4. Relational algebra

Relational Algebra Overview

The third part of the relational model, the manipulation part, states that access to relational data is performed using relational algebra or equivalent relational calculus.

In implementations of specific relational DBMS, neither relational algebra nor relational calculus is currently used in its pure form. Structured Query Language (SQL) has become the de facto standard for accessing relational data. The SQL language is a mixture of relational algebra operators and relational calculus expressions using a syntax that is close to English phrases and enhanced by additional features not found in relational algebra and relational calculus. In general, a data access language is called relationally complete if it is not inferior in its expressive power to relational algebra (or, equivalently, to relational calculus), i.e. any operator of relational algebra can be expressed by means of this language. That's exactly what SQL is.

This chapter will cover the basics of relational algebra.

Closed Relational Algebra

Relational algebra is a set of operators that use relations as arguments and return relations as a result. So the relational operator looks like a function with relations as arguments:

Relational algebra is closed because As arguments, you can substitute other relational operators of the following type into relational operators:

Thus, in relational expressions, you can use nested expressions of arbitrarily complex structure.

Each relationship must have a unique name within the database. The name of the relationship obtained as a result of a relational operation is defined in the left part of the equation. However, you can not require the presence of names from relations derived from relational expressions, if these relations are substituted as arguments in other relational expressions. Such relations will be called unnamed relations . Unnamed relationships do not really exist in the database, but are only calculated at the time of calculating the value of the relational operator.

Traditionally, following Codd [43], eight relational operators are defined, combined into two groups.

Set theoretic operators:

• Union
• Intersection
• Subtraction
• Cartesian product

Special relational operators:

• Sample
• Projection
• Compound
• Division

Not all of them are independent, i.e. Some of these operators can be expressed through other relational operators.

Type-compatible relationships

Some relational operators (for example, union) require that relations have the same headers. Indeed, the relationship consists of a heading and a body. The operation of combining two relations is simply the union of two sets of tuples taken from the bodies of the corresponding relations. But will the result be an attitude? First, if the initial relations have a different number of attributes, then it is obvious that the set, which is the union of such heterogeneous tuples, cannot be represented as a relation. Secondly, even if the relationship has the same number of attributes, but the attributes have different names. How then to determine the title of the relationship, resulting from the union of sets of tuples? Third, let relations have the same number of attributes, attributes have the same name, but are defined on different domains. Then again the union of the tuples will not form a relation.

Definition 1 . We will call relationships compatible by type if they have identical headings, namely,

• Relationships have the same set of attribute names , i.e. for any attribute in one respect there is an attribute with the same name in another respect,
• Attributes with the same name are defined on the same domains .

Some relationships are not compatible by type, but become so after some renaming of attributes. In order for such relationships to be used in relational operators, an auxiliary attribute renaming operator is introduced.

Attribute Renamer

The attribute renaming operator has the following syntax:

Where

- attitude

- the original attribute names,

- new attribute names.

As a result of applying the attribute renaming operator, we get a new relationship, with the attribute names changed.

Example 1

The following statement returns an unnamed relationship in which the attribute renamed to :

Set theoretic operators

Union

Definition 2 . Combining two compatible relationships and called a relationship with the same heading as the relationship and and body consisting of tuples belonging to or , or or both relationships.

The syntax of the merge operation:

Remark A union, like any relation, cannot contain identical tuples. Therefore, if some tuple is included in and attitude , it enters the union once.

Example 2 Let two relationships be given and with employee information:

Personnel Number	Surname	Salary
one	Ivanov	1000
2	Petrov	2000
3	Sidorov	3000

Table 1 Attitude A

Personnel Number	Surname	Salary
one	Ivanov	1000
2	Furs	2500
four	Sidorov	3000

Table 2 Attitude B

Merging relationships and will look like:

Personnel Number	Surname	Salary
one	Ivanov	1000
2	Petrov	2000
3	Sidorov	3000
2	Pushnikov	2500
four	Sidorov	3000

Table 3 Attitude A UNION B

Remark As can be seen from the above example, potential keys that were in a relationship and not inherited by combining these relationships. Therefore, in the union of relationships and The attribute "Personnel Number" may contain duplicate values. If this were not the case, and the keys would be inherited, then this would contradict the notion of union as "union of sets". Of course, the union of relationships and has, like any relationship, a potential key, for example, consisting of all attributes.

Intersection

Definition 3 . The intersection of two compatible by type of relationship and called a relationship with the same heading as the relationship and , and the body, consisting of tuples belonging simultaneously to both relationships and .

Intersection operation syntax:

Example 3 For the same relationship and , as in the previous example, the intersection has the form:

Personnel Number	Surname	Salary
one	Ivanov	1000

Table 4 Attitude A INTERSECT B

Remark It would seem that, unlike the union operation, the potential keys could be inherited by the intersection of relations. However, it is not. In general, no relational operators convey any data about potential keys to the resulting relation . The reason for this could be a trivial consideration that this is more simple and symmetrical - all operators are arranged in the same way. In fact, the reason is deeper, and lies in the fact that the potential key is a semantic concept that reflects the distinguishability of objects in the domain. The presence of potential keys is not derived from the structure of the relationship, but is explicitly defined for each relationship, based on its meaning. Relational operators are formal operations on relationships and are executed in the same way, regardless of the meaning of the data contained in the relationship. Therefore, relational operators cannot “know” anything about the meaning of data. The interpretation of the result of relational operations is the user's business.

Subtraction

Definition 4 . Subtracting two compatible relationships and called a relationship with the same heading as the relationship and , and body consisting of tuples belonging to the relation and not belonging to the relation .

Subtract operation syntax:

Example 4 For the same relationship and , as in the previous example, the subtraction is:

Personnel Number	Surname	Salary
2	Petrov	2000
3	Sidorov	3000

Table 5: A MINUS B Ratio

Cartesian product

Definition 5 . Cartesian product of two relationships and is called a relationship whose title is a concatenation of relationship headers and :

,

and the body consists of tuples that are a concatenation of relations tuples and :

,

such that , .

The syntax of a Cartesian product is:

Remark Power of the work equal to the product of the power relations and because every tuple relationship connects with every tuple relationship .

Remark If in a relationship and If there are attributes with the same name, then before performing the Cartesian product, such attributes must be renamed.

Remark You can multiply any two relationships, compatibility by type is not required.

Example 5 Let two relationships be given and with information about suppliers and details:

Vendor number	Supplier name
one	Ivanov
2	Petrov
3	Sidorov

Table 6 Attitude A (Suppliers)

Detail number	the name of detail
one	Bolt
2	Nut
3	Screw

Table 7 Ratio B (Details)

Cartesian product of relationships and will look like:

Vendor number	Supplier name	Detail number	the name of detail
one	Ivanov	one	Bolt
one	Ivanov	2	Nut
one	Ivanov	3	Screw
2	Petrov	one	Bolt
2	Petrov	2	Nut
2	Petrov	3	Screw
3	Sidorov	one	Bolt
3	Sidorov	2	Nut
3	Sidorov	3	Screw

Table 8: A TIMES B Ratio

Remark The Cartesian product operation itself is not very important, since she does not give any new information, compared with the original relationship. For real requests, this operation is almost never used. However, the operation of a Cartesian product is important for performing special relational operations, which will be discussed below.

Special relational operators

Sampling (restriction, selection)

Definition 6 . Sampling (restriction, selection) on the relationship with the condition called a relationship with the same heading as the relationship , and by the body consisting of tuples, the attribute values ​​of which, when substituted into the condition give the value TRUE. is a boolean expression that can include relationship attributes and (or) scalar expressions.

In the simplest case, the condition has the appearance where - one of the comparison operators ( etc.), and and - relationship attributes or scalar values. Such samples are called - selections ( theta sampling ) or - restrictions - selection .

Sampling syntax:

,

or

Example 6 Let given the relation with employee information:

Personnel Number	Surname	Salary
one	Ivanov	1000
2	Petrov	2000
3	Sidorov	3000

Table 9 Ratio A

Sample result will look like:

Personnel Number	Surname	Salary
one	Ivanov	1000
2	Petrov	2000

Table 10 Ratio A WHERE Salary <3000

The meaning of the sampling operation is obvious - choose tuples of a relation that satisfy a certain condition. Thus, the sampling operation gives a " horizontal slice " of the relationship for some condition.

Projection

Definition 7 . Relationship projection by attributes where each of the attributes belongs to the relation called the relationship with the title and body containing many tuples of view such for which there are tuples with an attribute value equal , attribute value equal , ..., attribute value equal .

The projection operation syntax is:

Remark The projection operation gives a " vertical cut " of the relationship, in which all duplicates of tuples arising from this cut are removed.

Example 7 Let given the relation with information about suppliers, including name and location:

Vendor number	Supplier name	Supplier City
one	Ivanov	Ufa
2	Petrov	Moscow
3	Sidorov	Moscow
four	Sidorov	Chelyabinsk

Table 11 Attitude A (Suppliers)

Projection will look like:

Supplier City
Ufa
Moscow
Chelyabinsk

Table 12 Attitude A [Supplier City]

Compound

The operation of connecting relations, along with the operations of sampling and projection, is one of the most important relational operations.

Usually, several types of join operations are considered:

• General connection operation
• -connection (theta-compound)
• Equi connection
• Natural compound

The most important of these particular cases is the operation of a natural compound. All types of compounds are special cases of the general operation of the connection.

General connection operation

Definition 8 . Connection relationship and by condition called attitude

is a boolean expression that can include relationship attributes and and (or) scalar expressions.

Thus, the join operation is the result of the sequential application of the Cartesian product and the sample operation. If in a relationship and If there are attributes with the same name, then such attributes must be renamed before making the connection.

Theta compound

Definition 9 . Let the attitude contains an attribute attitude contains an attribute , but - one of the comparison operators ( etc.). Then - connection of the relation by attribute with attitude by attribute call attitude

This is a special case of a generic join operation.

Sometimes, for surgery -connections apply the following, shorter syntax:

Example 8 Consider a company that stores data on suppliers and parts supplied. Let suppliers and parts assigned a status. Let the business of the company be organized in such a way that suppliers have the right to supply only those parts whose status is not higher than the status of the supplier (this may be that a good supplier with a high status can supply more types of parts, and a poor supplier with a low status can supply only a limited list of parts, the importance of which (the status of the part) is not very high).

Vendor number	Supplier name	X (Supplier Status)
one	Ivanov	four
2	Petrov	one
3	Sidorov	2

Table 13 Attitude A (Suppliers)

Detail number	the name of detail	Y (Status details)
one	Bolt	3
2	Nut	2
3	Screw	one

Table 14 Ratio B (Details)

The answer to the question "which suppliers have the right to supply what parts?" gives -connection :

Vendor number	Supplier name	X (Supplier Status)	Detail number	the name of detail	Y (Status details)
one	Ivanov	four	one	Bolt	3
one	Ivanov	four	2	Nut	2
one	Ivanov	four	3	Screw	one
2	Petrov	one	3	Screw	one
3	Sidorov	2	2	Nut	2
3	Sidorov	2	3	Screw	one

Table 15 Relationship "Which suppliers supply which parts"

Equi connection

The most important special case of a -connection is the case when there is simply equality.

Equivalent syntax :

Example 9 Let there be relationships , and , storing information about suppliers, parts and deliveries, respectively (for convenience, we introduce the brief names of the attributes):

Vendor number PNUM	Supplier name PNAME
one	Ivanov
2	Petrov
3	Sidorov

Table 16 P ratio (Suppliers)

Detail number DNUM	the name of detail DNAME
one	Bolt
2	Nut
3	Screw

Table 17 Attitude D (Details)

Vendor number PNUM	Detail number DNUM	Quantity supplied VOLUME
one	one	100
one	2	200
one	3	300
2	one	150
2	2	250
3	one	1000

Table 18 PD ratio (Supply)

The answer to the question, what parts are supplied by suppliers, gives equi-connection .In fact, because in a relationship there are identical attributes, then you must first rename the attributes, and then perform an equi-connection. Recording becomes more cumbersome:

Usually, such a complex form of recording is not used. But be that as it may, the result is:

Vendor number PNUM1	Supplier name PNAME	Vendor number PNUM2	Detail number DNUM	Quantity supplied VOLUME
one	Ivanov	one	one	100
one	Ivanov	one	2	200
one	Ivanov	one	3	300
2	Petrov	2	one	150
2	Petrov	2	2	250
3	Sidorov	3	one	1000

Table 19 Attitude "What parts are supplied by which suppliers"

The disadvantage of an equi-connection is that if a connection occurs by attributes with the same name (and most often it happens!), Then two attributes with the same values ​​appear in the resulting relation. In our example, the attributes PNUM1 and PNUM2 contain duplicate data. You can get rid of this drawback by taking a projection on all attributes, except for one of the duplicates. This is how a natural connection works.

Natural compound

Definition 10 . Let given the relationship and that have the same attributes (i.e., attributes with the same name and defined on the same domains).

Then the natural mix of relationships and called a relationship with a header and a body containing many tuples such that and .

The natural connection is so important that a special syntax is used for it:

RemarkThe syntax of a natural connection does not indicate which attributes are being joined. A natural connection is made for all the same attributes.

Remark A natural connection is equivalent to the following sequence of relational operations:

1. Rename the same attribute in the relationship
2. Execute the Cartesian product of relations
3. Fetch by matching values ​​of attributes that have the same name.
4. Выполнить проекцию, удалив повторяющиеся атрибуты
5. Переименовать атрибуты, вернув им первоначальные имена

Remark Можно выполнять последовательное естественное соединение нескольких отношений. Нетрудно проверить, что естественное соединение (как, впрочем, и соединение общего вида) обладает свойством ассоциативности , т.е.

поэтому такие соединения можно записывать, опуская скобки:

Пример 10 . В предыдущем примере ответ на вопрос "какие детали поставляются поставщиками", более просто записывается в виде естественного соединения трех отношений (для удобства просмотра порядок атрибутов изменен, это является допустимым по свойствам отношений):

Номер поставщика PNUM	Supplier name PNAME	Номер детали DNUM	the name of detail DNAME	Поставляемое количество VOLUME
one	Ivanov	one	Болт	100
one	Ivanov	2	Nut	200
one	Ivanov	3	Screw	300
2	Петров	one	Болт	150
2	Петров	2	Nut	250
3	Сидоров	one	Болт	1000

Table 20 P JOIN PD JOIN D ratio

Division

Definition 11 . Let given the relationship and , and the attributes are common to the two relationships. The division of relationships on called a relationship with a header and a body containing a multitude of tuples , such that for all tuples there is a tuple in relation to .

The relation acts as a dividend , the relation acts as a divider . Division of relations is similar to division of numbers with the remainder.

The syntax of the division operation:

RemarkTypical inquiries implemented through a division operation usually have in their wording the word "all" - "which suppliers supply all the parts?".

Example 11In the example with suppliers, parts and deliveries, we answer the question "which suppliers supply all the parts?".

As a dividend, we take a projection containing the numbers of the suppliers and the numbers of the parts supplied by them:

Vendor number PNUM	Detail number DNUM
one	one
one	2
one	3
2	one
2	2
3	one

Table 21 Projection X = PD [PNUM, DNUM]

As a divider, take a projection containing a list of the numbers of all parts (not necessarily supplied by someone):

Detail number DNUM
one
2
3

Table 22 Projection Y = D [DNUM]

Division gives a list of vendor numbers that supply all parts:

Vendor number PNUM
one

Table 23 The ratio of X DEVIDEBY Y

It turned out that only supplier number 1 supplied all the parts.

Examples of using relational operators

Example 12 . Get the names of the vendors that supply part number 2.

Decision:

Example 13 . Get the names of suppliers supplying at least one nut.

Decision:

The answer to this request can be obtained in another way:

Example 14 Get the names of suppliers supplying all the details.

Decision:

Example 15 Get the names of suppliers that do not supply part number 2.

Decision:

The answer to this request can be obtained step by step:

- get a list of numbers of all suppliers

- connect data on suppliers and supplies

- in the data on suppliers and deliveries, leave only the data on deliveries of part number 2.

- get a list of vendor numbers that supply part number 2.

- get a list of supplier numbers that do not supply part number 2.

- connect the list of suppliers who do not supply part number 2 with data on suppliers (complete information will be obtained on suppliers who do not supply part number 2).

- the answer you are looking for (names of suppliers that do not supply part number 2).

Dependent relational operators

As stated at the beginning of the chapter, not all relational algebra operators are independent — some of them are expressed through other relational operators.

Connection operator

The join operator is defined in terms of the Cartesian product and the sample operator. For the natural join operator, a projection operator is added.

Intersection operator

The intersection operator is expressed by subtracting as follows:

Division operator

The division operator is expressed in terms of the subtraction, cartesian product and projection operators as follows:

Thus, it is shown that join , intersection and division operators can be expressed through other relational operators, i.e. these operators are not primitive.

Primitive relational operators

The remaining relational operators ( union , subtraction , Cartesian product , selection , projection ) are primitive operators - they cannot be expressed through each other.

Cartesian operator

The Cartesian product operator is the only operator that increases the number of attributes , so it cannot be expressed in terms of union, subtraction, selection, projection.

Оператор проекции

Оператор проекции - единственный оператор, уменьшающий количество атрибутов, поэтому его нельзя выразить через объединение, вычитание, декартово произведение, выборку.

Sampling operator

Оператор выборки - единственный оператор, позволяющий проводить сравнения по атрибутам отношения, поэтому его нельзя выразить через объединение, вычитание, декартово произведение, проекцию.

Операторы объединения и вычитания

Доказательство примитивности операторов объединения и вычитания более сложны и мы их здесь не приводим.

Запросы, невыразимые средствами реляционной алгебры

Несмотря на мощь языка реляционной алгебры, имеется ряд типов запросов, которые принципиально нельзя выразить только при помощи операторов реляционной алгебры. Это вовсе не означает, что ответы на эти запросы нельзя получить вообще. Просто, для получения ответов на подобные запросы приходится применять процедурные расширения реляционных языков.

Плохая нормализация отношений

Данный пример взят из книги Гилуа М.М. [6, стр.43].

Example 16 Пусть имеется отношение ХИМИЧЕСКИЙ_СОСТАВ_ВЕЩЕСТВ с набором атрибутов (Наименование вещества, Водород, Гелий, …, 105_элемент). Значением атрибута "Вещество" являются наименования химических веществ, значениями остальных атрибутов - процентный состав соответствующих элементов в этом веществе. Такое отношение могло бы иметь, к примеру, следующий вид:

Наименование вещества	Hydrogen	Helium	...	105 элемент
Дезоксирибону-клеиновая кислота	five	3	...	0.01
Petrol	50	0	...	0
...	...	...	...	...

Таблица 24 Отношение ХИМИЧЕСКИЙ_СОСТАВ_ВЕЩЕСТВ

Рассмотрим запрос "Найти все химические элементы, содержание которых в каком-либо из веществ превышает заданный процент (скажем, 90)".

С алгоритмической точки зрения этот запрос выполняется элементарно - просматриваются все столбцы таблицы, если в столбце присутствует хотя бы одно значение, большее 90, то запоминается заголовок этого столбца. Набор наименований запомненных столбцов и является ответом на запрос.

Формально невозможно выразить этот запрос в рамках реляционной алгебры, т.к. ответом на этот запрос должен быть список атрибутов отношений, удовлетворяющих определенному условию. В реляционной алгебре нет операторов, манипулирующих с наименованиями атрибутов.

На самом деле, этот пример показывает, что таблица плохо нормализована (нормализация отношений рассматривается в гл.6 и 7). В таблице есть набор однотипных атрибутов ("Водород", "Гелий" и т.д. в количестве 105 столбцов).

Правильнее разбить это отношение на три различных отношения:

1. ВЕЩЕСТВО( НОМ_ВЕЩЕСТВА , ВЕЩЕСТВО),
2. ЭЛЕМЕНТЫ( НОМ_ЭЛЕМЕНТА , ЭЛЕМЕНТ),
3. ХИМИЧЕСКИЙ_СОСТАВ_ВЕЩЕСТВ( НОМ_ВЕЩЕСТВА , НОМ_ЭЛЕМЕНТА , ПРОЦЕНТ).

НОМ_ВЕЩЕСТВА	ВЕЩЕСТВО
one	Deoxyribonucleic acid
2	Petrol

Таблица 25 Отношение ВЕЩЕСТВО

НОМ_ЭЛЕМЕНТА	ЭЛЕМЕНТ
one	Hydrogen
2	Helium
...	...
105	...

Таблица 26 Отношение ЭЛЕМЕНТЫ

НОМ_ВЕЩЕСТВА	НОМ_ЭЛЕМЕНТА	ПРОЦЕНТ
one	one	five
one	2	3
one	105	0.01
2	one	50

Таблица 27 Отношение ХИМИЧЕСКИЙ_СОСТАВ_ВЕЩЕСТВ

Для отношений, нормализованных таким образом, исходный запрос реализуется следующей последовательностью операторов:

1. R1(НОМЕР_ВЕЩЕСТВА,НОМ_ЭЛЕМЕНТА,ПРОЦЕНТ)= ХИМИЧЕСКИЙ_СОСТАВ_ВЕЩЕСТВ[ПРОЦЕНТ>90]. (Выборка из отношения).
2. R2(НОМ_ЭЛЕМЕНТА) = R1[НОМ_ЭЛЕМЕНТА]. (Проекция отношения).
3. R3(НОМ_ЭЛЕМЕНТА,ЭЛЕМЕНТ)= R2[НОМ_ЭЛЕМЕНТА=НОМ_ЭЛЕМЕНТА]ЭЛЕМЕНТЫ. (Естественное соединение)
4. ОТВЕТ(ЭЛЕМЕНТ) = R3[ЭЛЕМЕНТ]. (Проекция таблицы).

На языке SQL такой запрос реализуется одной командой:

 SELECT ЭЛЕМЕНТЫ.ЭЛЕМЕНТ
  FROM ЭЛЕМЕНТЫ, ХИМИЧЕСКИЙ_СОСТАВ_ВЕЩЕСТВ
  WHERE
    ЭЛЕМЕНТЫ.НОМ_ЭЛЕМЕНТА=ХИМИЧЕСКИЙ_СОСТАВ_ВЕЩЕСТВ.НОМ_ЭЛЕМЕНТА
    AND ХИМИЧЕСКИЙ_СОСТАВ_ВЕЩЕСТВ.ПРОЦЕНТ>90 ;

Невыразимость транзитивного замыкания реляционными операторами

Следующий пример иллюстрирует класс запросов, невыразимых средствами реляционной алгебры или реляционного исчисления по причине невыразимости средствами реляционной алгебры транзитивного замыкания отношений (см. гл. 1).

Пример 17 . Рассмотрим отношение, описывающее сотрудников некоего предприятия. Отношение содержит данные о табельном номере сотрудника, фамилии, должности и табельном номере руководителя сотрудника – СОТРУДНИКИ ( ТАБ_НОМ , ФАМИЛИЯ, ДОЛЖНОСТЬ, ТАБ_НОМ_РУК):

ТАБ_НОМ	ФАМИЛИЯ	ДОЛЖНОСТЬ	ТАБ_НОМ_РУК
one	Ivanov	Директор	one
2	Петров	Глав.бухгалтер	one
3	Сидоров	Accountant	2
four	Vasiliev	Начальник цеха	one
five	Сухов	Master	four
6	Шарипов	Working	five
...	...	...	...

Таблица 28 Отношение СОТРУДНИКИ

Рассмотрим запрос "Перечислить всех руководителей (прямых и непрямых) данного сотрудника".

Ответом на запрос может быть получен при помощи понятия транзитивного замыкания. Однако транзитивное замыкание не может быть выражено операторами реляционной алгебры.

Кросс-таблицы

Одной из задач, связанных с представлением табличных данных является построение так называемых кросс-таблиц.

Пусть имеется отношение с тремя атрибутами и потенциальным ключом, включающим первые два атрибута. Примером такого отношения могут быть данные с объемами продаж различных товаров за некоторые промежутки времени:

Товар	Month	amount
Computers	January	100
Printers	January	200
Scanners	January	300
Computers	February	150
Printers	February	250
Scanners	February	350
...	...	...

Таблица 29 Данные о продажах

Требуется представить эти данные в виде таблицы, по строкам которой идут наименования товаров, по столбцам - месяцы, а в ячейках содержатся объемы продаж. Это и будет кросс-таблицей:

Товар	January	February	...
Computers	100	150	...
Printers	200	250	...
Scanners	300	350	...

Таблица 30 Кросс-таблица

Построение кросс-таблицы средствами реляционной алгебры невозможно, т.к. для этого требуется превратить данные в ячейках таблицы в наименования новых столбцов таблицы.

findings

Доступ к реляционным данным возможен при помощи операторов реляционной алгебры. Реляционная алгебра представляет собой набор операторов, использующих отношения в качестве аргументов, и возвращающие отношения в качестве результата. Реляционная алгебра замкнута таким образом, что результаты одних реляционных выражений можно использовать в других выражениях.

Традиционно определяют восемь реляционных операторов, объединенных в две группы.

Теоретико-множественные операторы: объединение , пересечение , вычитание , декартово произведение .

Специальные реляционные операторы: выборка , проекция , соединение , деление .

Для выполнения некоторых реляционных операторов требуется, чтобы отношения были совместимы по типу .

Не все операторы реляционной алгебры являются независимыми - некоторые из них выражаются через другие реляционные операторы. Операторы соединения , пересечения и деления можно выразить через другие реляционные операторы, т.е. эти операторы не являются примитивными. Оставшиеся реляционные операторы ( объединение , вычитание , декартово произведение , выборка , проекция ) являются примитивными операторами - их нельзя выразить друг через друга.

There are several types of queries that cannot be expressed by means of relational algebra. These include queries that require giving a list of attributes that satisfy certain conditions, building a transitive closure of relations, and building cross-tables. To get answers to such queries, you have to use procedural extensions of relational languages.