I have 2 tables:
Class (PK = Tutor)
Columns: ID / Tutor / Room
Teacher (PK FK = Tutor)
Columns: Tutor / Contact
I want to join both tables.
Does it matter what order the tables are joined?
ie.
SELECT Class.ID, Class.Tutor, Teacher.Contact
FROM **Class INNER JOIN Teacher** ON Class.Tutor=Teacher.Tutor
or could I do it the other way round and Teacher INNER JOIN Class instead?
Both tables also share a common column name Tutor.
Does it matter if I use Class.Tutor or Teacher.Tutor in the query?
For an INNER JOIN, the order does not matter; the result set you will get will contain all the rows from both Class and Teacher where the common field Tutor matches, whichever order you specify Teacher and Class.
When you use an OUTER JOIN, the order does matter; I would recommend looking at the answers to What is the difference between Left, Right, Outer and Inner Joins?.
Yes, the order matters.
Selecting from class and joining Teacher will show only those classes that have a teacher
Selecting from teacher and joining Class will show only those teachers that have a class
Depending on what you are trying to achieve you'll know which is most relevant.
I'd recommend using "left join" though, it will give you all the classes even if they don't have teacher assigned etc
FWIW "class" is likely to be a reserved word in your database, server side script and client side scripting languages. I'd find another word for that table name.
Related
I'm having problems understanding the class table inheritance structure that you can implement using database tables. Info on class table inheritance. I have a use case where I have quite different types of persons that I need to model, but they have very minor differences. For example, all of these persons, like Student, Professor and so on, have a surname and a lastname. My first thought was to move theses attributes into a different table inside a base table like you would do in Object Oriented Programming. Here to illustrate further:
Right now, a Professor can only have one person, for example, otherwise it wouldn't make sense in my use case. Also, I have a school table that has two foreign keys, one for the Professor and one for the Student. Lets assume that a school can also have only one professor and one student. This is not the real use case that I have. This example just represents the relation in my real use case which would be too much to explain here.
What I don't understand is how you would collect data based on that. I'm trying to make a SQL Server View where I want to load the Person of the Professor and the Person of the Student from the view point of the School Table. For example:
SELECT
School.professor_id
surname,
lastname
FROM dbo.School AS school
INNER JOIN dbo.Professor as prof
ON school.professor_id = prof.ID
INNER JOIN dbo.Person as prof_person
ON prof.person_id = prof_person.ID
I can output the surname and lastname of the professor, but now I am stuck since I can't figure out how to get the person of the student.
A subtype table typically shares a key with the supertype table, instead of having its own PK and a FK. EG Student.ID is both the PK and the FK.
Then just join Student>Person in addition to Professor>Person, eg
SELECT
School.Id,
prof_person.surname prof_surname,
student_person.surname student_surname
FROM dbo.School AS school
INNER JOIN dbo.Professor as prof
ON school.professor_id = prof.ID
INNER JOIN dbo.Person as prof_person
ON prof.ID = prof_person.ID
INNER JOIN dbo.Student as student
ON school.student_id = student.ID
INNER JOIN dbo.Person as student_person
ON student.ID = student_person.ID
INNER JOIN is associative, so no need for special ordering or parentheses.
We have to select authors that havent written a book but there are 3 different tables which makes me confused about how to write the join expression.
We have tables:
authors: author_id
authorships: author_id, book_id
books: book_id.
Obviously I selected the names from authors and tried inner join but it wont work for me. Help would be appreciated!
Since this sounds like a school assignment I won't give the full answer.
Try using an outer join between authors and authorship. Make sure you retrieve the book I'd from the authorship.
Try to work out what an author who has not published looks like the. You can use this to formulate the query for the answer you are looking for with an appropriate where clause.
This is a good spot to use the LEFT JOIN antipattern:
SELECT a.*
FROM authors a
LEFT JOIN authorships s ON s.author_id = a.author_id
WHERE s.author_id IS NULL
Rationale: when the LEFT JOIN comes up empty, it means that the author has no corresponding record in the authorships table. The WHERE clause filters out on unmatched authors records only (ie authors that have no books). This is called an antipattern because the purpose of a JOIN is usually to match records, whereas here we use it to detect unmatched records.
Its really easy, just check which column seems to be having common value between all this three tables if something is common atleast within two tables then put inner join on those two and an outer join on the uncommon data table.
Remember your Aliases will always matter when you join between different tables, also the ON and WHERE should be properly mentioned.
According to a simple scenario ER diagram as follows.
Based on that, I developed a database mapping as follows.
In there, orange color columns shows the primary key of the tables and yellow color columns shows foreign key of the tables. Is this mapping correct? Now my problem is I need to retrieve students data who learned from some teacher, which means need to retrieve some teacher's sudents who learned from his/her.
You need to learn about how to join tables using different types of available JOINs.
For your scenario , try like following, it will give you Student and the Teacher mapping.
select s.*
tbl_student s
inner join tbl_Course_Subjects tcs on s.Course_Id= tcs.Course_Id
inner join tbl_Subjects_Teacher tst on tst.SubjectId=tcs.Subject_Id
inner join tbl_Teacher t on t.Teacher_Id=tst.Teacher_Id
where t.Teacher_Id = #SomeTeacherId
Which SQL query could I write to satisfiy this need:
"List the names of the students who take a course from instructor named John."
Not sure that you can, from the depicted relations.
You can identify tutors by selecting on InstructorID and filtering on Instructor.FirstName.
You can join that subset onto course, via the InstructorCourses Join Table - join InstructorID to that and join the result to Courses using CourseID
In this way, Instructor.InstructorID -> (InstructorCourses.InstructorID , InstructorCourses.CourseID ) -> Courses.CourseID.
This lets you find information about the courses taught by instructors filtered on their name.
You don't present any link between students and courses in your diagram. I suspect you're missing a relation StudentCourses, which ought to be similar to InstructorCourses, but rather links students to courses. With that data in the mix, you can extend the join to match students to the courses from the relationship you already have.
Your diagram implies a relation between Student and InstructorCourses, which seems incorrect - both because there is no key to join on, and also because the logical relationship would not be correct. I think this is probably an error.
It is impossible to satisfy the SQL query you need because your conception does not allow it in that there is no relationship between the 2 tables Student and InstructorCourses.
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.