I have a table that I want to make which is a bridge table for teacher and class table. This is the ERD
I initially thought I'm going to create this table by
CREATE TABLE class_teacher
(
teacher_id number(3),
class_id number(2),
CONSTRAINT class_teacher_pk
PRIMARY KEY(teacher_id, class_id),
CONSTRAINT class_teacher_teacher_fk
FOREIGN KEY(teacher_id) REFERENCES teacher(teacher_id),
CONSTRAINT class_teacher_class_fk
FOREIGN KEY(class_id) REFERENCES class(class_id)
);
But on the web I see people just having two foreign keys and no primary key, or table with no foreign key and having a primary key for two columns.
Am I doing it incorrectly?
Am I doing it incorrectly?
No, it looks correct.
Although I would question the size of the numeric data types as you are restricted to only have 1999 teachers and 199 classes (including negative numbers); this may be enough for immediate use but after several years when classes get re-organised or when the syllabus is re-written and new classes are created then you may run out of primary keys.
Does create statement returns any error? Otherwise you should be good.
Try to insert some data in all 3 tables and run some delete statements to see how it goes.
Related
I create 3 tables just like image. Each students can be enrolled multiple class I tried to build one to many relation.
What I want to do is, when a student is deleted from the "Student" table, the course in which the student is registered in the "Bridge" table returns to null. How can I do this operations with postgresql (pgAdmin 4), can you help me please? Thank you...
You are describing the on delete set null option to foreign keys constraints. The create table statement for bridge would look like:
create table bridge (
std_id int references students(std_id) on delete set null,
class_id int references class(class_id)
);
I am unsure that set null is your best pick for such a bridge table though. This leaves "gaps" in your data that do not make a lot of sense. on delete cascade would probably make more sense - and you could apply it to both foreign keys:
create table bridge (
std_id int references students(std_id) on delete cascade,
class_id int references class(class_id) on delete cascade
);
That way, the bridge table is properly cleaned up when any parent record is dropped. This also opens the way to set up a composite primary key made of both columns in the bridge table.
I've been trying to create this table in my database. We were told to use Oracle-Apex for creating the database. So I keep getting this error that I cant solve:
If I remove the last line of the code, it creates the table fine without any errors.
Here are screenshots of the other tables being referenced here:
Company Table
Branch Table
IDK if this is a rookie mistake, I only learnt apex/sql in like an hour and went off to make the database. Thank you for helping me! :)
The column(s) referenced by a foreign key must have a unique index in the source table (or they must be the primary key of that table). Your code fails because of the following foreign key declaration, where the target is not unique:
foreign key (BranchNo) references Branch(BranchNo)
Here, I think that you want a compound foreign key that references the primary key of Branch rather than two different keys. Branch(CCode) references Company(CCode) already so there is no need to put that relationship in the Equipment table.
create table Equipment(
CCode int,
BranchNo int,
EquipNo it,
Description varchar2(50),
NumberOfEquip int,
primary key(CCode, BranchNo, EquipNo),
foreign key (CCode, BranchNo) references Branch(CCode, BranchNo)
);
CREATE TABLE Album (
name VARCHAR(50),
lenght FLOAT,
genre VARCHAR(30),
nrSongs INT,
PRIMARY KEY (name, writer),
FOREIGN KEY (writer) REFERENCES Musician(name) OR Band(name),
FOREIGN KEY (Studio) REFERENCES Studio(name)
);
Ok this is what I'm trying to do, I have two tables called Musician and Band, and in the foreign key called writer I want to use the name of a Musician or a Band, but this gives me an error in the OR statement, do you know the correct way to implement this? I can't find it.
Thanks :)
Not possible. A foreign key is a direct link between one field in one table, and another field in another table. It is 1:1. You cannot have n:1, 1:n, or n:n mappings in a foreign key definition.
Plus, your overall table definition is invalid. You have no writer field, so your primary key and the writer FK will fail anyways.
For this particular purpose, why not just making everyone a "band". A solo artist is simply a band that happens to have one member.
I have a SQL database opened with visual studio, and I need to add some constraints to a table already created. I need a foreign key, which already has a foreign key from a third table. To explain better ,
Table ANIMALI needs a foreign key from table GABBIA, which has already a foreign key from table STANZA. This was the code I came up with:
ALTER TABLE ANIMALE ADD CONSTRAINT REF_ANIMA_GABBI_FK FOREIGN KEY (n_stanza, n_gabbia) REFERENCES GABBIA(n_stanza, n_gabbia);
This gives me an error, n_stanza is a column id not valid. I think it's about the fact that the ID for the class GABBIA is taken from joining n_gabbia and n_stanza, the latter being a key in class STANZA.
Can anyone help me out?
In order for your ALTER TABLE statement to work as written, both tables (not classes) "ANIMALE" and "GABBIA" must include the columns "n_stanza" and "n_gabbia".
In addition, in the table "GABBIA", there must be either a primary key constraint or a unique constraint on the pair of columns "n_stanza" and "n_gabbia". That is, you need something like either primary key (n_stanza, n_gabbia) or unique (n_stanza, n_gabbia) in the table "GABBIA".
I'm creating a mock up database for the first time. I have created the Relational Model which consists of a many to many relationship. In the Relational Model it has a separate linking/joining table. When creating the database do I need to create this linking table as a separate table also? Or can I just put each foreign keys in the many to many tables?
If I need a separate table how do I link these tables together via syntax?
Thanks
We can only build foreign keys in a one-to-many fashion. So you need this intersection table. It is the sort of additional construct we introduce when transforming a logical data model into a physical one.
The intersection table often has just two columns, the referencing keys of the two tables you want to link in M:N fashion (there may also be some metadata columns to hold properties of the link). It usually has a compound primary key on the two columns, to avoid redundancy. It has a foreign key on each of the referenced tables, which must have defined primary keys on the referenced columns.
The syntax is pretty obvious; this sample builds two master tables and an intersection defining just the keys.
create table m1 (
id number not null
, constraint m1_pk primary key (id) );
create table m2 (
id number not null
, constraint m2_pk primary key (id) );
create table intersection_t (
m1_id number not null
, m2_id number not null
, constraint int_pk primary key (m1_id, m2_id)
, constraint int_m1_fk foreign key (m1_id)
references m1 (id)
, constraint int_m2_fk foreign key (m2_id)
references m2 (id)
);