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Joins

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An SQL join clause - corresponding to a join operation in relational algebra - combines columns from one or more tables in a relational database.

SQL is a domain-specific language used in programming and designed for managing data held in a relational database management system, or for stream processing in a relational data stream management system.

SQL Joins Explained |¦| Joins in SQL |¦| SQL Tutorial by Socratica

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It creates a set that can be saved as a table or used as it is.

SQL Joins Tutorial For Beginners | Inner, Left, Right, Full Join | SQL Joins With Examples | Edureka by edureka!

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A JOIN is a means for combining columns from one or more tables by using values common to each.

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ANSI-standard SQL specifies five types of JOIN: INNER, LEFT OUTER, RIGHT OUTER, FULL OUTER and CROSS.

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As a special case, a table can JOIN to itself in a self-join.

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A programmer declares a JOIN statement to identify rows for joining.

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If the evaluated predicate is true, the combined row is then produced in the expected format, a row set or a temporary table.

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Relational databases are usually normalized to eliminate duplication of information such as when entity types have one-to-many relationships.

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For example, a department may be associated with a number of employees.

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Joining separate tables for department and employee effectively creates another table which combines the information from both tables.

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All subsequent explanations on join types in this article make use of the following two tables.

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The rows in these tables serve to illustrate the effect of different types of joins and join-predicates.

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In the following tables the DepartmentID column of the Department table is the primary key, while Employee.

In the relational model of databases, a primary key is a specific choice of a minimal set of attributes that uniquely specify a tuple in a relation.

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DepartmentID is a foreign key.

In the context of relational databases, a foreign key is a set of attributes subject to a certain kind of inclusion dependency constraint, specifically a constraint that the tuples consisting of the foreign key attributes in one relation, R, must also exist in some other relation, S, and furthermore that those attributes must also be a candidate key in S. In simpler words, a foreign key is a set of attributes that references a candidate key.

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Note: In the Employee table above, the employee "Williams" has not been assigned to any department yet.

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Also, note that no employees are assigned to the "Marketing" department.

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This is the SQL statement to create the aforementioned tables.

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CROSS JOIN returns the Cartesian product of rows from tables in the join.

In mathematics, specifically set theory, the Cartesian product of two sets A and B, denoted A × B, is the set of all ordered pairs where a is in A and b is in B. In terms of set-builder notation, that is

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In other words, it will produce rows which combine each row from the first table with each row from the second table.

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Example of an explicit cross join:

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