I'm following a course which is declaring tables like this:
CREATE TABLE Artist (
id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT UNIQUE,
name TEXT UNIQUE
);
Doesn't PRIMARY KEY imply UNIQUE?
The SQLite documentation says:
Each row in a table with a primary key must have a unique combination of values in its primary key columns.
and
A UNIQUE constraint is similar to a PRIMARY KEY constraint, except that a single table may have any number of UNIQUE constraints.
So it looks like you don't need the UNIQUE constraint for primary keys?
Yes a primary key implies unique key, theres no need to use unique again.
for more kindly read:https://www.geeksforgeeks.org/difference-between-primary-key-and-unique-key/
Related
I need some advice on SQL structure on Postgresql.
I have those two tables :
DROP TABLE IF EXISTS "public"."attribute_value";
CREATE TABLE "public"."attribute_value"
(
"id" INTEGER NOT NULL,
"attribute_id" INTEGER NOT NULL,
"value" CHARACTER VARYING(100) NULL
);
--*****************************************************
DROP TABLE IF EXISTS "public"."product_attribute";
CREATE TABLE "public"."product_attribute"
(
"product_id" INTEGER NOT NULL,
"attribute_value_id" INTEGER NOT NULL,
"attribute_id" INTEGER NOT NULL
);
I added no constraints on purpose.
I need a foreign key on the child table product_attribute.attribute_value_id referencing the parent table attribute_value.id. The best practice is to create a primary key on the field attribute_value.id (maybe with a sequence), or to CREATE UNIQUE INDEX on attribute_value.id ?
I first thought indexes were only special lookup tables that the database search engine can use to speed up data retrieval. But when I played with foreign keys, I found that creating an unique index allowed me to avoid error "there is no unique constraint matching given keys for referenced table blablabla" because a foreign key is not supposed to point to a non unique value. Should indexes be used to create foreign keys then ?
I also need a foreign key on the child table product_attribute.attribute_id referencing parent table attribute_value.attribute_id. The problem is that attribute_value.attribute_id is not unique. But all the rows in product_attribute.attribute_id must not take any value out of attribute_value.attribute_id's possible values. How should I do ?
Every table should have a primary key. Don't join the legion of people who complain about duplicate rows in their tables.
So make id the primary key of attribute_value, then you can create a foreign key constraint.
Constraints are implemented by unique indexes, so technically they are almost the same. Still you need a constraint and not just a unique index as the target of a foreign key constraint.
About attribute_id: that should not be a foreign key constraint between the two tables from your question, but both tables should have a foreign key referencing yet another table (attribute?).
I am reading about primary keys and at a lot of tutorials, technical blogs etc., and I found that a primary key cannot be null. I think it's totally wrong because I was able to insert null value in the column.
I suppose a primary key can have a not null value only if the column is declared as not null. But again this is not a feature of primary keys.
My question is why do we have a concept of primary key because I find only one difference between primary key and unique key is that "Primary key can be declared only on one column whereas unique key can be declared on multiple columns". So my understanding is that why can't we also declare the primary key as a unique key if we don't have any other difference.
I suppose a primary key can have a not null value only if the column
is declared as not null.But again this is not a feature of primary
key.
Primary key can't have a null values. By definition of primary key, it is UNIQUE and NOT NULL.
My another question is that why do we have a concept of primary key
because I find only one difference between primary key and unique key
is that "Primary key can be declared only on one column whereas unique
key can be declared on multiple columns"
This is completely wrong. You can create primary key on multiple columns also, the difference between Primary Key and Unique Key is Primary Key is not null and Unique key can have null values.
The main purpose of primary key is to identify the uniqueness of a row, where as unique key is to prevent the duplicates, following are the main difference between primary key and unique key.
Primary Key :
There can only be one primary key for a table.
The primary key consists of one or more columns.
The primary key enforces the entity integrity of the table.
All columns defined must be defined as NOT NULL.
The primary key uniquely identifies a row.
Primary keys result in CLUSTERED unique indexes by default.
Unique Key :
There can be multiple unique keys defined on a table.
Unique Keys result in NONCLUSTERED Unique Indexes by default.
One or more columns make up a unique key.
Column may be NULL, but on one NULL per column is allowed.
A unique constraint can be referenced by a Foreign Key Constraint.
I suggest you read this primary key and unique key
You forgot Indexing. When it comes to large data to find particular data raw it need to travel through memory record by record. To overcome that the concept of indexing is there. Primary key helps in this. So it will help to get your data access faster. After that there is concept of binary search which will helps further in that task.
A primary key is a special relational database table column (or combination of columns) designated to uniquely identify all table records.
A primary key’s main features are:
It must contain a unique value for each row of data.
It cannot contain null values.
A primary key is either an existing table column or a column that is specifically generated by the database according to a defined sequence.
The primary key concept is critical to an efficient relational database. Without the primary key and closely related foreign key concepts, relational databases would not work.
A primary key, also called a primary keyword, is a key in a relational database that is unique for each record.
One Table Have Only One Primary Key.
I want to know what is the difference between unique key and composite primary key in SQL Server.
According to w3c school:
The UNIQUE constraint uniquely identifies each record in a database table.
The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.
A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.
Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.
We can create composite primary key by using this:
CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
UNIQUE (P_Id)
)
For composite primary key syntax:
CREATE TABLE Persons
(
P_Id int,
C_Id int,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
Primary Key (P_Id,C_Id)
);
The UNIQUE constraint uniquely identifies each record in a database table. This provide a guarantee for uniqueness for a column or set of columns. We can define(point) a single row with this.
A PRIMARY KEY has a UNIQUE constraint by default.
While in some tables, there won't be any columns with a unique value to define a row. In such cases COMPOSITE KEYs are used. In such cases two or more columns are combined together so that this combination is unique.
As you said yourself, the only difference between a unique and a primary key is that there may only be 1 primary key on a table while it can have more unique keys.
Furthermore, the values of a primary key may not be null.
Both unique- and primary keys can be composed of multiple columns (composed key).
By the ways, in your example you had a unique key on the P_Id column and a composed primary key that includes that very same column.
This has no sense.
I would suggest to create only a simple primary key on that P_Id column.
Composite keys are formed of more than one column
Primary key prevents a second row on that table with same key column values.
A Primary key can be formed of a single column and as well as of more than one column. So primary key can be defined as a composite key too.
The primary key does not accept the any duplicate and NULL values.
A primary key of one table can be referenced by foreign key of another table. A table can have more than one unique key unlike primary key. Unique key constraints can accept only one NULL value for column. Unique constraints are also referenced by the foreign key of another table. A composite key is having two or more attributes that together can uniquely identify a tuple in a table. Such a key is also known as Compound Key, where each attribute creating a key is a foreign key in its own right.
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
While reading articles on w3schools about SQL primary keys I read the following:
Each table should have a primary key, and each table can have only ONE primary key.
http://www.w3schools.com/sql/sql_primarykey.asp
Yet I have this SQL file, for making a table, which I ran and worked:
CREATE TABLE accessLog (
memberId SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
pageUrl VARCHAR(255) NOT NULL,
numVisits MEDIUMINT NOT NULL,
lastAccess TIMESTAMP NOT NULL,
PRIMARY KEY (memberId, pageUrl)
);
Now according the the primary key quote above, the line of code: " PRIMARY KEY(memberId, pageUrl)," should not have worked.
Any help on this about how you can have more than ony primary key in a table. Note:I already know about the "UNIQUE", "UNIQUE KEY" statement.
This is a composite primary key - it's still one key, but it's comprised of multiple columns. Here's a good, short description of composite primary keys.
It is a composite primary key . When you define more than one column as your primary key on a table, it is called a composite primary key.
Your quote
"Each table should have a primary key, and each table can have only ONE primary key."
is misleading. It's equivalent to saying that each state can have only one capital city, or that each company can have only one Chief executive officer.
Of course each table can have only one PRIMARY key. But any table can have multiple Unique Keys. And, frankly, does not have to have any one of them designated as "PRIMARY". In fact designating one key as PRIMARY key does absolutely nothing at all. There is only one distinction (except as noted below) attached to the Primary Key which is not also associated with all keys.
EDIT ... except for one small distinction. If a key is designated as the Primary key, then all the fields used in that key must be non-nullable. Other Unique keys are not required to honor this rule, although rows in the table which contain nulls must still be unique, in that there cannot be more thab one row with the same values for all the non-nullable field and a null value in a nullable field.