairs (i.e. rows) before grouping. This explains why you are getting a count of 7 per manager name.
Besides joining the tables, I would suggest you to group on the manager id instead of the manager name. The manager id is known to be unique per manager, but not the name. This then requires you to either group on the name in addition, because the name is in the select list or to apply an aggregate function on the name. Each additional grouping slows down the query; therefore I prefer the aggregate function.
SELECT
COUNT(*) AS StaffCount,
FIRST(Manager.Name) AS ManagerName
FROM
Stafflink
INNER JOIN Employee AS Manager
ON StaffLink.ManId = Manager.Id
GROUP BY
StaffLink.ManId
I don't know if it makes a performance difference, but I prefer to group on StaffLink.ManId than on Employee.Id, since StaffLink is the main table here and Employee is just used as lookup table in this query.
My task is to find the number of occurrences of late timesheet submissions for each employee in our database. There are two tables which I have primarily been looking at, but I'm having trouble putting the two together and coming up with a decent view of the COUNT of occurrences and the employee ID for which they are associated with.
I have created this query which provides me with the EmployeeID for each occurrence.
SELECT db.Employee.EmployeeID
FROM db.LateTimesheets
INNER JOIN db.Employee ON Employee.LastName = LateTimesheets.LastName AND Employee.FirstName = Late Timesheets.FirstName
Now, with this simple query I have a view of the EmployeeID repeated however many times these incidents have occured. However, what I ultimately want to end up with is a table that displays a count for each occurance, along with the EmployeeID for which this count is associated with.
I would assume I would need to use the COUNT() function to count the amount of rows for each EmployeeID, and then select that value along with EmployeeID. However, I am having trouble structuring the subquery correctly, and everything I have tried thus far has only generated errors with MS SQL Server Management Studio.
A simpler version of usr's answer would be the following which avoids the construction of the derived table:
Select db.Employee.EmployeeID, Count( db.LateTimesheets.somecolumn ) As Total
From db.Employee
Left Join db.LateTimesheets
On LateTimesheets.LastName = Employee.LastName
And Late Timesheets.FirstName = Employee.FirstName
Group By db.Employee.EmployeeID
I may have misunderstood the question, but wouldn't GROUP BY solve your problem?
SELECT COUNT(db.LateTimesheets.somecolumn), db.Employee.EmployeeID
FROM db.LateTimesheets
INNER JOIN db.Employee ON Employee.LastName = LateTimesheets.LastName
AND Employee.FirstName = Late Timesheets.FirstName
GROUP BY db.Employee.EmployeeID
Just replace somecolumn with the name of a column that's actually in the table.
select e.*, isnull(lt.Count, 0) as Count
from Employee e
left join (
select LastName, count(*) as Count from LateTimesheets
) LateTimesheets lt on e.LastName = lt.LastName
The trick is to do the grouping in a derived table. You don't want to group everything, just the LateTimesheets.
We need a left join to still get employees with no LateTimesheets.