![]() ![]() For the data only matching the left table, the columns of the left table are output, while the columns of the right table are null. For the data in the set intersection of the left and right tables, the column values of the left and right tables are output. The result of Full Join contains three parts of data: the data in the set intersection of the left and right tables (in which the data matches the left and right tables at the same time), the data only matching the left table, and the data only matching the right table. A left join B is equivalent to A right join B.įull join is a combination of Left Join and Right Join, which returns the set union of the left and right tables. Therefore, a Left Join can be converted into a Right Join. However, Left Join and Right Join are symmetrical. Right Join does not allow the Interchanging of the left and right tables. For rows that are not in the intersection, the columns of the right table are normally output, while the columns of the left table are null. For the data in the intersection of the left and right tables that matches the left table, the columns of the left and right tables are output. These data are divided into two categories: the data that matches the left table and the data that does not match the left table. It returns all rows of the right table, regardless of whether these data rows match the left table. Right Join and Left Join are symmetrical. Left Join does not allow the Interchanging of the left and right tables. For the data that is not in the intersection nor does not match the right table, the column value of the left table is output, and the column value of the right table is null. For the data in the intersection of the left and right tables that matches the right table, the columns of the left and right tables are output respectively. The data is divided into two categories: the data that matches the right table and the data that does not match the right table. Left Join returns all rows in the left table, regardless of whether these rows match the right table. Please refer to the figure above for the different points of each join. Outer Join includes Left Join, Right Join, and Full Join. The join condition of Theta Join is not the equality condition but may be greater-than or less-than conditions. The difference is that Equal Join is a certain field in the left table that is equal to a field in the right table under the join condition. Inner Join is divided into Equal Join and Non Equal Join (Theta Join). A inner join B is equivalent to B join A. Inner Join does not distinguish the order of the left and right tables. The syntax is select A.x, B.y from A join B on A.x = B.y. It means the rows are in the set intersection of the left and right tables. Only the rows that appear in both the left table and the right table meet the condition. The left and right tables are joined by predicates. Inner Join is one of the most frequently used joins. In general, SQL Joins are classified into Inner Join, Outer Join, Self Join, and Cross Join. In addition, it aims to help you understand how to write a high-performance SQL Join by learning its internal implementation. This article aims to thoroughly sort the usage and internal implementation of joins. ![]() Different ways of usage and implementation will cause many differences in performance. Joins have many types and different ways of usage and implementation. Joins are indispensable for data analysis. However, you need to associate multiple tables through joins when analyzing data. In data models of relational databases, different data is often placed in different tables to avoid redundant data storage, which are divided into fact tables and dimension tables. Join is one of the most common concepts in databases and data warehouses.
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