Constraints in SQL

12 Key excerpts on "Constraints in SQL"

• 

  eBook - PDF

  SQL: 1999

  Understanding Relational Language Components

  • Jim Melton, Alan R. Simon(Authors)
  • 2001(Publication Date)
  • Morgan Kaufmann

    (Publisher)

  Constraints, Assertions, and Referential Integrity Introduction Your organization's databases are only as good as the integrity of the data con- tained within their tables and records. That is, the reliability and consistency of the data is critical to your applications, decision support and query tools, and other aspects of your information systems environment that utilize your data- base. Outright errors in the data are an obvious problem, although not the only one. Missing information, lost linkages among data elements, and other prob- lems can result in incorrect reports and can lead the database users to lose confi- dence in the overall database environment. The integrity of the data can be ensured by applying constraints, or rules, to the structure of the database and its contents. There are two primary methods by which these constraints can be specified, applied, and enforced: 9 through the applications programs and software systems that use the data- base 9 through the database management system itself Traditionally, application programs have been the medium through which such constraints have been applied. Examples include 355 356 Chapter 10 Constraints, Assertions, and Referential Integrity 9 Verifying that an input value for a particular data item is within the correct range of values (e.g., a new employee's age must be between 16 and 90, and hourly wage must be greater than the legal minimum wage rate) or is from the correct list of values (e.g., a new movie's type can only be CHILDREN, HORROR, COMEDY, OTHER). 9 Verifying that the relationship between two or more values is correct (e.g., comparing the number of copies of a particular CD that we have on hand with the number requested in an incoming order). The ever-growing use of relational database management systems intro- duced the concept of database-enforced constraints to many information systems users.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Handbook of Data Management

  1999 Edition

  • Sanjiv Purba(Author)
  • 2019(Publication Date)
  • Auerbach Publications

    (Publisher)

  Chapter 16 , “Data Quality: An Architectural Solution,” discusses the process of integrating data from multiple data sources into a data warehouse while maintaining data quality. This chapter provides advice on resolving problems such as the existence of data from multiple data sources that are inconsistent or from data stores that utilize different data models. Such differences must be resolved when building a data warehouse.

  Passage contains an image

  Chapter 14 Programming Data Constraints using Microsoft SQL Server

  M

  ICROSOFT® STRUCTURED QUERY LANGUAGE (SQL) SERVER

  was one of the first widely released database servers to support the functionality of coding data constraints directly inside the data dictionary itself — as far back as 1987 in colloboration with Sybase. These constraints are attached to tables or table fields with the ANSI ‘98 Data Definition Language (DDL) commands, “create table” and “alter table.” This chapter provides examples of data scripts that will support the following:

  • Identity Constraints: These constraints can be applied to a single column in a table so that the value in the field is incremented automatically when records are inserted.
  • Key Constraints: SQL Server supports two types of key constraints, namely “Primary” key and “Foreign” key constraints. Primary key constraints ensure that duplicate records are not inserted into a table. Foreign key constraints ensure that records are not inserted into a table if a foreign key is missing.
  • Default Constraints: Default contracts are associated with specific columns in tables. If a record is inserted into a table without a data value for a column that has an associated default constraint, the default value is saved in the column with the rest of the record. Default values can be overridden at insert time by a user-supplied data value.

  Create Table Constraints

  The table creation script developed in the “Database Servers and Universal Servers” section did not take advantage of table constraints or related features. These are used to enforce table data integrity or to provide additional functionality that would otherwise need to be programmed at the client side or within stored procedures. There are several schools of thought regarding the use of these features. From a performance and practicality perspective, these constraints and features are highly useful. From a pure object-oriented perspective, they separate logic from data so that some classes are no longer pure. The author’s preference is to make use of the constraints on a case-by-case basis. Constraints use the “sysconstraints” and the “sys-references” system tables in the Microsoft SQL Server environment.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Oracle SQL

  Jumpstart with Examples

  • Gavin JT Powell, Carol McCullough-Dieter(Authors)
  • 2004(Publication Date)
  • Digital Press

    (Publisher)

  447 20 Constraints In this chapter: What is Referential Integrity? What are constraints and how are they used to implement Referential Integrity? How are constraints placed into tables? What else can constraints do? This chapter shows you how to define constraints on tables. Constraints perform validation on data both within and between tables and, to a lesser degree, with views. Primary and foreign key constraints are probably the most significant of constraints. These constraints can be used to enforce the validity (integrity) of relationships (references) between rows in tables (entities). A primary key reference is placed on a unique identifying column in a superset entity. A foreign key is placed on a subset table, which contains a copy of the primary key value from the superset entity. Oracle uses primary and foreign key con-straints to validate values between superset and subset tables. Referential Integrity can be enforced using constraints or triggers, or can be coded at the application level. My personal recommendation is to use constraints. Note: A superset contains a subset. The superset contains a primary key and the subset contains a foreign key. A foreign key can have many iterations of a primary key. So let’s move on to looking at constraints in detail. 448 20.1 What Are Constraints? 20.1 What Are Constraints? A constraint , as the word implies, constrains or applies a rule to an object or a part of an object. In Oracle Database 10 g , constraints are used to restrict values in tables or make validation checks on one or more columns in a table, or check values between columns in different tables. 20.1.1 Types and Uses of Constraints There are two levels of constraints you can place on relational tables: Inline Constraint . A constraint that applies to an individual column in a table. Out-of-Line Constraint . A constraint that applies to a table as a whole or to multiple columns in a table.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Dynamic and Mobile GIS

  Investigating Changes in Space and Time

  • Roland Billen, Elsa Joao, David Forrest(Authors)
  • 2006(Publication Date)
  • CRC Press

    (Publisher)

  Within the context of VR systems, constraints are often implemented as the behaviour of objects. An illustration is the constraint ‘two trees (objects) cannot grow on the same location’, which is realised (hard coded in the edit environment) by collision detection, a well-known computer graphics technique. Referential integrity, topological correctness and collision detection are just a few examples of constraint types, but the available solutions may only work in certain subsystems. Other subsystems may not be aware of these and may have different ‘opinions’ of correct data. So constraints must be implemented at various levels (or subsystems), including application (edit, simulate,…) level, data exchange (communication) level and database level. Although support for integrity constraints is patchy, there has been some research in this area. Primarily, integrity constraints are related to data quality (Hunter, 1996) and the source of errors (Collins and Smith, 1994) such as during data collection, data input, data storage, data manipulation, data output and the use of results. Cockcroft (1997) was one of the first researchers presenting a taxonomy of (spatial) integrity constraints. A contribution of the current chapter is a refinement of this taxonomy. Cockcroft (2004) advocated an integrated approach to handling integrity, based on a repository that contains the model together with the constraints. Cockcoft (2004) concluded that the constraints should be part of the object class definition, similar to other aspects of the definition. The repository is used both by the database and the application as a consistent source of integrity constraints. The current chapter continues these investigations into the possibilities of managing constraints in an integrated system-wide manner and adds data communication as an additional part of the system where constraints are important.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  The SQL Workshop

  A New, Interactive Approach to Learning SQL

  • Frank Solomon, Prashanth Jayaram, Awni Al Saqqa(Authors)
  • 2019(Publication Date)
  • Packt Publishing

    (Publisher)

  referential constraints . Referential constraints are important internal database objects that help maintain data integrity.

  Primary Key Constraints

  A primary key constraint on a column instructs the database engine to keep the entries in a column unique. For example, if we were to create a table with information about all the human beings on Earth, we could use the tongue print of human beings as unique identification. If tongue prints were in a column, it would be the primary key.

  It is possible to have a duplicate tongue print; however, it is rare. In such a case, you could create a primary key across multiple columns. Therefore, you could combine the tongue print, fingerprint, and the retinal signature to make a primary key. In such a case, the combination of these values in these columns should be unique across the table. In other words, there may be a duplicate tongue print, a duplicate fingerprint, and a duplicate retinal signature in the table—the database engine will allow that. However, there cannot be a duplicate combination of all three. Alternatively, there can be no two human beings whose tongue prints, fingerprints, and retinal signatures exactly match. This is called a composite primary key .

  Foreign Key Constraints

  Let's look at this in the context of a primary key. When this primary key is referenced by a column in another table, this primary key becomes the foreign key of the other table. For example, consider the previously created database PACKT_ONLINE_SHOP

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  SQL in 7 Days

  A Quick Crash Course in Manipulating Data, Databases Operations, Writing Analytical Queries, and Server-Side Programming (English Edition)

  • Alex Bolenok(Author)
  • 2022(Publication Date)
  • BPB Publications

    (Publisher)

  If we define a foreign key constraint on this table, the situation changes. Now, there is only a subset of integer numbers that this field will accept. The integers outside this subset are deemed incorrect by the system. The system would reject them, just as it would reject a date, string, or any other non-integer type.

  In a sense, by putting a foreign key constraint on a field, we are defining a new type, which is narrower than the original integer type.

  From the entity-relationship model's point of view, this constraint ensures the validity of the relationship. If the entity modeled by this table is in a one-to-one, one-to-many or many-to-many relationship with some other entity, we can make sure that this other entity's record is present in the database.

  Check

  Check constraints are defined using some predicate over fields in a single record. If this predicate is false, the record would be rejected. You can use the check constraints to reject records that do not have a real-world meaning.

  NOT NULL is, in fact, a special case of a check constraint. If you have an integer field foo and do not want null values in it, you can define it as foo INT CHECK (foo IS NOT NULL) to the same effect. The fact that null values or lack thereof are so ubiquitous and play such a special role in SQL called for a short-hard syntax for such a constraint.

  Digression: constraints and relational model

  SQL has built-in syntax for constraints, but constraints do not really belong to the relational model. This is the higher-level, real-world entity-relationship model spilling down to the lower level. You can totally have records for entities that have duplicates, violate physical laws, and are related to non-existing entities, and you can still be able to write meaningful, real-world queries.

  Remember, the relational model allows us to define all the relationships at runtime. We can just use these constraints in our queries. We can handle the duplicates using set operators and DISTINCT , move the check constraints into the WHERE

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Database

  Principles Programming Performance

  • Patrick O'Neil(Author)
  • 2014(Publication Date)
  • Morgan Kaufmann

    (Publisher)

  While the constraint makes the update refusal auto-matic, the alternative action to be performed must be supplied by the appli-cation programmer. As another example, we might have a rule to accept only valid name and address data from order entry clerks—for example, to guarantee that the zip code is correct for the state named. (Zip codes do not cross state borders—this constraint could be implemented using refer-ential integrity.) If such a check fails, however, we wouldn't want to simply refuse to accept the data. We would presumably want to call a routine that asked the clerk to re-enter the information, and if the error is repeated, accept it provisionally, possibly in a separate table. After all, data that is not totally accurate may still be of some value to an enterprise, and it might be possible for later analysis to correct the entry. In both of these examples, specifying the test for the non-procedural constraint is the sim-plest part of the job. The hard part is specifying the course to take if the test fails; clearly a procedural approach is needed for this. 6.1 Integrity Constraints Guaranteeing Transactional Consistency. Another common example of 413 an important constraint is that of consistency, one of the guarantees made to the programmer in update transactions. For example, in moving money between different account records in a commercial transaction, a consis-tency rule might be that money should be neither created nor destroyed. Application programming managers who are responsible for such update transactions would be extremely happy about a constraint capability to catch subtle consistency-breaking program bugs at runtime. However, no major commercial product provides constraints that apply to interactive effects between two or more updates at the end of a transaction.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Web Applications with Javascript or Java

  Volume 1: Constraint Validation, Enumerations, Special Datatypes

  • Gerd Wagner, Mircea Diaconescu(Authors)
  • 2017(Publication Date)
  • De Gruyter Oldenbourg

    (Publisher)

  Part IIConstraint Validation

  For catching various cases of flawed data, we need to define suitable integrity constraints that can be used by the application’s data validation mechanisms. Integrity constraints may take many different forms. The most important type of integrity constraints are property constraints , which define conditions on the admissible property values of an object.

  Passage contains an image

  7Integrity Constraints and Data Validation

  7.1Introduction

  For detecting non-admissible and inconsistent data and for preventing such data to be added to an application’s database, we need to define suitable integrity constraints that can be used by the application’s data validation mechanisms for catching these cases of flawed data. Integrity constraints are logical conditions that must be satisfied by the data entered by a user and stored in the application’s database.

  For instance, if an application is managing data about persons including their birth dates and their death dates, then we must make sure that for any person record with a death date, this date is not before that person’s birth date.

  Since integrity maintenance is fundamental in database management,the data definition language part of the relational database language SQL supports the definition of integrity constraints in various forms. On the other hand, however, there is hardly any support for integrity constraints and data validation in common programming languages such as PHP, Java, C# or JavaScript. It is therefore important to take a systematic approach to constraint validation in web application engineering, like choosing an application development framework that provides sufficient support for it.

  Unfortunately, many web application development frameworks do not provide sufficient support for defining integrity constraints and performing data validation. Integrity constraints should be defined in one (central) place in an app, and then be used for configuring the user interface and for validating data in different parts of the app, such as in the user interface and in the database. In terms of usability, the goals should be:

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Understanding Databases

  Concepts and Practice

  • Suzanne W. Dietrich(Author)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  Figure 6.1 shows an abstract specification of column and table-level constraints. Any column or table constraint can be optionally named. The alternatives for column constraints include primary key, not null, unique, check and referential integrity. A check constraint specifies an expression that must evaluate to true for valid data. The specification of a single-attribute for- eign key uses the references clause that indicates the referenced table and column name. Recall that referential integrity specifies that if the value of a foreign key attribute is not null, then its value must appear as a primary key value in the referenced table. Therefore, if a foreign key value does not exist as a primary key value, then the database cannot allow the insert or update operation to occur; otherwise, referential integrity would be violated. There are optional clauses when specifying referential integrity that include actions to take when a primary key value is updated or deleted. The decision of what action to take depends on the database application that is being implemented. The no action on an update or delete of a primary key means that if the update/delete would result in a referential integrity violation, then the operation would not be allowed. The cascade option indicates that the update/delete of a referenced primary key would also be applied to the occurrences of the value as a foreign key. (Beware that cascading a delete is a dangerous choice and should only be used when the database application truly war- rants this behavior.) The set null action allows the update/delete of the primary key and changes any foreign key references to null, which would not violate referential integrity. Note that this option is only viable if the foreign key attribute is allowed to be null. Another alternative is to

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Learn SQL Database Programming

  Query and manipulate databases from popular relational database servers using SQL

  • Josephine Bush(Author)
  • 2020(Publication Date)
  • Packt Publishing

    (Publisher)

  constraint would allow nulls since not all people have a year of death:

  Passage contains an image

  The primary key

  The primary key is used to ensure that all values in a column are not null and unique. This key combines the unique and not null constraint properties into one key. This type of key can be applied to any data type and is used to avoid missing and duplicate data. You can only have one primary key per table.

  If you create a primary key on a table, it will force the table to have unique, not null values in that specific column. If the values don't comply, then the row will not be able to be inserted or updated. You can also create a primary key on multiple columns. This is considered a composite key. In this case, the composite key would have to be unique for each row, otherwise the row could not be inserted or updated.

  In the following screenshot, the playerID constraint would be the primary key because it's unique and not null for every row in the table:

  In the following screenshot, the playerID, yearID, and teamID constraints could be the composite primary key because the combination of those three columns is unique and not null for every row in the table:

  Passage contains an image

  Referential integrity

  Referential integrity refers to the consistency and accuracy between tables that can be linked together. By having a primary key on the parent table and a foreign key on the child table, you achieve referential integrity. A foreign key on the child table creates a link between one or more columns in the child table and the primary key on the parent table. When a foreign key is present, it must reference a valid, existing primary key in the parent table. This way, the data in both tables can maintain a proper relationship. You will learn more about this in the following example.

  If you don't set up referential integrity, you wind up with orphaned records. For example, let's say that you delete a player from the first table here:

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Relational Database Design and Implementation

  • Jan L. Harrington(Author)
  • 2016(Publication Date)
  • Morgan Kaufmann

    (Publisher)

  There are additional constraints that you can place on columns in a table beyond primary and foreign key constraints. These include requiring unique values and predicates in CHECK clauses.

  Requiring Unique Values

  If you want to ensure that the values in a non-primary key column are unique, then you can use the UNIQUE keyword. UNIQUE verifies that all non-null values are unique. For example, if you were storing social security numbers in an employee table that used an employee ID as the primary key, you could also enforce unique social security numbers with

  The UNIQUE clause can also be placed at the end of the CREATE TABLE statement, along with the primary key and foreign key specifications. In that case, it takes the form

  Check Clauses

  The CHECK clause to which you were introduced earlier in the chapter, in the “Domains” section, can also be used with individual columns to declare column-specific constraints. To add a constraint, you place a CHECK clause after the column declaration, using the keyword VALUE in a predicate to indicate the value being checked.

  For example, to verify that a column used to hold true-false values is limited to T and F, you could write a CHECK clause as

  Modifying Database Elements

  With the exception of tables, database elements are largely unchangeable. When you want to modify them, you must delete them from the database and create them from scratch. In contrast, just about every characteristic of a table can be modified without deleting the table, using the ALTER TABLE statement.

  Adding Columns

  To add a new column to a table, use the ALTER TABLE statement with the following syntax:

  For example, if Antique Opticals wanted to add a telephone number column to the producer table, they would use

  To add more than one column at the same time, simply separate the clauses with commas:

  Adding Table Constraints

  You can add table constraints, such as foreign keys, at any time. To do so, include the new constraint in an ALTER TABLE statement:

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  OCA: Oracle Database 12c Administrator Certified Associate Study Guide

  Exams 1Z0-061 and 1Z0-062

  • Biju Thomas(Author)
  • 2014(Publication Date)
  • Sybex

    (Publisher)

  Chapter 7 Creating Tables and Constraints

  Oracle Database 12c: SQL Fundamentals exam objectives covered in this chapter:

  • Introduction to Data Definition Language
    • Categorize the main database objects.
    • Explain the table structure.
    • Describe the data types that are available for columns.
    • Create a simple table.
    • Explain how constraints are created at the time of table creation.
    • Describe how schema objects work.

  An Oracle database has many different types of objects. Related objects are logically grouped together in a schema, which consists of various types of objects. The basic types of objects in an Oracle database are tables, indexes, constraints, sequences, and synonyms. Although this chapter discusses tables and constraints, we will begin it with an overview of the main database objects in Oracle.

  The table is the basic structure of data storage in Oracle. A table has columns as part of the definition and stores rows of data. In a relational database, the data in various tables may be related. A constraint can be considered as a rule or policy defined in the database to enforce data integrity and business rules. In this chapter, we will discuss creating tables and using constraints. Because the table is the most important type of object in an Oracle database, knowing how to create tables and constraints on tables is important.

  Database Objects Overview

  Data in an Oracle database is stored in tables. A table is the main database object. Many other database objects, whether or not they store data, are generally based on the tables.

  Figure 7-1 shows a screenshot from SQL Developer. The left side shows the various object types available in Oracle Database 12c .

  Let’s review the main database objects in Oracle that are relevant for this certification exam:

  1. Table A table is defined with columns, and it stores rows of data. A table should have at least one column. In Oracle, a table normally refers to a relational table. You can also create object tables. Object tables are created with user-defined datatypes. Temporary tables (called global temporary tables in Oracle) are used to hold temporary data specific to a transaction or session. A table can store a wide variety of data. Apart from storing text and numeric information, you can store date, timestamp, binary, or raw data (such as images, documents, and information about external files). A table can have virtual columns

  Sign up to read

  Learn more about book