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Differences between "foreign key" and "constraint foreign key"
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Closed 9 years ago.
I am not much familiar with the SQL and please explain me the difference of following two and what is the best way to use. Is there any advantage using one over a another.
CREATE TABLE Employee
(
Emp_Id int NOT NULL,
Dep_Id int NOT NULL,
...
FOREIGN KEY (Dep_Id) REFERENCES Department(Dep_Id)
);
And
CREATE TABLE Employee
(
Emp_Id int NOT NULL,
Dep_Id int NOT NULL,
...
CONSTRAINT fk_EmpDept FOREIGN KEY (Dep_Id) REFERENCES Department(Dep_Id)
);
I always name my keys and constraints, generally with enough information that anybody looking at the key name will be able to understand what the key does.
Foreign keys I name FK_FieldName#TableName.
Primary keys I name PKC_TableName or PKN_TableName ("c" for "clustered" and "n" for "nonclustered", although this is not critical).
For indexes, I name them based upon their uniqueness, their clusteredness, and an "I" for index, e.g., UNI_FieldName#TableName.
The reason for naming is for the convenience both if you ever need to drop the object and, more importantly, if something in your code attempts to violate the relationship / constraint. It's immediately apparent where the problem is, if you get a message back saying that you've had a key violation of key FK_FieldName#TableName but not so clear if the name doesn't make sense.
The only difference is in naming. If you don't explicitly set a name for constraint, it will has auto generated name. For example: FK_Employee_Dep_Id__164452B1 . And you'll see this name in description of table keys, in different exceptions and so on.
If you do not name a foreign key, the db implicitly assign a foreign key name for the field.
And if you name a foreign key, the db uses that as foreign key constant name.
You can use drop FOREIGN KEY foreign_key_name for dropping the constraint (for MySql).
You can use drop constraint foreign_key_name for dropping the constraint (for Oracle, MS Sql etc).
You can use
ALTER TABLE TableName
ADD CONSTRAINT foreign_key_name
FOREIGN KEY (field)
REFERENCES foreign_key_table(Foreign_key_field)
First example with FOREIGN KEY (Dep_Id) REFERENCES Department(Dep_Id) is used in MySQL and it does not allow you to give this constraint specific name.
In the second example with CONSTRAINT fk_EmpDept FOREIGN KEY (Dep_Id) REFERENCES Department(Dep_Id) you can change fk_EmpDept to basically any name you want.
It can be used in MySQL, SQL Server, Oracle, MS Access.
Using second version you can also created Foreign Key constraint on several columns.
Related
I'm wondering if there's any (maybe subtle) difference between these two SQL statements:
CREATE TABLE profiles (
profile_id SERIAL PRIMARY KEY NOT NULL,
bio TEXT,
user_id INTEGER NOT NULL,
FOREIGN KEY (user_id) REFERENCES users(user_id)
);
and
CREATE TABLE profiles (
profile_id SERIAL PRIMARY KEY NOT NULL,
bio TEXT,
user_id INTEGER NOT NULL REFERENCES users(user_id)
);
I've noticed that when I create a table in Postico with the first notation, but look at the DDL of the created profiles table later, the FOREIGN KEY is removed and I end up with the shorter second notation.
Create table with FOREIGN KEY:
DDL view doesn't show FOREIGN KEY:
So, I'm wondering (and seeking confirmation) that the two statements are in fact 100% equivalent or if there are some subtle differences in what they do to the DB.
Any pointer to official resources (and maybe also how that differs from MySQL) would be appreciated.
The two samples you show do the same thing, just with a different syntax.
The first method is called table constraint, the second column constraint, but the latter name is somewhat misleading because the constraint is on the table as well.
The main difference is that the column constraint syntax is shorter, but cannot be used for all constraints: if you have for example a primary key that contains two columns, you have to write it in the table constraint syntax.
DDL view doesn't show FOREIGN KEY
DDL view created by unknown third-party tool in not an argument.
See fiddle. Foreign key exists in both cases. Moreover, I do not see the result difference for both DDL queries.
PS. As a recommendation - always specify the constraint name explicitly. What if you need to delete it? It is problematic without the constraint name...
In PostgreSQL, you define a foreign key through a foreign key constraint. A foreign key constraint indicates that values in a column or a group of columns in the child table match with the values in a column or a group of columns of the parent table. We say that a foreign key constraint maintains referential integrity between child and parent tables.
This may explain to you better or you can read about Foreign Keys documentation .
I am reading migrations of an existing web app:
...
create table BillableTime (
id int8 not null,
...
project int8,
primary key (id),
unique (employee, project, date)
);
...
create table Project (
id int8 not null,
...
primary key (id)
);
...
alter table BillableTime
add constraint FK3EBA06E37BE2CBE
foreign key (project)
references Project;
I do not understand 2 things:
1) why not to use just a simple reference declaration
create table BillableTime (
id int8 not null,
...
project int8 REFERENCES project (id),
primary key (id),
unique (employee, project, date)
);
What are the benefits of their method?
2) why constraint name is so weird: FK3EBA06E37BE2CBE?
Is there a reason for that?
1) why not to use just a simple reference declaration
Your ORM probably works with MySQL too, and MySQL ignores inline foreign key declarations, so you'd have to do it the long way.
There's less code to support adding foreign keys when you create the table and adding foreign keys later.
2) why constraint name is so weird: FK3EBA06E37BE2CBE? Is there a
reason for that?
It's perhaps a hash of the referencing and referenced table and column names. Don't forget that sometimes you need to DROP foreign keys, and you have to do so by name. This would make it slightly easier I suppose.
The constraint's name is so weird most likely as a result of using an ORM. I believe, the name of a constraint must be unique throughout the database... so the ORM which has provided the name, aimed to assure uniqueness.
And refering to the first part of your question - it is a result of ORM's migration system implementation - that it generates two statements instead of one... it must have been easier to implement this way - and for Postgres - it does exactly the same
What is the difference between using or not using the CONSTRAINT keyword when working with Foreign Keys on SQL Server?
I noticed that apparently both worked the same in this specific case, without CONSTRAINT:
CREATE TABLE ClientsPhones
(
ClientPhone varchar(10) NOT NULL,
ClientID smallint NOT NULL,
PRIMARY KEY (ClientPhone),
FOREIGN KEY (ClientID) REFERENCES Clients(ClientID)
);
And with CONSTRAINT:
CREATE TABLE ClientsPhones
(
ClientPhone varchar(10) NOT NULL,
ClientID smallint NOT NULL,
PRIMARY KEY (ClientPhone),
CONSTRAINT fk_ClientID
FOREIGN KEY (ClientID) REFERENCES Clients(ClientID)
);
Both didn't let me add records to the table unless the ClientID already existed on the Clients table, and the same ClientID and ClientPhone weren't already on the ClientsPhones table.
Is there any real difference between the two besides the fact that I'm able to name the constraint?
If you don't create constraint.it will automatically create own constraint name
the foreign key index name is generated using the name of the referencing foreign key column Automatically.
So there is no way to see difference of using and not using Constraint keyword. by default constraint name will be defined.
I did some research and don't believe Hell Boy's answer was as clear as it could be and had some misinformation.
Every constraint you add to a database has a name set by default. This includes PRIMARY KEY, FOREIGN KEY, DEFAULT, NOT NULL. It isn't necessarily the name of the column(s) used.
You can imagine that when you don't use the CONSTRAINT keyword SQL Server puts it there as well as generates a name for you.
If you want to remove or change a constrain you would either have to delete the entire table and recreate it with the correct constraints or you can reference the constraint by name and then alter it somewhat like a column using the ALTER keyword. This can be useful for when you need to delete a table with a foreign key. If you name the foreign key constraint you can delete it and then the table instead of having to delete the table the foreign key points to.
I am very confused about those two terms. Are they the same or different?
Some books and people say they are the same and others say they are different.
I tried but couldn't find a conclusive answer.
I am supposing that you are talking about using the REFERENCES where the FOREIGN KEY keyword is not used when constraining a column inline, which is called a column-level foreign key constraint, eg.
author_id INTEGER REFERENCES author(id)
... instead of the table-level foreign key constraint, which is placed after the column declarations ...
author_id INTEGER,
FOREIGN KEY(author_id) REFERENCES author(id)
The answer is, that it is simply shorthand syntax for the same thing. The main concern when altering between the two should be readability.
For more advanced use, it might be relevant that only table-level foreign key constraints can describe constraints on multiple keys at once, where all must be present in the referenced table.
Do note that MySQL 'parses but ignores “inline REFERENCES specifications” (as defined in the SQL standard) where the references are defined as part of the column specification', meaning that only the table-level foreign key constraint will work.
Both Postgres and Microsoft's SQL Server respect both column- and table-level foreign key constraints.
A foreign key must refer to a primary key.
When using REFERENCES constraint simply, then it isn't necessary that the referenced key be a primary key.
"Reference key" isn't a normal technical term in relational modeling or in SQL implementation in US English.
A foreign key "references" a key in some other table; could that be where the confusion comes from?
You don't really call something a reference key... They are the same thing... you might see the word references used for example in sqlite: you might use syntax like this to start a db of authors and books. This lets you show that one author can have many books. This tells the db that the books.author_id (defined a couple of lines up) references author.id
CREATE TABLE 'author' (
id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
firstname varchar(255)
lastname varchar(255)
);
CREATE TABLE 'books' (
id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
author_id INTEGER,
title varchar(255),
published date,
FOREIGN KEY(author_id) REFERENCES author(id)
);
In terms of standard SQL, both result in a foreign key constraint.
One form is a table constraint, meaning it can apply to one or more columns. You would need this to reference a table that has a multi-column primary key:
CREATE TABLE child (
id int PRIMARY KEY,
parent_id int,
date date,
FOREIGN KEY (parent_id, date) REFERENCES parent(id, date)
);
The other form is a column constraint, meaning it can only apply to the single column it is defined with. It cannot be used to reference a table with a multi-column primary key.
CREATE TABLE child (
id int PRIMARY KEY,
parent_id int REFERENCES parent(id)
);
The above syntax works exactly the same as if you declared a table constraint for a single column (supposing the RDBMS supports this type of column constraint), as follows:
CREATE TABLE child (
id int PRIMARY KEY,
parent_id int,
FOREIGN KEY (parent_id) REFERENCES parent(id)
);
It frequently causes confusion for users of MySQL and its InnoDB storage engine, that the latter column-constraint style is not supported. You must define a table-level constraint for a foreign key, even if it is a single-column constraint. This has been a strange behavior of MySQL since its earliest days, that some constraint syntax is valid, but results in no constraint. See discussion here: https://bugs.mysql.com/bug.php?id=17943
The only and most important difference between the two keywords 'FOREIGN KEY" and "REFERENCES" keywords is though both of them make the data to be child data of the parent table, the "FOREIGN KEY" is used to create a table level constraint whereas REFERENCES keyword can be used to create column level constraint only. Column level constraints can be created only while creating the table only. But table level constraints can be added using ALTER TABLE command.
Perhaps you are using the term "reference key" somewhat loosely?
A foreign key value in one row is said to "reference" the row that contains the corresponding key value. Note the word "reference" in the prior sentence is a verb, so we may say we have a referencing foreign key value and a referenced key value.
Although it is the key values, rather than the table key constraint, that is being referenced, I suppose loosely speaking we could say "referenced key" to mean the rows that comprise the values that may potentially be referenced. I then see how "referenced key" could become "referenced key" but not belie its origin.
There are 2 ways to declare a foreign key(s):
if the foreign key is a SINGLE attribute:
REFERENCES ()
if foreign keys are a LIST of attributes
FOREIGN KEY () REFERENCES
A foreign key "references" a key in some other table. That key in some other table is called Referenced key. You'll probably hear a lot about this if you're using Graphic feature on phpmyadmin.
The Reference Key is the primary key that is referenced in the other table.
On the other hand, Foreign Key is how you link the second table to the primary tables Primary Key (or Reference Key).
I am aware of Oracle's create table syntax
CREATE TABLE MyTable(
id int primary key,
...
);
This will create a table called MyTable with an int primary key. So, nothing new here.
but I am having difficulties understanding the following query:
CREATE TABLE departament (
cod_dept INTEGER CONSTRAINT dept_key PRIMARY KEY,
dept_name CHAR(15) NOT NULL,
admission DATE NOT NULL,
localization CHAR(20))
When I look up on Oracle's SQL Developer software on departement's table, I can see 4 columns: cod_dept, dept_name, admission and localization. On the constraints tab, I can also see dept_key, but I am confused as to what this might mean. What is dept_key purpose here?
Edit
Ok, seems it is a way to define the name of the constraint you're adding to the table. My next question is why don't you just call it the same name as the primary key column? From what I've seen it seems Oracle by default just creates a random name for the constraint!
Thanks
When you write id int primary key, Oracle will create a primary key constraint to ensure uniqueness of primary key values. All constraints have names, so in this case Oracle assigns an autogenerated name to this constraint. But you can set a name of this constraint explicitly using the CONSTRAINT syntax:
cod_dept INTEGER CONSTRAINT dept_key PRIMARY KEY
This name may be used later to refer to the constraint, for example, to delete or modify it:
ALTER TABLE department DROP CONSTRAINT dept_key;
EDIT:
Constraint names are unique across the schema, so Oracle can't just use the name of primary key column as a constraint name.
Primary keys can be explicitly be named. dept_key is just a name.
dept_key is the name of the primary key constraint. That means cod_dept is the unique identifier for your table, the mechanism for identifying a row, and so it can only have one occurrence of any given value.
That is the constraint you created representing the primary key.
A table is made up of:
Columns (where the data lives)
Indexes (indexed copies of the data used for faster searching)
Constraints (rules about what data can be in the table, including PK, FK, and check constraints).
dept_key is the name of the constraint. You specified the name here : "INTEGER CONSTRAINT dept_key PRIMARY KEY," so it will create a constraint with the name dept_key.
Another syntax for the same would be to write the following after your CREATE TABLE instruction.
ALTER TABLE department
ADD CONSTRAINT dept_key PRIMARY KEY (cod_dept)
dept_key is then the name of the constraint you created to be the primary key for this table. In order for a database engine to know the primary key, and to index it for fastest results and so forth, it needs to create a known constraint that is indexed. Here, it is you who has given the name which is dept_key.
For you kind information, it is often seen to write PK_[table name] for primary keys constraints and FK_[current_table_name]_[foreign_table_name] for foreign keys constraints.
Hope this helps! =)
I think whenever we create a Primary Key value then by default Oracle will crate constraint for it with the same name but it looks like that u are creating constraint with some other name.
Thank You