What is the difference between a primary key and a unique constraint?

Question

Someone asked me this question on an interview...

Answer 1

Primary keys can't be null. Unique keys can.

Answer 2

A primary key is a unique field on a table but it is special in the sense that the table considers that row as its key. This means that other tables can use this field to create foreign key relationships to themselves.

A unique constraint simply means that a particular field must be unique.

Answer 3

1. Primary key can not be null but unique can have only one null value.
2. Primary key create the cluster index automatically but unique key not.
3. A table can have only one primary key but unique key more than one.

Answer 4

TL;DR Much can be implied by PRIMARY KEY (uniqueness, reference-able, non-null-ness, clustering, etc) but nothing that can't be stated explicitly using UNIQUE.

I suggest that if you are the kind of coder who likes the convenience of SELECT * FROM... without having to list out all those pesky columns then PRIMARY KEY is just the thing for you.

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a relvar can have several keys, but we choose just one for underlining and call that one the primary key. The choice is arbitrary, so the concept of primary is not really very important from a logical point of view. The general concept of key, however, is very important! The term candidate key means exactly the same as key (i.e., the addition of candidate has no real significance—it was proposed by Ted Codd because he regarded each key as a candidate for being nominated as the primary key)... SQL allows a subset of a table's columns to be declared as a key for that table. It also allows one of them to be nominated as the primary key. Specifying a key to be primary makes for a certain amount of convenience in connection with other constraints that might be needed

What Is a Key? by Hugh Darwen

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it's usual... to single out one key as the primary key (and any other keys for the relvar in question are then said to be alternate keys). But whether some key is to be chosen as primary, and if so which one, are essentially psychological issues, beyond the purview of the relational model as such. As a matter of good practice, most base relvars probably should have a primary key—but, to repeat, this rule, if it is a rule, really isn't a relational issue as such... Strong recommendation [to SQL users]: For base tables, at any rate, use PRIMARY KEY and/or UNIQUE specifications to ensure that every such table does have at least one key.

SQL and Relational Theory: How to Write Accurate SQL Code By C. J. Date

In standard SQL PRIMARY KEY

• implies uniqueness but you can specify that explicitly (using UNIQUE).
• implies NOT NULL but you can specify that explicitly when creating columns (but you should be avoiding nulls anyhow!)
• allows you to omit its columns in a FOREIGN KEY but you can specify them explicitly.
• can be declared for only one key per table but it is not clear why (Codd, who originally proposed the concept, did not impose such a restriction).

In some products PRIMARY KEY implies the table's clustered index but you can specify that explicitly (you may not want the primary key to be the clustered index!)

For some people PRIMARY KEY has purely psychological significance:

• they think it signifies that the key will be referenced in a foreign key (this was proposed by Codd but not actually adopted by standard SQL nor SQL vendors).
• they think it signifies the sole key of the table (but the failure to enforce other candidate keys leads to loss of data integrity).
• they think it implies a 'surrogate' or 'artificial ' key with no significance to the business (but actually imposes unwanted significance on the enterprise by being exposed to users).

Answer 5

Every primary key is a unique constraint, but in addition to the PK, a table can have additional unique constraints.

Say you have a table Employees, PK EmployeeID. You can add a unique constraint on SSN, for example.

Answer 6

Unique Key constraints:

1. Unique key constraint will provide you a constraint like the column values should retain uniqueness.
2. It will create non-clustered index by default
3. Any number of unique constraints can be added to a table.

4. It will allow null value in the column.

   ALTER TABLE table_name ADD CONSTRAINT UNIQUE_CONSTRAINT UNIQUE (column_name1, column_name2, ...)

Primary Key:

1. Primary key will create column data uniqueness in the table.
2. Primary key will create clustered index by default
3. Only one Primay key can be created for a table
4. Multiple columns can be consolidated to form a single primary key

5. It wont allow null values.

   ALTER TABLE table_name ADD CONSTRAINT KEY_CONSTRAINT PRIMARY KEY (column_name)

Answer 7

In addition to Andrew's answer, you can only have one primary key per table but you can have many unique constraints.

Answer 8

1. Primary key's purpose is to uniquely identify a row in a table. Unique constraint ensures that a field's value is unique among the rows in table.
2. You can have only one primary key per table. You can have more than one unique constraint per table.

Answer 9

A primary key is a minimal set of columns such that any two records with identical values in those columns have identical values in all columns. Note that a primary key can consist of multiple columns.

A uniqueness constraint is exactly what it sounds like.

Answer 10

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

Answer 11

Primary key can't be null but unique constraint is nullable. when you choose a primary key for your table it's atomatically Index that field.

Answer 12

Primary keys are essentially combination of (unique +not null). also when referencing a foreign key the rdbms requires Primary key.

Unique key just imposes uniqueness of the column.A value of field can be NULL in case of uniqe key. Also it cannot be used to reference a foreign key, that is quite obvious as u can have null values in it

Answer 13

There are several good answers in here so far. In addition to the fact that a primary key can't be null, is a unique constraint itself, and can be comprised of multiple columns, there are deeper meanings that depend on the database server you are using.

I am a SQL Server user, so a primary key has a more specific meaning to me. In SQL Server, by default, primary keys are also part of what is called the "clustered index". A clustered index defines the actual ordering of data pages for that particular table, which means that the primary key ordering matches the physical order of rows on disk.

I know that one, possibly more, of MySql's table formats also support clustered indexing, which means the same thing as it does in SQL Server...it defines the physical row ordering on disk.

Oracle provides something called Index Organized Tables, which order the rows on disk by the primary key.

I am not very familiar with DB2, however I think a clustered index means the rows on disk are stored in the same order as a separate index. I don't know if the clustered index must match the primary key, or if it can be a distinct index.

Answer 14

Both guarantee uniqueness across the rows in the table, with the exception of nulls as mentioned in some of the other answers.

Additionally, a primary key "comes with" an index, which could be either clustered or non-clustered.

Answer 15

A great number of the answers here have discussed the properties of PK vs unique constraints. But it is more important to understand the difference in concept.

A primary key is considered to be an identifier of the record in the database. So these will for example be referenced when creating foreign key references between tables. A primary key should therefore under normal circumstances never contain values that have any meaining in your domain (often automatically incremential fields are used for this).

A unique constraint is just a way of enforcing domain specific business rules in your database schema.

Becuase a PK it is an identifier for the record, you can never change the value of a primary key.