Constraints in SQL

4 Key excerpts on "Constraints in SQL"

• 

  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.

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  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.

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  eBook - ePub

  Web Applications with Javascript or Java

  • 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.

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  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:

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  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

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  eBook - ePub

  IBM Db2 11.1 Certification Guide

  Explore techniques to master database programming and administration tasks in IBM Db2

  • Mohankumar Saraswatipura, Robert Collins(Authors)
  • 2018(Publication Date)
  • Packt Publishing

    (Publisher)

  Implementing Business Rules

  This chapter will prepare you for creating and using constraints to enforce business rules at the database layer. You will also learn how to use triggers and constraints to enforce data integrity rules. This chapter will introduce you to the key system catalog tables, which describe the physical and logical structure of the data stored in the database.

  After the completion of this chapter, you will be able to demonstrate the ability to:

  • Understand various constraints, such as NOT NULL, DEFAULT, CHECK, UNIQUE, referential integrity, and NOT ENFORCED informational constraints and to determine when and how to use them
  • Create views with CHECK OPTION
  • Use the SET INTEGRITY command
  • Create and use triggers
  • Understand constraint behavior in a column-organized database
  • Examine the content of the system catalog tables

  Certification test: Number of questions: 6 Percentage in the exam: 10%

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  Business rules

  A business rule is a statement that defines some characteristics of the business. In most businesses, the data must adhere to a specific set of rules and restrictions such as an employee having a valid employee ID. These restrictions can be effectively implemented at the database layer using constraints.

  The types of constraints available within Db2 are:

  • NOT NULL
  • DEFAULT
  • CHECK
  • UNIQUE
  • NOT ENFORCED informational
  • Referential integrity

  The constraints are usually defined during table creation, however they can also be added to existing tables by using the ALTER TABLE statement.

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  NOT NULL constraints

  In Db2, you can use the NULL value to represent an unknown state or missing information for a column. By default, every column in a table will accept a NULL value. However, some business rules might dictate that a value must always be provided for a column. In those situations, the NOT NULL constraint can be used to ensure that a given column of a table is never assigned the NULL value. Once a NOT NULL constraint has been defined on a column or set of columns, any INSERT or UPDATE operation that attempts to insert a NULL value into that column will fail. The following diagram shows how to use the NOT NULL constraint to avoid NULL values:

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