Please correct if im wrong. And kindly point me to articles on this concept.
When we create a primary key, in the background there is automatically a unique index, clustered index, and a not null constraint created on that coloumn.
Does this also mean that if we create a not null constraint, [clustered index or non clustered index] and unique index on a column, then that column becomes a primary key?
I want to understand the core concept/relation between primary key, index and constrains.
The primary key is the one that is declared as the "primary" key. Just having the characteristics doesn't make a key "primary". It has to be explicitly declared as such.
Different databases implement primary keys in different ways. Although primary keys are usually implemented with a clustered unique index, that is not a requirement.
The primary key is exactly what its name suggests: "primary". Any other column or group of columns can be declared both unique and not null. That does not make them primary keys. In some databases, you could even define another column or group of columns as not null, unique and clustered -- without that being the primary key.
In summary:
You can have any number of unique indexes on a table.
You can have any number of unique indexes on non-NULL columns on a table.
You can have at most one clustered index. In almost all cases, this would be the primary key. But is not required in all databases.
You can have at most one primary key. In almost all cases, this would be clustered, although that is not required in all databases.
For more detail, you should refer to the documentation of the database you are using.
If you have multiple columns comprising non-NULL, unique keys, then only one is "primary" -- that one that has been explicitly declared as primary.
Why would you have a non-clustered primary key? I can give one scenario. Imagine a database where UUIDs are the keys for rows. The company does not want to use auto-generated sequence numbers, because they provide information in the number.
However, UUIDs are remarkably bad candidates for clustered indexes, because inserts are almost never at the end. In this case, you might want to design the table with a clustered auto-generated sequential key, to speed inserts You might make this key the primary key. But, you want all foreign key references to use the UUID -- and you want all foreign key references to be to the primary key of the table.
No.
All the columns could be added with Not null and Non-clustered index and Unique But only ONE column could be PK.
And the Unique allows NULL while Primary Key does not.
You might be talking about Candidate Key, here is the ref:
https://www.techopedia.com/definition/21/candidate-key
Related
Primary key can be clustered or non clustered. It is unique and not null.
Suppose if I create an unique index/constraint and keyword NOT NULL then is this technically the same as primary key?
It depends what you mean by "technically" the same. The physical index is identical with a primary key, unique constraint, or unique index. A foreign key can reference a unique constraint or unique index as well as a primary key.
Differences that come to mind are:
Only one primary key is allowed per table
SQL Server will enforce the NOT NULL requirement for primary key columns but not for unique constraints/indexes
Unique indexes allow additional options such as INCLUDE and WHERE clauses, although cannot be referenced by FK constraints when a WHERE clause is specified
What is the difference between Primary and Unique key (in MySQL)? How these can be considered as a foreign key? please explain.
I tried creating a database table and don't know how to make a primary key as a foreign key. Does it take Joins concept where the separating attribute automatically create a foreign key?
A table in MySQL can only have one Primary Key at most, while you can create as many unique Keys or indexes as you want.
Also a primary key is not nullable while a unique key can have NULL as value.
But the biggest difference is the purpose:
You want to have a primary key because you need an identifier
The unique Key/Index on the other hand is usefull to controll values that are automatically getting inserted into your table (e.g. to avoid duplicates where none are allowed)
If you want to use a column as a forgein key, you need to define it as a primary key first. Unique Constraint can not be related with other tables as a foreign key.
I am building my third database and I am unsure how to structure primary keys and unique constraints to ensure data integrity.
The database is updated periodically when public data is released, some monthly, some annually and will have a low query load.
I have some tables where a combination of columns makes each row unique. I am aware that I can use a composite primary key but I am unsure if this is best practise as many of the articles I read on this have conflicting views.
For example, one table requires five columns for uniqueness with datatypes:
smallint
char(7)
varchar(7)
smallint
varchar(3)
I believe this would make the primary key up to 25 bytes and all of these five columns will regular be selected and often used in the where clause. One of the smallints is also a foreign key but no foreign keys will reference this table.
The only alternative I am aware of is to create a unique identifier for each row, setting this as the primary key and creating a unique constraint over the five columns.
What is the best option and if it is to use a composite primary key should it be clustered or non-clustered?
Is there a recommended maximum number of columns and bytes for a composite primary key?
I don't know what is the purpose of your database, but first advise if you can add ,id column as primary key. It'll be easier for every query.
Then If you want just be sur of unique combination of columns in the same table, the easy way is a index with is the keyword UNIQUE:
CREATE UNIQUE NONCLUSTERED INDEX [IX_COLUMN1_COLUMN2]
ON TABLE(COLUMN1 ASC, COLUMN2ASC);
In SQL Server, I have a non nullable column with a unique clustered index on it.
If I make this column a Primary Key the exact same index is created automatically plus
the column is recognized as a Primary Key.
I understand the abstract/semantic difference.
(Primary Key identifies the entity, while any other column with this index may not.
For example, a Person can have Email field which is Unique,Non-nullable... but can be changed)
But what bothers me is the actual difference when it comes to the DB engine itself.
What will happen if I will just create an Id column, make it non-nullable, create a unique clustered index for it, make it Identity Increment, but without the Primary Key constraint?
In what scenarios the Primary Key constraint comes into play?
(I've looked at many related questions before asking this, but all the answers I saw ended up with an abstract/theoretical explanation).
Nothing will be different really. You specify PRIMARY KEY to relay your intentions, not so that the engine does anything differently. When constructing a query plan, the optimizer will still use the uniqueness for all of its properties, and will still use the clustered index for all of its properties, regardless of whether you technically created it as a PRIMARY KEY. When creating a FOREIGN KEY, you can still reference the column(s) specified as unique (clustered or not). The difference is solely in the metadata (sys.indexes.is_primary_key) and in SSMS' representation to you (oh and the fact that you can create a unique clustered index on a NULLable column, but you can't create a PRIMARY KEY on that column).
In fact there are many cases where you want to completely separate the clustered index from the PRIMARY KEY. If you have a table where the PK is a GUID, for example, and you are typically running date range queries against the table, you are probably better off having the PK be non-clustered and have a clustered index on a naturally increasing column (the datetime column) - both to minimize page splits on heavy insert activity and also to best assist date range queries. The non-clustered index will be perfectly fine for looking up individual GUIDs. (I wanted to mention that because a lot of people think the primary key has to be clustered. Not true.)
Also interesting to note that if you create a PRIMARY KEY constraint, then create a unique clustered index with the same name using DROP_EXISTING, the is_primary_key column will still be 1 and Object Explorer will still show the index name under Keys.
Here is one scenario - a lot of code to data mapping frameworks look at the database metadata (what are the primary keys, foreign keys, etc) to determine how code is executed. For example Hibernate requires a primary key.
A typical scenario might be generating a where clause for an update.
Can anyone tell me what is the difference between a primary key and index key. And when to use which?
A primary key is a special kind of index in that:
there can be only one;
it cannot be nullable; and
it must be unique.
You tend to use the primary key as the most natural unique identifier for a row (such as social security number, employee ID and so forth, although there is a school of thought that you should always use an artificial surrogate key for this).
Indexes, on the other hand, can be used for fast retrieval based on other columns. For example, an employee database may have your employee number as the primary key but it may also have an index on your last name or your department.
Both of these indexes (last name and department) would disallow NULLs (probably) and allow duplicates (almost certainly), and they would be useful to speed up queries looking for anyone with (for example) the last name 'Corleone' or working in the 'HitMan' department.
A key (minimal superkey) is a set of attributes, the values of which are unique for every tuple (every row in the table at some point in time).
An index is a performance optimisation feature that enables data to be accessed faster.
Keys are frequently good candidates for indexing and some DBMSs automatically create indexes for keys, but that doesn't have to be so.
The phrase "index key" mixes these two quite different words and might be best avoided if you want to avoid any confusion. "Index key" is sometimes used to mean "the set of attributes in an index". However the set of attributes in question are not necessarily a key because they may not be unique.
Oracle Database enforces a UNIQUE key or PRIMARY KEY integrity constraint on a table by creating a unique index on the unique key or primary key. This index is automatically created by the database when the constraint is enabled.
You can create indexes explicitly (outside of integrity constraints) using the SQL statement CREATE INDEX .
Indexes can be unique or non-unique. Unique indexes guarantee that no two rows of a table have duplicate values in the key column (or columns). Non-unique indexes do not impose this restriction on the column values.
Use the CREATE UNIQUE INDEX statement to create a unique index.
Specifying the Index Associated with a Constraint
If you require more explicit control over the indexes associated with UNIQUE and PRIMARY KEY constraints, the database lets you:
1. Specify an existing index that the database is to use
to enforce the constraint
2. Specify a CREATE INDEX statement that the database is to use to create
the index and enforce the constraint
These options are specified using the USING INDEX clause.
Example:
CREATE TABLE a (
a1 INT PRIMARY KEY USING INDEX (create index ai on a (a1)));
http://docs.oracle.com/cd/B28359_01/server.111/b28310/indexes003.htm
Other responses are defining the Primary Key, but not the Primary Index.
A Primary Index isn't an index on the Primary Key.
A Primary Index is your table's data structure, but only if your data structure is ordered by the Primary Key, thus allowing efficient lookups without a requiring a separate data structure to look up records by the Primary Key.
All databases (that I'm aware of) have a Primary Key.
Not all databases have a Primary Index. Most of those that don't build a secondary index on the Primary Key by default.