95 lines
3 KiB
Markdown
95 lines
3 KiB
Markdown
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# Relationale Algebra
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Die Rechen-Operationen, die vom Datenbank-Server ausgeführt werden, um Datensätze abzufragen, heisst Relationale Algebra.
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Relationale Algebra ist im Prinzip die Computer-Sprache, zu der SQL-Abfragen vor der Ausführung umgewandelt werden.
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## Operation Vereinigung $\cup$
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Vereinigt zwei Relationen und entfernt alle Duplikate.
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**_Beispiel:_**
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**ActionActors:**
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| FirstName | LastName | YearOfBirth |
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| --------- | -------- | ----------- |
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| Jackie | Chan | 1954 |
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| Jason | Statham | 1967 |
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| Vin | Diesel | 1967 |
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**ComedyActors:**
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| FirstName | LastName | YearOfBirth |
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| --------- | -------- | ----------- |
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| Jackie | Chan | 1954 |
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| Ryan | Reynolds | 1976 |
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| Jack | Black | 1969 |
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**ActionActors $\cup$ ComedyActors:**
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| FirstName | LastName | YearOfBirth |
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| --------- | -------- | ----------- |
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| Jackie | Chan | 1954 |
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| Jason | Statham | 1967 |
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| Vin | Diesel | 1967 |
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| Ryan | Reynolds | 1976 |
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| Jack | Black | 1969 |
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> **_In Worten:_**
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> Alle Schauspieler, die in Action- oder in Comedy-Filmen spielen.
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## Operation Intersection $\cap$
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Alle Elemente, die in beiden Relationen vorkommen.
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**_Beispiel:_**
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**ActionActors:**
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| FirstName | LastName | YearOfBirth |
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| --------- | -------- | ----------- |
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| Jackie | Chan | 1954 |
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| Jason | Statham | 1967 |
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| Vin | Diesel | 1967 |
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**ComedyActors:**
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| FirstName | LastName | YearOfBirth |
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| --------- | -------- | ----------- |
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| Jackie | Chan | 1954 |
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| Ryan | Reynolds | 1976 |
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| Jack | Black | 1969 |
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**ActionActors $\cap$ ComedyActors:**
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| FirstName | LastName | YearOfBirth |
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| --------- | -------- | ----------- |
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| Jackie | Chan | 1954 |
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> **_In Worten:_**
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> Alle Schauspieler, die in Action- und in Comedy-Filmen spielen.
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## Operator Differenz $\setminus$
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Alle Elemente, die in der 1. Relation vorkommen und in der 2. Relation nicht vorkommen.
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**_Beispiel:_**
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**ActionActors:**
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| FirstName | LastName | YearOfBirth |
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| --------- | -------- | ----------- |
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| Jackie | Chan | 1954 |
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| Jason | Statham | 1967 |
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| Vin | Diesel | 1967 |
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**ComedyActors:**
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| FirstName | LastName | YearOfBirth |
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| --------- | -------- | ----------- |
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| Jackie | Chan | 1954 |
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| Ryan | Reynolds | 1976 |
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| Jack | Black | 1969 |
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**ActionActors $\setminus$ ComedyActors:**
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| FirstName | LastName | YearOfBirth |
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| --------- | -------- | ----------- |
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| Jason | Statham | 1967 |
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| Vin | Diesel | 1967 |
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> **_In Worten:_**
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> Alle Schauspieler, die in Action-Filmen, aber in keinen Comedy-Filmen mitspielen.
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