I'm a beginner trying to learn and practice SQL with tables based on this schema:
EMPLOYEE - ID, Name
ASSIGNMENT - ID, Country, Start, End
The primary keys are Employee.ID and all four columns shown for ASSIGNMENT; and ASSIGNMENT.ID is a reference to EMPLOYEE.ID. The domain of start and end is in years.
Problem: I'm trying to write a query that will display all the employees (by name) where they were assigned to an assignment in the USA directly after they had completed an assignment in Canada.
This is my current attempt, which fails to compute. I believe I am heading in the correct direction but there are syntactical mistakes.
SELECT
E.Name
FROM
EMPLOYEE E
INNER JOIN
ASSIGNMENT A ON E.ID = A.ID
WHERE
(SELECT End FROM ASSIGNMENT
WHERE Country = 'Canada') = (SELECT Start FROM ASSIGNMENT
WHERE COUNTRY = 'USA')
GROUP BY
E.Name;
Any critique to benefit my understanding of my misconceptions are welcome. My errors are coming from the combination of the subqueries in the WHERE clause
At most one record can be returned by this subquery.
Perhaps someone can show me another way to compute this?
This query is being tested in MS Access since I found it easy to build a database and relationships quickly.
Instead of subqueries use another join and add constraints to the join conditions:
SELECT
E.Name
FROM
EMPLOYEE E
INNER JOIN
ASSIGNMENT A ON ( E.ID = A.ID
AND A.Country = 'Canada' )
INNER JOIN
ASSIGNMENT B ON ( E.ID = B.ID
AND B.Country = 'USA'
AND B.Start = A.End )
GROUP BY
E.Name;
Update
The OP reported an error from MS Access complaining about the composite join condition for the above version. However, you may safely move inner join conditions to the where clause. The interim resultsets will grow, though, since the product of the tables is produced first with less constraints and filtered thereafter (A good query optimizer might avoid the unnecessary generation of records, but I do not know about the capabilities of MS Access in this regard).
SELECT
E.Name
FROM
EMPLOYEE E
INNER JOIN
ASSIGNMENT A ON ( E.ID = A.ID )
INNER JOIN
ASSIGNMENT B ON ( E.ID = B.ID )
WHERE
A.Country = 'Canada'
AND B.Country = 'USA'
AND B.Start = A.End
GROUP BY
E.Name;
Related
I am facing an issue in terms of understanding the joins. Lets say for an example we have two tables employee and sales and now I have a query where we have sales of an employee using the id of the employee
select e.employeename
,s.city
,SUM(s.sales)
from employee e
left join (select sales,eid from sales) s on s.eid = e.id
group by 1,2
I'd like to understand why s.city wasn't showing up? and also would like to understand what is this concept called? Is it co related sub queries on Joins? Please help me down over here.
select
e.employeename
,s.city
,SUM(s.sales)
from employee e
left join (select sales,eid,city from sales) s on s.eid = e.id
group by 1,2
in the left join above you have to add city as well. The query Imagine select sales,eid,city from sales is a table itself and then from this table you are selecting city (your second column s.city) this will run error as your table doesn't have a city column yet.
It is much easier to use CTE (common table expressions than CTE's) You can also do the above question as
select
e.employeename
,s.city
,SUM(s.sales)
from employee e
left join sales as s
on e.id = s.id
group by 1,2
here I have added e.id = s.id instead of s.id = e.id it is better to reference the key of the main table first.
you could use CTE (although used when you have to do a lot of referencing but you can see how it works):
With staging as (
select
e.employeename
,s.city
,s.sales
from employee e
left join sales as s
on e.id = s.id
),
sales_stats as (
select
staging.employeename,
staging.city,
sum(staging.sales)
from staging
group by 1,2
#here you will select from staging again consider staging as a separate table so you will have to have all the columns in the staging that you want to use further. Also you will have to reference columns using staging.x
)
select * from sales_stats
-- here you could have combined the steps but I wanted to show you how cte works, Hope this works for you
I have two tables. First one is name of all members and second is all projects and its team members of different roles.
Table 1 : [members] id, name
Table 2 : [projects] id, proj_name, sponsor (fk1_tbl_1), proj_mgr(fk2_tbl_1) , proj_co (fk3_tbl_1)
I created a query to show the project name and names of all project roles.
I am doing three joins with two sub-queries in order to achieve this.
I want to know if there is better ways to do this (in pure sql, NOT script languages like pl/sql).
select f.proj_name, f.proj_sponsor, f.proj_mgr, e.name proj_co
from
name e,
(
select
d.proj_name, d.proj_sponsor, c.name proj_mgr, d.proj_co
from
members c,
(
select
b.proj_name, a.name proj_sponsor, b.proj_mgr mgr, b.proj_co co
from
members a, projects b
where
b.sponsor = a.id
) d
where
c.id = d.mgr
) f
where
e.id = f.proj_co
Use join and join again:
select p.*, ms.name as sponsor, mm.name as manager, mc.name as co_name
from projects p left join
members ms
on p.sponsor = ms.id left join
members mm
on p.manager = mm.id left join
members mo
on p.proj_co = mo.id;
Notes:
This uses left join in case any values are missing. The project will still be returned.
Never use commas in the FROM clause.
Always use proper, explicit, standard JOIN syntax.
Use meaningful table aliases, rather than arbitrary letters.
I have four tables :
Applicant (aid, aname)
entrance_test (Etid, etname)
etest_centre (etcid, location)
etest_details (aid, etid, etcid, etest_dt)
I want to select the number of applicants who have appeared for each test, test center wise.
This is my current query:
select
location, etname, count(Aid) as number of applicants
from
applicant as a
inner join
etest_details as d on a.aid = d.aid
inner join
Entrance_Test as t on t.Etid = d.Etid
inner join
Etest_Centre as c on c.Etcid = d.Etcid
group by
Location, Etname
This is the error I am getting :
Ambiguous column name 'Aid'
You have the column aid in multiple tables, and it doesn't know which to pick from. You should specify which table it is from using the aliases you defined.
In this case, since a.Aid is the same as d.Aid (due to the JOIN), I'm using the a alias, but do keep in mind if location and etname also appear in multiple tables, you need to specify which table it should pick from.
Select c.location, t.etname, Count(a.Aid)
From Applicant As a
Inner Join etest_details As d On a.aid = d.aid
Inner Join Entrance_Test As t On t.Etid = d.Etid
Inner Join Etest_Centre As c On c.Etcid = d.Etcid
Group By c.Location, t.Etname
As a rule of thumb, when you have multiple sources in one query, you should always be explicit about which table it should come from. Even if you're sure it only exists in one of them, it's a good habit to get into to avoid issues like this in the future.
You need to mention the alias in the COUNT clause. Since you are using aliases, it would be better if you use them in the SELECT and GROUP BY sections as well. In this case, it should be :
SELECT a.location,
a.etname,
COUNT(d.Aid)
FROM applicant AS a
INNER JOIN etest_details AS d ON a.aid = d.aid
INNER JOIN Entrance_Test AS t ON t.Etid = d.Etid
INNER JOIN Etest_Centre AS c ON c.Etcid = d.Etcid
GROUP BY a.Location,
a.Etname
I have this schema :
When I want to display the Attend table, instead of DoctorId and PatientId, Employee Name And Patient Name display.
Like this:
id
Patient name
Doctor Name [== Employee name]
Start
End
First way is to use an inner join :
select
a.Id, p.Name, a.Name, a.Start, a.End
from
Patient as p
inner join
(select
e.Name, at.Id, at.Start, at.End, at.PatientId
from
Attend as at
INNER JOIN
Employee as e on at.DoctorId = e.Id) as a on p.Id = a.PatientId
Second way is to use a function or stored procedure - send in the id and get back name
select
a.Id,
FindDoctor(a.DoctorId) as Doctor,
FindPatient(a.PatientId) as Patient,
a.Start, a.EndTime
from
Attend AS a
Which is the better? Which is the optimized approach?
In general, SQL engines do a better job of optimizing queries than of optimizing function calls. I would suggest that you stick with the query, at least at first. This also makes it easier to understand performance issues that might arise.
By the way, there is no need for a subquery for what you want to do. SQL Server is pretty smart, so it probably doesn't affect performance. However, you can write the query as:
select at.Id , p.Name , e.Name , at.Start, at.End
from Patient p inner join
Attend at
on p.Id = at.PatientId inner join
Employee as e
on at.DoctorId = e.Id;
Some people like to embed such queries in stored procedures. If you want to encapsulate this logic, I would suggest a view or table-valued function instead.
I asked a similar question a few weeks ago, but now the requirements have changed.
Considering the following tables:
http://www.maroisconsulting.com/Temp/query.png
I need to create a query that returns the percentages of employees who are managers (Titles.IsManager) and who have a date in the Certified field (Employees.Certified). The results need to be grouped by the Group each store is in.
So far I have this:
SELECT d.GroupId,
Sum(d.cert_complete) AS SumOfcert_complete,
Count(d.cert_complete) AS CountOfcert_complete
FROM (SELECT DISTINCT
s.GroupId,
e.EmployeeID,
IIf(e.Certified Is Null,0,1) AS cert_complete
FROM
((Stores AS s
INNER JOIN EmployeeStores AS es ON s.StoreId = es.StoreId)
INNER JOIN Employees AS e ON es.EmployeeId = e.EmployeeID)
INNER JOIN Titles AS t ON e.TitleId = t.TitleId
) AS d
WHERE t.IsManager
GROUP BY d.GroupId;
And then this
SELECT q.GroupId,
(SumOfcert_complete/CountOfcert_complete)*100 AS percent_certified,
Groups.GroupName
FROM qryGroupCert_base AS q
INNER JOIN Groups ON q.GroupId = Groups.GroupId;
You can see in the first query where I added the Titles table.
1) I get prompted for the IsManager, although I don't know why
2) The results coming back are not different than before I added the IsManager
Anyone see what's wrong here?
Many thanks
Within your first query, you have this subquery which includes Titles aliased as "t":
(SELECT DISTINCT
s.GroupId,
e.EmployeeID,
IIf(e.Certified Is Null,0,1) AS cert_complete
FROM
((Stores AS s
INNER JOIN EmployeeStores AS es ON s.StoreId = es.StoreId)
INNER JOIN Employees AS e ON es.EmployeeId = e.EmployeeID)
INNER JOIN Titles AS t ON e.TitleId = t.TitleId
) AS d
Then, after the definition of the subquery, you have this WHERE clause:
WHERE t.IsManager
The problem is the "t" alias and IsManager column only exist within the the subquery --> they are unknown to the outer (parent) query. In cases where the Access database engine encounters something it doesn't recognize as an object name, function, literal value, or SQL keyword, it thinks that something must be a parameter ... so pops up the input box asking you to provide a value for the (IsManager) parameter.
I think you should move the WHERE clause inside the subquery definition.
SELECT d.GroupId,
Sum(d.cert_complete) AS SumOfcert_complete,
Count(d.cert_complete) AS CountOfcert_complete
FROM [SELECT DISTINCT
s.GroupId,
e.EmployeeID,
IIf(e.Certified Is Null,0,1) AS cert_complete
FROM
((Stores AS s
INNER JOIN EmployeeStores AS es ON s.StoreId = es.StoreId)
INNER JOIN Employees AS e ON es.EmployeeId = e.EmployeeID)
INNER JOIN Titles AS t ON e.TitleId = t.TitleId
WHERE t.IsManager = True
]. AS d
GROUP BY d.GroupId;
Perhaps you need to supply a criteria for t.IsManager, such as t.IsManager = TRUE. If the where clause doesn't have a value to set it equal to, Access probably isn't resolving it to the actual column, but thinks it's a query parameter.