Everything You Need to Know about Relational Databases

Adatabaseis a set of data stored somewhere, organized in a schema (we’ll discuss this in more detail later on). Arelational databaseis a database that allows administrators and users to set up connections among different data records, and to use those connections to view and manage data. Adatabase management system(DBMS) is the software used to query and view the data in the database, and arelational database management system(RDBMS)软件用于做同样的relational database. While there are separate functions for the database (data storage, data schema) and the DBMS (storage management, query processing), the two parts work hand-in-glove and are generally considered a single unit.

What Are Relational Databases Used For?

Relational databases can be used for everything from simple tasks like storing artist, album, and song information for a music retailer, to more complex activities like a financial institution’s records of stock and bond trades and other transactions for its customer base.

Relational databases are a good tool for managing a large amount of information because of their built-in scalability and flexibility. They also allow you to easily add new categories to the schema, don’t require you to update the software, and won’t impact existing data.

RDBMSs allow businesses to work with managerial data, like employees or customers, as well as transactional data, like sales or purchases.Management Information Systemsare specialized implementations of databases.

RDBMSs are flexible in many ways and allow users to do the following:

• View and manipulate data to fit their own needs (they don’t restrict the user to preset views).
• Easily add or delete data categories as business needs change.

Relational Database Terms

Below are the unique terms and specific definitions that will help you understand what a RDB can do and how it works:

• Row:A set of data constituting a single item. For example, the data for a single employee (e.g. first name, last name, employee ID, hire date, work location, etc.) of a company would be displayed in a row. A row can also be called a record, an entity, or a tuple.
• Column:Labels for elements of rows. A column gives context to the information contained in rows. For an employee database, the column headers could be the items listed above for employees. A column is also known as an attribute or a field.
• Table:一群行匹配的参数设置or the table. The data in a table must all be related. An employee database may have separate tables for active employees, retired employees, and former employees. A table is also known as a relation or base revelar.
• View:A set of data based on a query via the RDBMS; also known as result set or derived revelar.
• Domain:The set of possible values for a given column. For example, the phone number and ZIP code columns would be numbers, while first and last names would be limited to letters.
• Constraint:A narrowing of a domain. For example, the domain of the work location on a employee record would be alphanumeric, but it could be restricted to a predefined list rather than being a free-form field. The phone number field would be constrained to 10 digits.
• Primary key:The unique identifier of a row in a table.
• Foreign key:The unique identifier of a row in another table.
• Distributed Database:A database that stores data in multiple locations, rather than on a single hard drive or server.

More About Keys

Primary keys and foreign keys are used to identify rows or records in a table. With a unique value to identify each record, it’s easy to build relationships between records in different tables. Rows can be in a table in any order, but are easy to find because of keys.

There are a few ways to ensure a unique value for each key when a new data record is added to a table:

• Generate One:Keys can be created based on an algorithm. It could be a random or sequential number, or based on the data in the record. An employee ID might be generated by taking the first and last initials of the employee’s name, and then adding a number. For example, Hans Henderson might get employee ID HH001, and Hiroko Hirai HH002.
• Create One:You can combine columns to create a unique key. To create a key for a list of work locations, you should be able to create a unique value by combining the name of the building and its ZIP code.
• Enter One:A user can type a value when entering the data. However, ensure uniqueness by putting business rules and data checks into place.

A key that has no relation to the data (such as a random or sequential number) is called asurrogate key. One based on the data is called anatural key.

Analternate key是一个次要的惟一标识符记录或row. These are used for sorting, filtering, or viewing data within a table, but not for creating a relationship with data in another table. Alternate keys are often formed by combining the values in several columns.

A row or rows in one table can be linked to a row or rows in other tables by adding a column that contains the key for the linked row(s), thus creating relationships. Foreign keys in the referenced table match the primary key in the base table. The foreign key in the referenced table doesn’t have to be unique, which allows the creation of one-to-many and many-to-many relationships (we’ll talk more about relationships in the next section).

Primary keys become foreign keys when they are referenced in tables other than the one where they originated.

The power of a relational database is in the links and relations. By connecting rows in different tables through the use of primary and foreign keys, you can create views, reports, and other slices of information to serve your organization. There are three primary types of database relationships:

• One-to-One:One row in one table is connected to one and only one row in another table. For example, a Social Security number is linked to a single employee.
• One-to-Many:One row in one table is connected to zero, one, or more than one rows in another table. For example, one work location can be linked to many employees.
• Many-to-Many:Zero, one, or many rows in one table are linked to zero, one, or many rows in another table. For example, multiple employees can be assigned to multiple projects.

In the examples below, data is grouped into four tables: Employee, HR Data, Work Location, and Projects.

The example tables also demonstrate three ways to have a unique primary key:

• The primary key for theEmployeetable isEmployee ID, which is generated. Two people could have the same name. A phone number is unique, but if someone moves to a new work location, that might change. A generated value is static and unique.
• TheLocationstable’s primary key is a calculated value. TheBuilding IDcolumn combines the building name and the ZIP code. Buildings in different cities might have the same name or address, so this calculated values guarantees that it’s unique.
• TheProjectstable uses theNamecolumn as the primary key, which would be entered by an employee.

The tables in this example also contain illustrations of each kind of relationship.

One-to-One Relationship:

• Employee to HR data.This could easily be added as a column to the Employee table, but for security reasons, some data is stored in a separate table. A person using the database to find all employees working in one location doesn’t need to see their salaries or SSNs.
• Building managers to locations.Because the building manager is an employee, it makes sense to link to the employee data rather than recreating it in the building table.

One-to-Many Relationship:

• Project managers to projects.Each manager can be assigned to multiple projects.
• Work locations to employees.Each location can have multiple people working there.

Many-to-Many Relationship:

• Employees to projects.Employees can be assigned to multiple projects, and each project can have multiple people working on it.

More detailed information on database relationships can be foundhere.

Using Data to Create Views

With the example tables, admins could create different views for different user groups. Views often have basic query functionality built in, so users can customize details like dates, amounts, or locations. Below are some examples of different user needs:

• Directors want to see all employees assigned to their key project, their work location, and the phone number, plus who the project manager is.

• Corporate real estate wants to see how many employees are assigned to a particular work location, plus their contact information.

• HR wants to know which employees make a certain salary or more, plus their contact information, but doesn't want to display their Social Security number.

• A VP wants to see each project, the project manager, their contact information, and their work location.

• Another VP wants to see the same information, but with the project manager as the key piece of data.

SQL (Structured Query Language)

SQL (pronounced “sequel”) is the most commonly used RDBMS. It’s based on SEQUEL (Structured English QUEry Language), a language created at IBM by Donald D. Chamberlin and Raymond F. Boyce, based on Edgar F. Codd’s relational model (more information below). In 1979, Relational Software, Inc. (later known as Oracle) released the first commercial SQL version. SQL has been in use for so long because it’s easy to learn, ubiquitous, and stable, and it performs well. There are open source variations, so the barrier to entry is low. Because it’s so popular, there is a lot of support available for people learning it or needing help.

Variations of SQL

有几个不同版本的SQL。把differences are mostly in syntax rather than function. Here is a list of common versions:

• MySQL (open source)
• SQL Server (Microsoft)
• SQLite (open source)
• Oracle Database (Oracle)

Other DBMSs

There are many DBMSs — some of these DBMS types are explored later in this article. Some popular non-relational databases include:

• PostgreSQL, an object-relational DBMS
• MongoDB, a document-oriented DBMS
• Redis, an in-memory key-value DBMS. In-memory means the data is stored in a computer’s main memory rather than on a disk, which allows for faster operations.
• Cassandra, a NoSQL DBMS

Others include IBM DB2 (relational), IBM Informix (relational), SAP Sybase Adaptive Server Enterprise (relational), SAP Sybase IQ (relational), Teradata (relational), Microsoft Access (relational), and Elasticsearch (search engine).

What Are the Most Popular DBMSs?

The answer to this question depends on whom you ask and what methodology each source uses. Here’s a review of rankings from Stack Overflow, ServerWatch, and DB-Engines:

Notes:

• The top 10 from each source are included (Stack Overflow only listed 7).
• DB-Engine uses a multiparameterrankingsystem.
• Stack Overflowsurveyedits readers.

ServerWatchdid not list its ranking criteria.

While RDBMSs are powerful and useful, they aren’t all sunshine and rainbows. The following are problems to be aware of with setup and use:

• RDBMSs can be complicated to implement.
• Performance issues can be difficult to predict, especially when the data is shared among multiple applications.
• It’s difficult to maintain data integrity when the data is shared among multiple applications.
• Different design strategies are required for operational databases versus reporting databases.

The term “database” was first used in the 1960s. The history of databases can be divided into three eras, based on the dominant data model:

• Navigational:Data is stored in files, and is accessed by navigating through a tree-like structure. There were two versions:hierarchical(data is organized in a tree structure) andnetwork(data is organized in a lattice structure).
• Relational:数据存储在表和访问通过逗号nds that display and combine records.
• Post-relational:This will be explored later in this article.

Early DBMSs were GE’s Integrated Data Store and IBM’s Information Management System.

Computer scientist Edgar F. Codd created the principles for the relational model while working at IBM in the 70s; this model led to the creation of relational databases. The model, outlined in a 1970 paper titledA Relational Model of Data for Large Shared Data Banks, says that data can be stored tuples (or rows) and grouped into relations (or tables); tuples can then be connected to tuples in other relations.

This paper led IBM to create System R in 1974, which was the first real-world application of the relational model and the first use of the language SEQUEL. The first commercial relational database was Oracle, released in 1979 by Relational Software Inc.; the company later changed its name to Oracle Systems Corporation.

Codd’s 12 Rules

In the 80s, E. F. Codd created a list of rules that defined his vision of a relational database. The list is called Codd’s 12 Rules (though in reality there are 13 because he started numbering at zero rather than one). The intent of the rules was to ensure that RDBMSs were in sync with the relational model he created. It’s hard to find a definitive source that lays out how well RDBMSs follow all the rules, but most online sources say that few commercial RDBMSs are fully compliant. Under a loose interpretation, an RDBMS can considered relational if it complies with at least two of Codd’s rules. Under stricter interpretations (by such people as database theorist Christopher J. Date and computer scientist Hugh Darween), systems that don’t comply with all rules are considered pseudo-RDBMSs.

The Third Manifesto

Date and Darween published a book in 2006 calledDatabases, Types, and The Relational Model: The Third Manifesto, now in its third edition. In their first two manifestos, they talked about their vision for databases, DBMSs, and data. In the third, they build on that and focus on the data integrity problems that arise when RDBMSs and other kinds of DBMSs are accessing the same data (this is known asimpedance mismatch). They also argue that SQL is inadequate and should be replaced.

Besides relational, there are a number of other database design models and plenty of reasons why a nonrelational database might be better. These include (but are not limited to) the following:

• A data model that isn’t supported by a relational schema
• Scalability and performance issues
• Records that don’t fit well into the relational model

Before diving into these alternative databases, a couple of terms need to be defined:

• Object:In a relational database, anobjectis a table, column, or relationship. In object-oriented models, objects can be defined by the developers to meet the needs of the business.
• Object-oriented:A modular design approach that relies on creating and reusing objects. In databases, data is organized into objects rather than tables.

Basic Functionality

The basic functions of a DBMS are to read (view data via queries), create (add data, tables, rows, or columns), update (change data, tables, rows, or columns), and delete data. Conditions can be added to perform more functions such as the following:

• View data that meet certain criteria.A store manager could view all items that have fewer than 10 units in stock, or that have been in the warehouse for more than three months.
• Create a new table that is a subset of a base table.The marketing department could create a table that shows only customers who live within a 10-mile radius of a new location.
• Connect the contents of two different tables.A contractor could view all the subcontractors who worked on a project and the amount each billed for their work, or a principal could see all students who have a GPA of 3.5 or better.
• Combine data in two tables:HR could view all actives employees and all retired employees to create an invitation list for the company holiday party.
• View data that have no relationship:This might be a list of all customers who created an account but who never placed an order, then delete them.
• Add new data to existing records:Find all customers who’ve ordered more than $1,000 worth of products in a year and give them an automatic discount on their next order.
• Delete data from existing records:Delete the automatic discount from customer accounts after its expiration date.

Some Common SQL Commands

To accomplish the actions above, database developers and admins use the commands listed below to query or update the database:

• ALTER TABLE: Adds new columns to an existing table.
• BETWEEN: Filters results based on two parameters — e.g., view employees whose birthdays are between June 1 and June 30, or all cars that cost between $20,000 and $25,000.
• CREATE TABLE: Adds a new table to the database. Parameters are added to the command to specify the number and names of the columns.
• DELETE: Removes rows from a table.
• JOIN: Allows the data from multiple tables to be combined. Read more about SQL JOINShere.
• INSERT: Adds a new row to a table. Parameters are added to the command to specify the data in the columns.
• LIMIT: Sets the maximum number of rows that will be displayed when querying a database. You could see the best-selling books of 2014, but limit it to the top 10.
• ORDER BY: Sorts the results by a certain column. For example, when looking at the 10 top-selling books of 2014, you could sort the list by title, by the author’s last name, or by sales volume.
• SELECT: All SQL queries begin with SELECT.
• UNION: Combines the results of multiple queries.
• UPDATE: Edit rows in a table.
• 地点:一个条件语句,允许查询呢, additions, deletions, and changes to be limited to data that meet certain criteria. The BETWEEN, LIMIT, and ORDER BY examples above would be accomplished by using WHERE. It could also be used to limit rows removed in a DELETE command.

Data Models, Designs, and Schemas

Adatabase modelis an abstract representation of how data will be stored. It shows how data elements will be organized and the relationships between them. The term is also used to describe high-level concepts like relational, flat-file, or object-relational database systems.

Adatabase schemais the implementation of the data model (the first sense above) in the database. It includes data types, constraints, and keys (both primary and foreign).

Database designis the production of a database model (again, in the first sense above) based on the needs of the users.

Locking

When data is being modified, locking prevents another user or transaction from altering that data. This prevents data from falling out of sync.

Database Normalization

Database normalizationis the foundation of relational databases. It’s the process that organizes data into tables that can then be connected to each other. Each table should be about a specific topic, and only those columns that support the topic are included in the table.

Database normalization provides the following benefits:

• Duplicate data is minimized, and data storage needs decrease.
• Changes to data have to occur in only one place.
• Access to the data is quicker and more efficient.

Once the data schema and business rules for the database are established, the data is processed in steps that create tables and primary keys, eliminate repetitive data, and build relationships between the data by splitting the data into new tables and creating foreign keys for those tables. The steps are called First Normal Form (1NF), Second Normal Form (2NF), and Third Normal Form (3NF). Each step attempts to further refine the data and narrow the focus of each table.

Complicated data schemas may require fourth and fifth normal forms. There are variations of forms, such as the Boyce–Codd Normal Form (BCNF), which is a more robust version of the 3NF and is used in unique situations.

A full description of the normalization process is beyond the scope of this article, but an excerpt from the 2002 bookAbsolute Beginner's Guide to Databasesby John Petersen, available on theinformIT website,涵盖了很好。

Null and Duplicate Values

Null valuesare data points that don’t exist. For example, not everyone has a middle name, a customer may not provide a phone number, or data may have been lost. A relational database must accommodate these exceptions to rules (it’s the third of Codd's 12 Rules). Null values do not equal zero or a space. However, a null row should not be allowed, even though some RDBMSs do.

Duplicate values will exist. There may be two employees with the same first and last name, and maybe even the same birthday. That’s OK. But having duplicate rows that match, including the key, is not. Again, some RDBMSs allow this, but it violates the basic principles of a relational database.

Database Security

There are a few facets to database security, all of which are vital to database operations:

• Ensuring that users can see only the data they are authorized to see and use only commands they are authorized to run
• Keeping unauthorized users from accessing the database
• Detecting and stopping malware and other attacks
• Preventing data corruption
• Ensuring database availability by preventing system crashes

Detailed information on the second and third concepts can be foundhere.

Views and Reports

Aviewis a generally a screen or a set of screens that is customized to show data that meet the needs of a specific user group, and prevents them from seeing data they don’t have access to. Views can allow users to query the contents of the database without having to know any SQL or other commands.

Reportsare queries that are run and delivered to a person or group that can be used to guide business decisions. Reports can be scheduled or ad-hoc, and can show either real-time data or data from the past (such as the previous week or quarter).

Transactions and Stored Procedures

Transactionsoccur when data is added, updated, or deleted. Relational database transactions should follow the ACID guidelines, meaning they should be:

• Atomic:事务是不可分割的,或者所有操作in the transaction must occur, or the transaction fails and all changes must be rolled back to their original state.
• Consistent:All steps must meet the defined rules for any changes to data fields, record uniqueness, data constraints, etc.
• Isolated:Data that’s being changed by the transaction can’t be updated by other transactions or by users until the first transaction is completed or cancelled.
• Durable:Once the transaction is complete, the changes must not be lost in the event of a power outage or other catastrophic event.

Examples of transactions could include adding an order record to a table once a customer hits the buy button, or changing the work location of an employee if they've moved from one office to another.

Stored procedures are compiled pieces of code that usually perform a number of defined steps. Code runs faster when it compiled (i.e., when it’s been translated from the developer's programming language into machine language). A stored procedure can include transactions. An example of a stored procedure might be a monthly sweep to move customer records from an active customer table to an inactive table if the customer hasn’t purchased anything in the last year.

Coupling:A measure of the amount of dependence between two items. A column of data (e.g., employee birth date) is tightly coupled to any stored procedure (e.g., “show all employees who have a birthday in the coming month”) that will reference the column, so changes to one will impact the other. If all dates are changed to a new format, but the stored procedure isn’t updated, employees won’t get their cake.

Data integrity:Also known asdata quality. Data should be complete, accurate, and correctly formatted; if it’s not, it can be useless (or even harmful). Ensuring data integrity is a key part of database design.

Referential integrity:This is an important subset of data integrity. Every column in a table that holds a foreign key must contain a null value or a value in another table that is considered that table’s primary key. These are the basic rules:

1. A key (primary or foreign) must match the key in a row in another table, or be null.
2. If a primary key in one table changes, all tables where that key is listed as a foreign key must be updated.
3. If a row is updated, all rows linked to that row must be deleted or relinked to other rows.

Metadata

Metadataare data about the database itself. Just like data, metadata are stored in tables, but only used by the DBMS. As data is added, deleted, or changed, the metadata is updated. The metadata allows the DBMS to find data, create new tables, manage resources, and enforce security.

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