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It creates a set that can be saved as a table or used as it is. If the evaluated predicate is true, the combined row is then produced in the expected format, a row set or a temporary table. For example, a Department may be associated with a number of Employees.
Joining separate tables for Department and Employee effectively creates another table which combines the information from both tables. All subsequent explanations on join types in this article make use of the following two tables.
The rows in these tables serve to illustrate the effect of different types of joins and join-predicates. Note: In the Employee table above, the employee «Williams» has not been assigned to any department yet. Also, note that no employees are assigned to the «Marketing» department.
This is the SQL statement to create the aforementioned tables. In other words, it will produce rows which combine each row from the first table with each row from the second table.
The cross join does not itself apply any predicate to filter rows from the joined table. F401, «Extended joined table», package. A Venn Diagram showing the inner overlapping portion filled. A Venn Diagram representing an Inner Join SQL statement between the tables A and B. The query compares each row of A with each row of B to find all pairs of rows which satisfy the join-predicate.
When the join-predicate is satisfied by matching non-NULL values, column values for each matched pair of rows of A and B are combined into a result row. Cartesian product is slower and would often require a prohibitively large amount of memory to store. SQL specifies two different syntactical ways to express joins: the «explicit join notation» and the «implicit join notation». The «implicit join notation» is no longer considered a best practice, although database systems still support it.
The queries given in the examples above will join the Employee and Department tables using the DepartmentID column of both tables. Where the DepartmentID does not match, no result row is generated.
The employee «Williams» and the department «Marketing» do not appear in the query execution results. However transaction databases usually also have desirable join columns that are allowed to be NULL. NULL join columns that an SQL query author cannot modify and which cause inner joins to omit data with no indication of an error. The choice to use an inner join depends on the database design and data characteristics.
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