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So I went through Odoo database design and I saw that they stored the relationship between 2 tables in a separate table that doesn't have primary key. How are you able to do this? I want to replicate this kind of behavior in SQL Server. Is it auto-inserted?
A table should always have a primary key. Look at the thousands of questions on Stackoverflow that ask how to delete one of two identical rows in a database table.
You typically model m-to-n relationships between tables with a separate table that has foreign keys to both tables:
CREATE TABLE person (
person_id bigint PRIMARY KEY,
name text,
...
);
CREATE TABLE game (
game_id bigint PRIMARY KEY,
name text NOT NULL,
...
);
CREATE TABLE likes_to_play (
person_id bigint REFERENCES person NOT NULL,
game_id bigint REFERENCES game NOT NULL,
PRIMARY KEY (person_id, game_id)
);
CREATE INDEX ON likes_to_play (game_id);
The primary key on the table makes sure there are no superfluous double entries and backs one of the foreign keys. The other index is created for the other foreign key.
What are the differences between Unique Key, Primary Key and Foreign Key with respect to concept of SQL?
How they are different from each other?
A PRIMARY Key and UNIQUE Key constraints both are similar and it provide unique enforce uniqueness of the column on which they are defined.
Primary Key
Primary key cannot have a NULL value.
Each table can have only one primary key.
By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index.
Primary key can be related with another table's as a Foreign Key.
We can generated ID automatically with the help of Auto Increment field. Primary key supports Auto Increment value.
Unique Key
Unique Constraint may have a NULL value.
Each table can have more than one Unique Constraint.
By default, Unique key is a unique non-clustered index.
Unique Constraint can not be related with another table's as a Foreign Key.
Unique Constraint doesn't supports Auto Increment value.
Foreign Key
Foreign key is a field in the table that is primary key in another table.
Foreign key can accept multiple null value.
Foreign key do not automatically create an index, clustered or non-clustered. You can manually create an index on foreign key.
We can have more than one foreign key in a table.
There are actual advantages to having a foreign key be supported with a clustered index, but you get only one per table. What's the advantage? If you are selecting the parent plus all child records, you want the child records next to each other. This is easy to accomplish using a clustered index.
Having a null foreign key is usually a bad idea. In the example below, the record in [dbo].[child] is what would be referred to as an "orphan record". Think long and hard before doing this.
Note: we use constraint for enforce data integrity
Primary Key
1)can't insert null value
2) one table have one primary key
Unique key
1) insert null value one at time
2)one table have multiple unique key
3) you can refereed as candidate key also
foreign key
1) maintain the relationship between two table and also multiple
Note: without any constraint you get data in multiple table but you can not get data peoperly
A note about Unique key
The parent table in a Primary Key-Foreign Key relation is normally called as Primary Key table but PK is not mandatory in a parent table. A unique key/constraint in parent table is sufficient. As PK is always unique, it is often used as foreign key in another table. see this SO post
I don't know if I created these tables in sql correctly. Can someone confirm I am doing this correctly or incorrectly? Can a table just have a foreign key or does it have to have a primary key? Thanks in advance.
CREATE TABLE Job(
JobId integer,
ToolName varchar(40),
status varchar(100),
start_time time,
finish_time time
PRIMARY KEY(JobId));
CREATE TABLE ErrorLog(
JobId integer,
ErrCode integer,
Description varchar(200),
PRIMARY KEY(ErrCode)
FOREIGN KEY(JobId) REFERENCES Job(JobId));
CREATE TABLE BLAST(
input varchar(100),
database varchar(100),
evalue varchar(100),
JobId integer,
FOREIGN KEY (JobId) REFERENCES Job(JobId));
CREATE TABLE MitoLoc(
input varchar(100),
specificity varchar(100),
JobId integer,
FOREIGN KEY (JobId) REFERENCES Job(JobId));
It is best practice to create a primary key column on a table in a SQL RDBMS. Although SQL does not require tables to have primary keys.
Primary keys allow for a unique identifier to be set for each row in the table. This is the only way you can uniquely identify a specific row in the table, and the only way you'll be able to utilize a foreign key relationship.
Database servers, like SQL Server, also optimize both the storage and querying of tables better when they have primary keys defined.
Primary keys are a very good idea, but not required. Almost every table that I create has an automatically incremented primary key. This is in addition to several other columns that I keep around, such as CreatedAt and CreatedBy.
Foreign key relationships are typically to primary keys but can also be to unique keys.
Why do you want a primary key?
So you can delete a row that is a duplicate, easily.
So you can readily update a single row.
An auto incremented primary key gives you an idea of the order that rows were inserted into the table.
A single column primary key is much easier to handle with foreign key references.
There are, undoubtedly, other reasons. But those are a few.
As for your tables, I think Mitoch and Blast should have id columns that are primary keys. You can also declare other columns (or combinations) to be unique, if appropriate.
Not needed. But It is not good for table structure.
you can create tables without primary key. and it can be have foreign key.
But some DBMS don't allow to save table without primary key
if you are not using primary key then
It is difficult to identify each record
Its record can't references to other tables
when creating table structure some time you have to create composite primary key that mean two or more columns together (combination ) can be primary key
I have to add a auto_Increment primary key column to the table. I used this script to create but its doing fine. but one thing is thtat I have alredy have a primary key in that table. so its throwing an error.
how to remove alredy existing primary and making a new auto_increamtn column has a primary key.
ALTER TABLE ABCD ADD ABCD_Id int NOT NULL IDENTITY (1,1) PRIMARY KEY
thanks
You need to drop the PRIMARY key constraint first, something like
ALTER TABLE DROP CONSTRAINT PK_Table_Col
I have about 5000 records in a table in SQL Server 2008
I need to add a primary key. I don't care what it is, maybe an autonumber would be best?
How do I add a primary key to every record ?
I also would like this primary key to be generated automatically when I add new rows!
Try this:
ALTER TABLE myTable ADD id INT IDENTITY
CONSTRAINT id _pk PRIMARY KEY;