What are database constraints

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What is a clear definition of database constraint   Why are constraints important for a database   What are the types of constraints

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Constraints dictate what values are valid for data in the database. For example, you can enforce the a value is not null (a NOT NULL constraint), or that it exists as a unique constraint in another table (a FOREIGN KEY constraint), or that it's unique within this table (a UNIQUE constraint or perhaps PRIMARY KEY constraint depending on your requirements). More general constraints can be implemented using CHECK constraints.

The MSDN documentation for SQL Server 2008 constraints is probably your best starting place.

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Constraints are part of a database schema definition    A constraint is usually associated with a table and is created with a CREATE CONSTRAINT or CREATE ASSERTION SQL statement   They define certain properties that data in a database must comply with  They can apply to a column  a whole table  more than one table or an entire schema  A reliable database system ensures that constraints hold at all times  except possibly inside a transaction  for so called deferred constraints    Common kinds of constraints are    not null - each value in a column must not be NULL unique   - value s  in specified column s  must be unique for each row in a table primary key - value s  in specified column s  must be unique for each row in a table and not be NULL  normally each table in a database should have a primary key - it is used to identify individual records foreign key - value s  in specified column s  must reference an existing record in another table  via it s primary key or some other unique constraint  check    - an expression is specified  which must evaluate to true for constraint to be satisfied

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Constraints are nothing but the rules on the data  What data is valid and what is invalid can be defined using constraints  So  that integrity of data can be maintained  Following are the widely used constraints    Primary Key   which uniquely identifies the data   If this constraint has been specified for certain column then we can t enter duplicate data in that column Check   Such as NOT NULL   Here we can specify what data we can enter for that particular column and what is not expected for that column  Foreign key   Foreign key references to the row of other table  So that data referred in one table from another table is always available for the referencing table

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To understand why we need constraints  you must first understand the value of data integrity   Data Integrity refers to the validity of data  Are your data valid  Are your data representing what you have designed them to   What weird questions I ask you might think  but sadly enough all too often  databases are filled with garbage data  invalid references to rows in other tables  that are long gone    and values that doesn t mean anything to the business logic of your solution any longer   All this garbage is not alone prone to reduce your performance  but is also a time-bomb under your application logic that eventually will retreive data that it is not designed to understand   Constraints are rules you create at design-time that protect your data from becoming corrupt  It is essential for the long time survival of your heart child of a database solution  Without constraints your solution will definitely decay with time and heavy usage   You have to acknowledge that designing your database design is only the birth of your solution  Here after it must live for  hopefully  a long time  and endure all kinds of  strange  behaviour by its end-users  ie  client applications   But this design-phase in development is crucial for the long-time success of your solution  Respect it  and pay it the time and attention it requires   A wise man once said   Data must protect itself    And this is what constraints do  It is rules that keep the data in your database as valid as possible   There are many ways of doing this  but basically they boil down to    Foreign key constraints is probably the most used constraint  and ensures that references to other tables are only allowed if there actually exists a target row to reference  This also makes it impossible to break such a relationship by deleting the referenced row creating a dead link  Check constraints can ensure that only specific values are allowed in certain column  You could create a constraint only allowing the word  Yellow  or  Blue  in a VARCHAR column  All other values would yield an error  Get ideas for usage of check constraints check the sys check constraints view in the AdventureWorks sample database Rules in SQL Server are just reusable Check Constraints  allows you to maintain the syntax from a single place  and making it easier to deploy your constraints to other databases    As I ve hinted here  it takes some thorough considerations to construct the best and most defensive constraint approach for your database design  You first need to know the possibilities and limitations of the different constraint types above  Further reading could include   FOREIGN KEY Constraints - Microsoft   Foreign key constraint - w3schools    CHECK Constraints  Good luck

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Constraints can be used to enforce specific properties of data  A simple example is to limit an int column to values  0-100000   This introduction looks good

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There are basically 4 types of main constraints in SQL    Domain Constraint  if one of the attribute values provided for a new tuple is not of the specified attribute domain Key Constraint  if the value of a key attribute in a new tuple already exists in another tuple in the relation Referential Integrity  if a foreign key value in a new tuple references a primary key value that does not exist in the referenced relation Entity Integrity  if the primary key value is null in a new tuple

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UNIQUE constraint  of which a PRIMARY KEY constraint is a variant   Checks that all values of a given field are unique across the table  This is X-axis constraint  records  CHECK constraint  of which a NOT NULL constraint is a variant   Checks that a certain condition holds for the expression over the fields of the same record  This is Y-axis constraint  fields  FOREIGN KEY constraint  Checks that a field s value is found among the values of a field in another table  This is Z-axis constraint  tables

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A database is the computerized logical representation of a conceptual  or business  model  consisting of a set of informal business rules  These rules are the user-understood meaning of the data  Because computers comprehend only formal representations  business rules cannot be represented directly in a database  They must be mapped to a formal representation  a logical model  which consists of a set of integrity constraints  These constraints     the database schema     are the logical representation in the database of the business rules and  therefore  are the DBMS-understood meaning of the data  It follows that if the DBMS is unaware of and or does not enforce the full set of constraints representing the business rules  it has an incomplete understanding of what the data means and  therefore  cannot guarantee  a  its integrity by preventing corruption   b  the integrity of inferences it makes from it  that is  query results      this is another way of saying that the DBMS is  at best  incomplete    Note  The DBMS-   understood    meaning     integrity constraints     is not identical to the user-understood meaning     business rules     but  the loss of some meaning notwithstanding  we gain the ability to mechanize logical inferences from the data    An Old Class of Errors  by Fabian Pascal

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constraints are conditions  that can validate specific condition  Constraints related with database are Domain integrity  Entity integrity  Referential Integrity  User Defined Integrity constraints etc

[sql] What are database constraints?

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