The cross-connection of two or more tables, that is, the so-called Cartesian product
If two or more tables are connected without using the WHERE clause. Then you have executed the Cartesol connection, also known as the Cartesol product. A equal connection equal connection - the purpose is to match the value of a list in a table to the corresponding values in the second table.
A table typically returning the minimum number of rows based on a given condition is called a drive table. Or a base table. To make data retrieval more efficient, non-base tables in the query are typically connected to the base table.
External connection and internal connection
The internal connection is the interconnection between the table in the table, and the number of new lines produced is equal to the product of the number of rows in each table. In addition, internal connection uses these lines to determine the result of the WHERE clause. External connections are combined with two tables in a less than a way. The external cable is all the data lines in a table, even if the matched data is not present in the connection table. Some SQL products use " " instead of the Outer Join statement. The simplicity of the " " is: "Show all information, including those lost" " " after the lack of matching data.
