Description of Types of Join in Relational Algebra
Description of Types of Join in Relational Algebra
Relational Algebra is a procedural query language that involves a set of operations that take one or two relations as input and produce a new relation as their result.
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Natural Join (NJ)
A natural join is a binary operation that is written as (relation1 ⋈ relation2). It operates on two relations and produces a new relation.
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Working of NJ
Natural Join compares all the rows of both the tables and forms a resultant table which consists of all the rows of both tables that have matching values in all the matched columns.
Use of NJ
Natural Join is used in scenarios where we need to combine rows from two or more tables based on the related columns between them.
Example of NJ
Let's suppose we have Employee and Department relations where EmployeeID is common in both, a Natural Join between these two returns all the rows where EmployeeID is matching.
Equi-Join
Equi-Join is a special type of join in which we use only equality ‘=’ operator. It's used to combine columns that have the same name in both tables.
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Working of Equi-Join
Equi-Join compares specified columns of two tables and forms a new table that has only those rows where the specified column's value matches.
Use of Equi-Join
Equi-Join is usually used when we need to combine related information from multiple tables.
Example of Equi-Join
If we have two relations Sales and Product, where ProductID is common, Equi-Join on ProductID will return all pairs of rows that have the same ProductID.
Theta Join
Theta join allows for arbitrary comparison relationships. That is, we can retrieve matching rows based on conditions other than equality.
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Working of Theta Join
In theta join, the condition is specified by θ (theta), which can be any binary operator like ≡, ≠, ≥, ≤, >, < etc.
Use of Theta Join
Theta join is used when we need to merge data bases on conditions other than equality.
Example of Theta Join
Suppose we have a Student relation and a Classroom relation where the ClassSize is common. A theta join with the condition (Student.ClassSize > Classroom.ClassSize) will return all the records where a class is larger than its classroom.
Types of join in Relational Algebra
Join operation in Relational Algebra is a combination of a Cartesian product followed by a select, making it among the most expensive operations in terms of performance. It combines tuples from two relations in a single relation if they satisfy certain conditions.
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Natural Join
Natural join is a binary operation that is written as (R ⨝ S) where R and S are either database relations or the result of an ongoing join operation. Its output includes all combinations of tuples in R and S that are equal on their common attribute names.
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Common Attribute Names
In a natural join, the joint operation happens based on the common attribute names between two relations. Here, the names of these attributes should be exactly the same.
Eliminate Duplicates
The natural join is a special case of equijoin in which, after the equijoin, the duplicate columns are eliminated in the result.
Application of Natural Join
Ideal when equating corresponding common attribute names, such as connecting the 'customer_id' field in a 'Sales' table to the 'customer_id' field in a 'Customer' table.
Theta Join
Theta join combines tuples from different relations provided they satisfy the theta condition. Theta join can use any kind of binary relational operator such as: <=, <, =, >, >=, etc.
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Definition of the Condition
The join condition is defined by the theta operator, which can be any relational operator such as equal to, greater than, and less than operations.
Flexibility
The theta join allows for more flexibility as it can use a range of operators to define the join condition.
Application of Theta Join
Great when you require the flexibility to define a join condition that isn't strictly equality, such as finding 'Products' that cost less than a certain amount in a 'Sales' record.
Equi Join
An equi join is a specific type of join in which two tables are joined on the equality condition.
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Equality Condition
The equi join combines relations based on the equality (=) condition, hence the name "equi" join.
Duplication of Columns
Although Equi Join considers only those tuples or rows with equal valued attributes, it creates duplication because it separately shows each of the joining attributes.
Application of Equi Join
Optimal when creating a condition where values of attributes must match strictly, such as pairing 'employee_id' on an 'Employee' table with 'manager_id' on a 'Department' table.
Types of Join in Relational Algebra
Relational Algebra is a procedural query language that involves a set of operations that take one or two relations as input and produce a new relation as their result. There are several types of join, including Natural Join (NJ), Equi-Join, and Theta Join.
Natural Join (NJ)
A natural join is a binary operation that is written as (relation1 ⋈ relation2). It operates on two relations and produces a new relation. Natural Join compares all the rows of both the tables and forms a resultant table which consists of all the rows of both tables that have matching values in all the matched columns.
Natural Join is used in scenarios where we need to combine rows from two or more tables based on the related columns between them.
For example, if we have Employee and Department relations where EmployeeID is common in both, a Natural Join between these two will return all the rows where EmployeeID is matching.
Equi-Join
Equi-Join is a special type of join in which we use only equality ‘=’ operator. It's used to combine columns that have the same name in both tables. Equi-Join compares specified columns of two tables and forms a new table that has only those rows where the specified column's value matches.
Equi-Join is usually used when we need to combine related information from multiple tables.
For instance, if we have two relations Sales and Product, where ProductID is common, Equi-Join on ProductID will return all pairs of rows that have the same ProductID.
Theta Join
Theta join allows for arbitrary comparison relationships. That is, we can retrieve matching rows based on conditions other than equality. In theta join, the condition is specified by θ (theta), which can be any binary operator like ≡, ≠, ≥, ≤, >, < etc.
Theta join is used when we need to merge data bases on conditions other than equality.
For example, suppose we have a Student relation and a Classroom relation where the ClassSize is common. A theta join with the condition (Student.ClassSize > Classroom.ClassSize) will return all the records where a class is larger than its classroom.
Understanding Types of Join
4 Major Types of Database Joins
The first type of join is the 'Inner join.' An inner join selects records that have matching values in both tables involved in the join.
The second one is 'Left/Right Outer join.' These types of join return all records from the 'left'/'right' table, and the matched records from the 'right'/'left' table.
The third type is 'Full Outer join.' This type of join returns all records when there is a match in either the left or the right table. Lastly, we have the 'Cross join,' which returns the Cartesian product of rows from the tables in the join.
SQL Cheat Sheet
Basic SQL Commands
SELECT
Used to select data from a database. The data returned is stored in a result table.
SELECT column1, column2 FROM table_name;%%%
UPDATE
Used to modify the existing records in a table.
UPDATE table_name SET column1 = value1, column2 = value2 WHERE some_column = some_value;%%%
DELETE
Used to delete existing records in a table.
DELETE FROM table_name WHERE some_column = some_value;%%%
SQL Functions
COUNT()
A function that returns the number of rows that matches a specified criterion.
SELECT COUNT(column_name) FROM table_name;%%%
SUM()
A function that returns the total sum of a numeric column.
SELECT SUM(column_name) FROM table_name;%%%
AVG()
A function that returns the average value of a numeric column.
SELECT AVG(column_name) FROM table_name;%%%
MAX() & MIN()
Functions that return the highest and lowest value of the selected column.
SELECT MAX(column_name) FROM table_name;
SELECT MIN(column_name) FROM table_name;%%%
SQL Constraints
NOT NULL
Ensures that a column cannot have NULL value.
CREATE TABLE table_name (column_name datatype NOT NULL);%%%
UNIQUE
Ensures that all values in a column are different.
CREATE TABLE table_name (column_name datatype UNIQUE);%%%
PRIMARY KEY
Uniquely identified each records in the database.
CREATE TABLE table_name (column_name datatype PRIMARY KEY);%%%
FOREIGN KEY
Uniquely identified a rows/records in any another database table.
CREATE TABLE table_name (column_name datatype,
FOREIGN KEY (column_name) REFERENCES other_table(column_name));%%%
SQL JOINs
INNER JOIN
Returns records that have matching values in both tables.
SELECT column_name(s) FROM table1 INNER JOIN table2 ON table1.column_name = table2.column_name;%%%
LEFT (OUTER) JOIN
Returns all records from the left table, and the matched records from the right table.
SELECT column_name(s) FROM table1 LEFT JOIN table2 ON table1.column_name = table2.column_name;%%%
RIGHT (OUTER) JOIN
Returns all records from the right table, and the matched records from the left table.
SELECT column_name(s) FROM table1 RIGHT JOIN table2 ON table1.column_name = table2.column_name;%%%
FULL (OUTER) JOIN
Returns all records when there is a match in either left or right table.
SELECT column_name(s) FROM table1 FULL OUTER JOIN table2 ON table1.column_name = table2.column_name;%%%