Can I have multiple primary keys in a single table

Question

User · Answer

Having two primary keys at the same time, is not possible. But (assuming that you have not messed the case up with composite key), may be what you might need is to make one attribute unique.

CREATE t1(
c1 int NOT NULL,
c2 int NOT NULL UNIQUE,
...,
PRIMARY KEY (c1)
);

However note that in relational database a 'super key' is a subset of attributes which uniquely identify a tuple or row in a table. A 'key' is a 'super key' that has an additional property that removing any attribute from the key, makes that key no more a 'super key'(or simply a 'key' is a minimal super key). If there are more keys, all of them are candidate keys. We select one of the candidate keys as a primary key. That's why talking about multiple primary keys for a one relation or table is being a conflict.

User · Answer

A primary key is the key that uniquely identifies a record and is used in all indexes  This is why you can t have more than one  It is also generally the key that is used in joining to child tables but this is not a requirement  The real purpose of a PK is to make sure that something allows you to uniquely identify a record so that data changes affect the correct record and so that indexes can be created   However  you can put multiple fields in one primary key  a composite PK   This will make your joins slower  espcially if they are larger string type fields  and your indexes larger but it may remove the need to do joins in some of the child tables  so as far as performance and design  take it on a case by case basis  When you do this  each field itself is not unique  but the combination of them is  If one or more of the fields in a composite key should also be unique  then you need a unique index on it  It is likely though that if one field is unique  this is a better candidate for the PK   Now at times  you have more than one candidate for the PK  In this case you choose one as the PK or use a surrogate key  I personally prefer surrogate keys for this instance   And  this is critical   you add unique indexes to each of the candidate keys that were not chosen as the PK  If the data needs to be unique  it needs a unique index whether it is the PK or not  This is a data integrity issue   Note this is also true anytime you use a surrogate key  people get into trouble with surrogate keys because they forget to create unique indexes on the candidate keys    There are occasionally times when you want more than one surrogate key  which are usually the PK if you have them   In this case what you want isn t more PK s  it is more fields with autogenerated keys  Most DBs don t allow this  but there are ways of getting around it   First consider if the second field could be calculated based on the first autogenerated key  Field1   -1 for instance  or perhaps the need for a second autogenerated key really means you should create a related table  Related tables can be in a one-to-one relationship  You would enforce that by adding the PK from the parent table to the child table and then adding the new autogenerated field to the table and then whatever fields are appropriate for this table  Then choose one of the two keys as the PK and put a unique index on the other  the autogenerated field does not have to be a PK   And make sure to add the FK to the field that is in the parent table  In general if you have no additional fields for the child table  you need to examine why you think you need two autogenerated fields

User · Answer

Some people use the term  primary key  to mean exactly an integer column that gets its values generated by some automatic mechanism   For example AUTO INCREMENT in MySQL or IDENTITY in Microsoft SQL Server   Are you using primary key in this sense   If so  the answer depends on the brand of database you re using   In MySQL  you can t do this  you get an error   mysql gt  create table foo     id int primary key auto increment     id2 int auto increment    ERROR 1075  42000   Incorrect table definition   there can be only one auto column and it must be defined as a key   In some other brands of database  you are able to define more than one auto-generating column in a table

User · Answer

A Table can have a Composite Primary Key which is a primary key made from two or more columns  For example   CREATE TABLE userdata     userid INT    userdataid INT    info char 200     primary key  userid  userdataid       Update  Here is a link with a more detailed description of composite primary keys

User · Answer

A table can have multiple candidate keys   Each candidate key is a column or set of columns that are UNIQUE  taken together  and also NOT NULL   Thus  specifying values for all the columns of any candidate key is enough to determine that there is one row that meets the criteria  or no rows at all   Candidate keys are a fundamental concept in the relational data model   It s common practice  if multiple keys are present in one table  to designate one of the candidate keys as the primary key   It s also common practice to cause any foreign keys to the table to reference the primary key  rather than any other candidate key     I recommend these practices  but there is nothing in the relational model that requires selecting a primary key among the candidate keys

User · Answer

You can only have one primary key  but you can have multiple columns in your primary key   You can also have Unique Indexes on your table  which will work a bit like a primary key in that they will enforce unique values  and will speed up querying of those values

User · Answer

Good technical answers were given in better way than I can do  I am only can add to this topic   If you want something that not allowed acceptable it is good reason to take step back    Understand the core of why it s not acceptable  Dig more in documentation journal articles web and etc  Analyze review current design and point major flaws  Consider and test every step during new design  Always look forward and try to create adaptive solution    Hope it will helps someone

User · Answer

This is the answer for both the main question and for  Kalmi s question of      What would be the point of having multiple auto-generating columns    This code below has a composite primary key  One of its columns is auto-incremented  This will work only in MyISAM  InnoDB will generate an error  ERROR 1075  42000   Incorrect table definition  there can be only one auto column and it must be defined as a key    DROP TABLE IF EXISTS  test   animals   CREATE TABLE   test   animals       grp  char 30  NOT NULL     id  mediumint 9  NOT NULL AUTO INCREMENT     name  char 30  NOT NULL    PRIMARY KEY   grp   id     ENGINE MyISAM   INSERT INTO animals  grp name  VALUES       mammal   dog     mammal   cat          bird   penguin     fish   lax     mammal   whale          bird   ostrich     SELECT   FROM animals ORDER BY grp id   Which returns    -------- ---- ---------    grp      id   name       -------- ---- ---------    fish      1   lax         mammal    1   dog         mammal    2   cat         mammal    3   whale       bird      1   penguin     bird      2   ostrich    -------- ---- ---------

User · Answer

Have been studying these  a lot   Candidate keys - A minimal column combination required to uniquely identify a table row  Compound keys - 2 or more columns    Multiple Candidate keys can exist in a table    Primary KEY - Only one of the candidate keys that is chosen by us Alternate keys - All other candidate keys   Both Primary Key  amp  Alternate keys can be Compound keys     Sources  https   en wikipedia org wiki Superkey https   en wikipedia org wiki Candidate key https   en wikipedia org wiki Primary key https   en wikipedia org wiki Compound key

User · Answer

Some people use the term  primary key  to mean exactly an integer column that gets its values generated by some automatic mechanism   For example AUTO INCREMENT in MySQL or IDENTITY in Microsoft SQL Server   Are you using primary key in this sense   If so  the answer depends on the brand of database you re using   In MySQL  you can t do this  you get an error   mysql gt  create table foo     id int primary key auto increment     id2 int auto increment    ERROR 1075  42000   Incorrect table definition   there can be only one auto column and it must be defined as a key   In some other brands of database  you are able to define more than one auto-generating column in a table

User · Answer

A table can have multiple candidate keys   Each candidate key is a column or set of columns that are UNIQUE  taken together  and also NOT NULL   Thus  specifying values for all the columns of any candidate key is enough to determine that there is one row that meets the criteria  or no rows at all   Candidate keys are a fundamental concept in the relational data model   It s common practice  if multiple keys are present in one table  to designate one of the candidate keys as the primary key   It s also common practice to cause any foreign keys to the table to reference the primary key  rather than any other candidate key     I recommend these practices  but there is nothing in the relational model that requires selecting a primary key among the candidate keys

User · Answer

You can only have one primary key  but you can have multiple columns in your primary key   You can also have Unique Indexes on your table  which will work a bit like a primary key in that they will enforce unique values  and will speed up querying of those values

User · Answer

Have been studying these  a lot   Candidate keys - A minimal column combination required to uniquely identify a table row  Compound keys - 2 or more columns    Multiple Candidate keys can exist in a table    Primary KEY - Only one of the candidate keys that is chosen by us Alternate keys - All other candidate keys   Both Primary Key  amp  Alternate keys can be Compound keys     Sources  https   en wikipedia org wiki Superkey https   en wikipedia org wiki Candidate key https   en wikipedia org wiki Primary key https   en wikipedia org wiki Compound key

User · Answer

Primary Key is very unfortunate notation  because of the connotation of  Primary  and the subconscious association in consequence with the Logical Model  I thus avoid using it  Instead I refer to the Surrogate Key of the Physical Model and the Natural Key s  of the Logical Model    It is important that the Logical Model for every Entity have at least one set of  business attributes  which comprise a Key for the entity  Boyce  Codd  Date et al refer to these in the Relational Model as Candidate Keys  When we then build tables for these Entities their Candidate Keys become Natural Keys in those tables  It is only through those Natural Keys that users are able to uniquely identify rows in the tables  as surrogate keys should always be hidden from users  This is because Surrogate Keys have no business meaning   However the Physical Model for our tables will in many instances be inefficient without a Surrogate Key  Recall that non-covered columns for a non-clustered index can only be found  in general  through a Key Lookup into the clustered index  ignore tables implemented as heaps for a moment   When our available Natural Key s  are wide this  1  widens the width of our non-clustered leaf nodes  increasing storage requirements and read accesses for seeks and scans of that non-clustered index  and  2  reduces fan-out from our clustered index increasing index height and index size  again increasing reads and storage requirements for our clustered indexes  and  3  increases cache requirements for our clustered indexes  chasing other indexes and data out of cache   This is where a small Surrogate Key  designated to the RDBMS as  the Primary Key  proves beneficial  When set as the clustering key  so as to be used for key lookups into the clustered index from non-clustered indexes and foreign key lookups from related tables  all these disadvantages disappear  Our clustered index fan-outs increase again to reduce clustered index height and size  reduce cache load for our clustered indexes  decrease reads when accessing data through any mechanism  whether index scan  index seek  non-clustered key lookup or foreign key lookup  and decrease storage requirements for both clustered and nonclustered indexes of our tables   Note that these benefits only occur when the surrogate key is both small and the clustering key  If a GUID is used as the clustering key the situation will often be worse than if the smallest available Natural Key had been used  If the table is organized as a heap then the 8-byte  heap  RowID will be used for key lookups  which is better than a 16-byte GUID but less performant than a 4-byte integer    If a GUID must be used due to business constraints than the search for a better clustering key is worthwhile  If for example a small site identifier and 4-byte  site-sequence-number  is feasible then that design might give better performance than a GUID as Surrogate Key    If the consequences of a heap  hash join perhaps  make that the preferred storage then the costs of a wider clustering key need to be balanced into the trade-off analysis   Consider this example    ALTER TABLE Persons ADD CONSTRAINT pk PersonID PRIMARY KEY  P Id LastName    where the tuple   P Id LastName   requires a uniqueness constraint  and may be a lengthy Unicode LastName plus a 4-byte integer  it would be desirable to  1  declaratively enforce this constraint as  ADD CONSTRAINT pk PersonID UNIQUE NONCLUSTERED  P Id LastName   and  2  separately declare a small Surrogate Key to be the  Primary Key  of a clustered index  It is worth noting that Anita possibly only wishes to add the LastName to this constraint in order to make that a covered field  which is unnecessary in a clustered index because ALL fields are covered by it   The ability in SQL Server to designate a Primary Key as nonclustered is an unfortunate historical circumstance  due to a conflation of the meaning  preferred natural or candidate key   from the Logical Model  with the meaning  lookup key in storage  from the Physical Model  My understanding is that originally SYBASE SQL Server always used a 4-byte RowID  whether into a heap or a clustered index  as the  lookup key in storage  from the Physical Model

User · Answer

Yes  Its possible in SQL  but we can t set more than one primary keys in MsAccess  Then  I don t know about the other databases   CREATE TABLE CHAPTER       BOOK ISBN VARCHAR 50  NOT NULL      IDX INT NOT NULL      TITLE VARCHAR 100  NOT NULL      NUM OF PAGES INT      PRIMARY KEY  BOOK ISBN  IDX

User · Answer

Yes  Its possible in SQL  but we can t set more than one primary keys in MsAccess  Then  I don t know about the other databases   CREATE TABLE CHAPTER       BOOK ISBN VARCHAR 50  NOT NULL      IDX INT NOT NULL      TITLE VARCHAR 100  NOT NULL      NUM OF PAGES INT      PRIMARY KEY  BOOK ISBN  IDX

User · Answer

This is the answer for both the main question and for  Kalmi s question of      What would be the point of having multiple auto-generating columns    This code below has a composite primary key  One of its columns is auto-incremented  This will work only in MyISAM  InnoDB will generate an error  ERROR 1075  42000   Incorrect table definition  there can be only one auto column and it must be defined as a key    DROP TABLE IF EXISTS  test   animals   CREATE TABLE   test   animals       grp  char 30  NOT NULL     id  mediumint 9  NOT NULL AUTO INCREMENT     name  char 30  NOT NULL    PRIMARY KEY   grp   id     ENGINE MyISAM   INSERT INTO animals  grp name  VALUES       mammal   dog     mammal   cat          bird   penguin     fish   lax     mammal   whale          bird   ostrich     SELECT   FROM animals ORDER BY grp id   Which returns    -------- ---- ---------    grp      id   name       -------- ---- ---------    fish      1   lax         mammal    1   dog         mammal    2   cat         mammal    3   whale       bird      1   penguin     bird      2   ostrich    -------- ---- ---------

User · Answer

A Table can have a Composite Primary Key which is a primary key made from two or more columns  For example   CREATE TABLE userdata     userid INT    userdataid INT    info char 200     primary key  userid  userdataid       Update  Here is a link with a more detailed description of composite primary keys

User · Answer

A table can have multiple candidate keys   Each candidate key is a column or set of columns that are UNIQUE  taken together  and also NOT NULL   Thus  specifying values for all the columns of any candidate key is enough to determine that there is one row that meets the criteria  or no rows at all   Candidate keys are a fundamental concept in the relational data model   It s common practice  if multiple keys are present in one table  to designate one of the candidate keys as the primary key   It s also common practice to cause any foreign keys to the table to reference the primary key  rather than any other candidate key     I recommend these practices  but there is nothing in the relational model that requires selecting a primary key among the candidate keys

User · Answer

Some people use the term  primary key  to mean exactly an integer column that gets its values generated by some automatic mechanism   For example AUTO INCREMENT in MySQL or IDENTITY in Microsoft SQL Server   Are you using primary key in this sense   If so  the answer depends on the brand of database you re using   In MySQL  you can t do this  you get an error   mysql gt  create table foo     id int primary key auto increment     id2 int auto increment    ERROR 1075  42000   Incorrect table definition   there can be only one auto column and it must be defined as a key   In some other brands of database  you are able to define more than one auto-generating column in a table

User · Answer

You can only have one primary key  but you can have multiple columns in your primary key   You can also have Unique Indexes on your table  which will work a bit like a primary key in that they will enforce unique values  and will speed up querying of those values

User · Answer

Some people use the term  primary key  to mean exactly an integer column that gets its values generated by some automatic mechanism   For example AUTO INCREMENT in MySQL or IDENTITY in Microsoft SQL Server   Are you using primary key in this sense   If so  the answer depends on the brand of database you re using   In MySQL  you can t do this  you get an error   mysql gt  create table foo     id int primary key auto increment     id2 int auto increment    ERROR 1075  42000   Incorrect table definition   there can be only one auto column and it must be defined as a key   In some other brands of database  you are able to define more than one auto-generating column in a table

User · Answer

Yes  Its possible in SQL  but we can t set more than one primary keys in MsAccess  Then  I don t know about the other databases   CREATE TABLE CHAPTER       BOOK ISBN VARCHAR 50  NOT NULL      IDX INT NOT NULL      TITLE VARCHAR 100  NOT NULL      NUM OF PAGES INT      PRIMARY KEY  BOOK ISBN  IDX

User · Answer

Yes  Its possible in SQL  but we can t set more than one primary keys in MsAccess  Then  I don t know about the other databases   CREATE TABLE CHAPTER       BOOK ISBN VARCHAR 50  NOT NULL      IDX INT NOT NULL      TITLE VARCHAR 100  NOT NULL      NUM OF PAGES INT      PRIMARY KEY  BOOK ISBN  IDX

User · Answer

Primary Key is very unfortunate notation  because of the connotation of  Primary  and the subconscious association in consequence with the Logical Model  I thus avoid using it  Instead I refer to the Surrogate Key of the Physical Model and the Natural Key s  of the Logical Model    It is important that the Logical Model for every Entity have at least one set of  business attributes  which comprise a Key for the entity  Boyce  Codd  Date et al refer to these in the Relational Model as Candidate Keys  When we then build tables for these Entities their Candidate Keys become Natural Keys in those tables  It is only through those Natural Keys that users are able to uniquely identify rows in the tables  as surrogate keys should always be hidden from users  This is because Surrogate Keys have no business meaning   However the Physical Model for our tables will in many instances be inefficient without a Surrogate Key  Recall that non-covered columns for a non-clustered index can only be found  in general  through a Key Lookup into the clustered index  ignore tables implemented as heaps for a moment   When our available Natural Key s  are wide this  1  widens the width of our non-clustered leaf nodes  increasing storage requirements and read accesses for seeks and scans of that non-clustered index  and  2  reduces fan-out from our clustered index increasing index height and index size  again increasing reads and storage requirements for our clustered indexes  and  3  increases cache requirements for our clustered indexes  chasing other indexes and data out of cache   This is where a small Surrogate Key  designated to the RDBMS as  the Primary Key  proves beneficial  When set as the clustering key  so as to be used for key lookups into the clustered index from non-clustered indexes and foreign key lookups from related tables  all these disadvantages disappear  Our clustered index fan-outs increase again to reduce clustered index height and size  reduce cache load for our clustered indexes  decrease reads when accessing data through any mechanism  whether index scan  index seek  non-clustered key lookup or foreign key lookup  and decrease storage requirements for both clustered and nonclustered indexes of our tables   Note that these benefits only occur when the surrogate key is both small and the clustering key  If a GUID is used as the clustering key the situation will often be worse than if the smallest available Natural Key had been used  If the table is organized as a heap then the 8-byte  heap  RowID will be used for key lookups  which is better than a 16-byte GUID but less performant than a 4-byte integer    If a GUID must be used due to business constraints than the search for a better clustering key is worthwhile  If for example a small site identifier and 4-byte  site-sequence-number  is feasible then that design might give better performance than a GUID as Surrogate Key    If the consequences of a heap  hash join perhaps  make that the preferred storage then the costs of a wider clustering key need to be balanced into the trade-off analysis   Consider this example    ALTER TABLE Persons ADD CONSTRAINT pk PersonID PRIMARY KEY  P Id LastName    where the tuple   P Id LastName   requires a uniqueness constraint  and may be a lengthy Unicode LastName plus a 4-byte integer  it would be desirable to  1  declaratively enforce this constraint as  ADD CONSTRAINT pk PersonID UNIQUE NONCLUSTERED  P Id LastName   and  2  separately declare a small Surrogate Key to be the  Primary Key  of a clustered index  It is worth noting that Anita possibly only wishes to add the LastName to this constraint in order to make that a covered field  which is unnecessary in a clustered index because ALL fields are covered by it   The ability in SQL Server to designate a Primary Key as nonclustered is an unfortunate historical circumstance  due to a conflation of the meaning  preferred natural or candidate key   from the Logical Model  with the meaning  lookup key in storage  from the Physical Model  My understanding is that originally SYBASE SQL Server always used a 4-byte RowID  whether into a heap or a clustered index  as the  lookup key in storage  from the Physical Model

User · Answer

A Table can have a Composite Primary Key which is a primary key made from two or more columns  For example   CREATE TABLE userdata     userid INT    userdataid INT    info char 200     primary key  userid  userdataid       Update  Here is a link with a more detailed description of composite primary keys

User · Answer

As noted by the others it is possible to have multi-column primary keys   It should be noted however that if you have some functional dependencies that are not introduced by a key  you should consider normalizing your relation   Example   Person id  name  email  street  zip code  area    There can be a functional dependency between id - gt  name email  street  zip code and area But often a zip code is associated with a area and thus there is an internal functional dependecy between zip code - gt  area   Thus one may consider splitting it into another table   Person id  name  email  street  zip code  Area zip code  name    So that it is consistent with the third normal form

User · Answer

Good technical answers were given in better way than I can do  I am only can add to this topic   If you want something that not allowed acceptable it is good reason to take step back    Understand the core of why it s not acceptable  Dig more in documentation journal articles web and etc  Analyze review current design and point major flaws  Consider and test every step during new design  Always look forward and try to create adaptive solution    Hope it will helps someone

User · Answer

Having two primary keys at the same time, is not possible. But (assuming that you have not messed the case up with composite key), may be what you might need is to make one attribute unique.

CREATE t1(
c1 int NOT NULL,
c2 int NOT NULL UNIQUE,
...,
PRIMARY KEY (c1)
);

However note that in relational database a 'super key' is a subset of attributes which uniquely identify a tuple or row in a table. A 'key' is a 'super key' that has an additional property that removing any attribute from the key, makes that key no more a 'super key'(or simply a 'key' is a minimal super key). If there are more keys, all of them are candidate keys. We select one of the candidate keys as a primary key. That's why talking about multiple primary keys for a one relation or table is being a conflict.

User · Answer

You can only have one primary key  but you can have multiple columns in your primary key   You can also have Unique Indexes on your table  which will work a bit like a primary key in that they will enforce unique values  and will speed up querying of those values

User · Answer

A Table can have a Composite Primary Key which is a primary key made from two or more columns  For example   CREATE TABLE userdata     userid INT    userdataid INT    info char 200     primary key  userid  userdataid       Update  Here is a link with a more detailed description of composite primary keys

User · Answer

As noted by the others it is possible to have multi-column primary keys   It should be noted however that if you have some functional dependencies that are not introduced by a key  you should consider normalizing your relation   Example   Person id  name  email  street  zip code  area    There can be a functional dependency between id - gt  name email  street  zip code and area But often a zip code is associated with a area and thus there is an internal functional dependecy between zip code - gt  area   Thus one may consider splitting it into another table   Person id  name  email  street  zip code  Area zip code  name    So that it is consistent with the third normal form

User · Answer

A primary key is the key that uniquely identifies a record and is used in all indexes  This is why you can t have more than one  It is also generally the key that is used in joining to child tables but this is not a requirement  The real purpose of a PK is to make sure that something allows you to uniquely identify a record so that data changes affect the correct record and so that indexes can be created   However  you can put multiple fields in one primary key  a composite PK   This will make your joins slower  espcially if they are larger string type fields  and your indexes larger but it may remove the need to do joins in some of the child tables  so as far as performance and design  take it on a case by case basis  When you do this  each field itself is not unique  but the combination of them is  If one or more of the fields in a composite key should also be unique  then you need a unique index on it  It is likely though that if one field is unique  this is a better candidate for the PK   Now at times  you have more than one candidate for the PK  In this case you choose one as the PK or use a surrogate key  I personally prefer surrogate keys for this instance   And  this is critical   you add unique indexes to each of the candidate keys that were not chosen as the PK  If the data needs to be unique  it needs a unique index whether it is the PK or not  This is a data integrity issue   Note this is also true anytime you use a surrogate key  people get into trouble with surrogate keys because they forget to create unique indexes on the candidate keys    There are occasionally times when you want more than one surrogate key  which are usually the PK if you have them   In this case what you want isn t more PK s  it is more fields with autogenerated keys  Most DBs don t allow this  but there are ways of getting around it   First consider if the second field could be calculated based on the first autogenerated key  Field1   -1 for instance  or perhaps the need for a second autogenerated key really means you should create a related table  Related tables can be in a one-to-one relationship  You would enforce that by adding the PK from the parent table to the child table and then adding the new autogenerated field to the table and then whatever fields are appropriate for this table  Then choose one of the two keys as the PK and put a unique index on the other  the autogenerated field does not have to be a PK   And make sure to add the FK to the field that is in the parent table  In general if you have no additional fields for the child table  you need to examine why you think you need two autogenerated fields

User · Answer

A table can have multiple candidate keys   Each candidate key is a column or set of columns that are UNIQUE  taken together  and also NOT NULL   Thus  specifying values for all the columns of any candidate key is enough to determine that there is one row that meets the criteria  or no rows at all   Candidate keys are a fundamental concept in the relational data model   It s common practice  if multiple keys are present in one table  to designate one of the candidate keys as the primary key   It s also common practice to cause any foreign keys to the table to reference the primary key  rather than any other candidate key     I recommend these practices  but there is nothing in the relational model that requires selecting a primary key among the candidate keys

[database] Can I have multiple primary keys in a single table?

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