SQL Full Outer Join Using Left and Right Outer Join and Union Clause
Subject
Database Management System
Standard
Computer Science Engineering
Views
498
Asked By
Alexandria
Answer / Solution
To perform a full outer join in SQL, you can utilize the combination of left outer join, right outer join, and the UNION clause. A full outer join returns all the rows from both tables, including the unmatched rows from each table. Here's how you can achieve it:
Left Outer Join: A left outer join returns all the rows from the left table (Table A) and the matching rows from the right table (Table B). The unmatched rows from Table A will have NULL values for the columns of Table B. The basic syntax for a left outer join is as follows:
SELECT * FROM Table_A LEFT OUTER JOIN Table_B ON Table_A.common_column = Table_B.common_column;
Right Outer Join: A right outer join returns all the rows from the right table (Table B) and the matching rows from the left table (Table A). The unmatched rows from Table B will have NULL values for the columns of Table A. The basic syntax for a right outer join is as follows:
SELECT * FROM Table_A RIGHT OUTER JOIN Table_B ON Table_A.common_column = Table_B.common_column;
UNION Clause: The UNION clause allows you to combine the results of two or more SELECT statements into a single result set. It removes any duplicate rows between the result sets. To perform a full outer join using left and right outer joins, you can use the UNION clause to combine the results of both joins. The basic syntax for a full outer join using UNION is as follows:
SELECT * FROM Table_A LEFT OUTER JOIN Table_B ON Table_A.common_column = Table_B.common_column UNION SELECT * FROM Table_A RIGHT OUTER JOIN Table_B ON Table_A.common_column = Table_B.common_column;
By using the UNION clause, the query combines the results of the left outer join and the right outer join, providing a result set that contains all the rows from both tables, including the unmatched rows.
It's important to note that the column names and data types from both tables should be compatible for the UNION clause to work correctly. If there are differences, you can explicitly specify the column names in the SELECT statements and ensure they match between the two tables.
In summary, by combining the left outer join, right outer join, and UNION clause, you can achieve a full outer join in SQL, which returns all rows from both tables, including unmatched rows from each table.
Answer / Solution
SQL provides the ability to perform a full outer join operation using a combination of left and right outer joins along with the UNION clause. A full outer join returns all rows from both the left and right tables, combining them based on a specified join condition.
If there is no match, NULL values are populated in the respective columns. Let's explore how this can be achieved.
To perform a full outer join using left and right outer joins and the UNION clause, you can follow these steps:
Start by writing a left outer join between the two tables, including the join condition. Example:
SELECT * FROM Table1 LEFT OUTER JOIN Table2 ON Table1.column = Table2.column;
Next, write a right outer join between the same tables, reversing the order of the tables and the join condition. Example:
SELECT * FROM Table2 RIGHT OUTER JOIN Table1 ON Table1.column = Table2.column;
Use the UNION clause to combine the results of the left outer join and right outer join queries. Example:
SELECT * FROM Table1 LEFT OUTER JOIN Table2 ON Table1.column = Table2.column UNION SELECT * FROM Table2 RIGHT OUTER JOIN Table1 ON Table1.column = Table2.column;
By following these steps, you will obtain a result set that includes all rows from both tables, matching them based on the join condition. If there is no match, NULL values will be populated in the columns from the non-matching table.
It's important to note that the columns selected in both the left and right outer join queries should be the same. Otherwise, you may need to specify the specific columns you want to select to ensure compatibility for the UNION operation.
The UNION clause combines the results of the left outer join and right outer join queries, eliminating duplicate rows. If you want to keep all rows, including duplicates, you can use the UNION ALL clause instead of UNION.
In conclusion, using left and right outer joins along with the UNION clause allows you to perform a full outer join in SQL. This technique combines the results of both joins to include all rows from both tables, matching them based on the join condition and populating NULL values where there is no match.
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