Hi i need to make a relation between tblPersoneel and tblWoonplaats but it is not working.
This is the code i am using
ALTER TABLE tblPersoneel
ADD CONSTRAINT FK_Perwoon FOREIGN KEY
REFERENCES
tblWoonplaats
GO
Try the following:
ALTER TABLE tblPersoneel ADD
CONSTRAINT FK_Perwoon FOREIGN KEY (woonplaatsId)
REFERENCES tblWoonplaats (id)
Of course you need to use the correct column names for the ids.
You should specify the columns:
FOREIGN KEY (<woonplaatsID within tblPersoneel>) REFERENCES tblWoonplaats (<woonplaatsID within tblWoonplaats>)
Related
So, I am applying foreign key constraint on a column in MySQL table.
What I noticed is that I am able to do that in two ways -
ALTER TABLE book ADD CONSTRAINT fk_code_id FOREIGN KEY(book_type) REFERENCES code(id);
and
ALTER TABLE book ADD FOREIGN KEY(book_type) REFERENCES code(id);
Why do we have two ways in place to achieve same thing?
Alter table with Constraint option for adding check constraints to MySQL database tables.
The add constraint function allows the user to add a constraint name and a constraint condition.
https://razorsql.com/features/mysql_add_constraint.html
And by this ALTER TABLE book ADD FOREIGN KEY(book_type) REFERENCES code(id) you just make fk between two tables.
create a FOREIGN KEY constraint on the "PersonID" column when the "Orders" table is already created, use the following SQL:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Orders
ADD FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);
To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
ALTER TABLE Orders
ADD CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID) REFERENCES Persons(PersonID);
I have successfully added my first FK in heidisql using the foreign key tab and adding all the appropriate sections.
I have tried to do the same to my second related column both by using the FK tab and by running a query but I keep getting an error.
SQL Error (1005): Can't create table sprout.#sql-430_3 (errno: 150 "Foreign key constraint is incorrectly formed")
sprout is my db name so I have no idea why it is saying cant create table sprout (because I'm not referencing it in my query).
sql query for my first FK(generated via heidisql):
ALTER TABLE `purchase_history`
ADD CONSTRAINT `bus_id` FOREIGN KEY (`bus_id`) REFERENCES `business` (`bus_id`);
sql query for my second FK(generated via heidisql)
ALTER TABLE `purchase_history`
ADD CONSTRAINT `bus_name_fk` FOREIGN KEY (`bus_name`) REFERENCES `business` (`bus_name`);
sql query I wrote to try and add second FK
Alter table purchase_history
Add constraint bus_name_fk
Foreign key (bus_name)
references business(bus_name);
Can someone help explain to me how my constraint is incorrectly formed? To my understanding I was able to add another constraint to the the table.
This is too long for a comment.
Huh? Why are you adding two foreign constraints to the same table . . . but using different columns? That doesn't really make sense. In general, the foreign key should be referencing the primary key of the other table, which I presume is bus_id.
Then, if you want the business name, you can use a join to get the name.
So I made all of my tables already and just found out about Foreign keys and now I know I need them.
How would I add a relation between 2 columns from 2 tables? I see multiple variations of how Foreign key is used, how would I use mine in my case?
Use an ALTER TABLE query to modify the table to add the foreign key constraint.
ALTER TABLE yourTable
ADD CONSTRAINT FOREIGN KEY (col_in_yourTable) REFERENCES otherTable (col_in_otherTable);
The syntax after ADD CONSTRAINT is essentially the same as the CONSTRAINT clause of CREATE TABLE.
I have a newbie question here in terms of database design and I've noticed 2 options.
1) include the foreign key constrain in the create table block
2) create table then Alter table by ADD CONSTRAINT FOREIGN KEY
Appreciate your experienced view on this
1. include the foreign key constrain in the create table block
self explanatory, You're creating foreign key constraint in create table there are not records.
2) create table then Alter table by ADD CONSTRAINT FOREIGN KEY
If you don't have any records in table then this will behave as 1. If you've any records in tables then you need to take care whether the available records doesn't violates the foreign key constraints.
Basically I'm rather new to MySQL and in an example I see this foreign key being added:
ALTER TABLE Department ADD FOREIGN KEY managerIsInDepartment
(manager,id) REFERENCES Employee(id,worksAt);
I suppose one foreign key is being added (of the name managerIsInDepartment).
But I thought the attributes in the parentheses were the attributes to turn into foreign keys?
Why is managerIsInDepartment being displayed?
managerIsInDepartment is simply a name for the key to help identify it. You can omit it and one will be automatically generated.
ALTER TABLE Department <-- Table that will have the foreign key constraint
ADD <-- Option
FOREIGN KEY managerIsInDepartment <-- Name of the key
(manager,id) <-- Columns included in the key
REFERENCES Employee <-- The table being referenced
(id,worksAt) <-- Columns being referenced in foreign table
Hope this helps.
A good reference.