Question

How are the OUTER JOIN operations different from the INNER JOIN operations? How is the OUTER UNION operation different from UNION?

Short answer

INNER JOIN only returns matched records from the tables, while OUTER JOIN returns all the records from one table and matched records from the other. In case of no match, OUTER JOIN returns NULL for every column of the table without a match. UNION operation combines the result-set of the select statements that have the same number of columns, similar data types, and same order while removing duplicates. OUTER UNION, however, returns all records from the tables including unmatched records and does not remove duplicates.

Step-by-step solution

Understanding INNER JOIN

The INNER JOIN operation returns only the matched records from two tables. Suppose there are two tables, Table A and Table B, when an INNER JOIN operation is performed on these tables, it will return only the records that have matching values in both tables.

Understanding OUTER JOIN

An OUTER JOIN operation, on the other hand, returns all the records from one table (Table A or Table B) and the matched records from the other table. If there is no match, it will result in NULL values for every column of the table that does not have a matching record.

Differences between INNER and OUTER JOIN

The main difference between INNER JOIN and OUTER JOIN lies in the way they handle the non-matching records: INNER JOIN disregards them while OUTER JOIN includes them.

Understanding UNION operation

The UNION operation is used to combine the result-set of two or more SELECT statements. However, for UNION to work, each SELECT statement within the UNION must have the same number of columns, the columns must also have similar data types, and they must also be in the same order.

Understanding OUTER UNION operation

The OUTER UNION operator concatenates the tuples of the participating relations, even if they do not match. If some tuples have no match or counterpart in the other relation, NULL values are used for the missing columns.

Differences between UNION and OUTER UNION

UNION only returns the matched records from the participating tables, eliminating the duplicate values. OUTER UNION, however, returns all the records, including the unmatched records, from the tables and does not eliminate duplicate values.

Key concepts

INNER JOIN

The INNER JOIN operation is a powerful tool used in databases to combine rows from two or more tables based on a related column. It works by identifying and returning rows that have matching values in both tables. When you perform an INNER JOIN, you are essentially filtering out any records that do not have corresponding entries in each table.
For example, consider two tables: one detailing customers and another showing their orders. An INNER JOIN on these tables will provide a list showing only the customers who have placed orders, excluding those who have not.
Key points about INNER JOIN:

• It focuses on matched records from both tables.
• Unmatched records are excluded from the result set.
• Typically used when you need data that satisfies specific conditions in both tables.

OUTER JOIN

In contrast to INNER JOIN, the OUTER JOIN operation ensures that all records from one table are included in the results, along with the matched records from the other table. If no matching record exists, the result will include NULL values for columns of the table that lacks a counterpart.
OUTER JOIN comes in three variations: LEFT OUTER JOIN, RIGHT OUTER JOIN, and FULL OUTER JOIN. Each serves different purposes:

• LEFT OUTER JOIN returns all records from the left table and the matched records from the right table. If no match is found, NULL values are added for the right table's columns.
• RIGHT OUTER JOIN returns all records from the right table and the matched records from the left table. NULL values are filled in for columns from the left table if no match is located.
• FULL OUTER JOIN includes all records when there is a match in either the left or the right table records. Unmatched rows will contain NULL values for columns with no corresponding data.

OUTER JOIN is beneficial when you want to get a comprehensive view of how two datasets relate, including records that do not directly correlate.

UNION operation

The UNION operation is designed to combine the results of two or more SELECT statements, yielding a singular result set. Unlike JOINs, the rows from each SELECT statement are added into the final results consecutively rather than combined based on matching columns.
For UNION to function effectively, each SELECT statement must return the same number of columns, with each column in corresponding positions having similar data types. The order of columns must also match across the queries involved.
Important considerations for UNION:

• Duplicate rows are automatically removed from the final result. If you want to include duplicates, you can use UNION ALL.
• The columns must align in terms of number and data types.
• UNION operations are useful for gathering data from different queries into a single, unified result set.

OUTER UNION operation

The OUTER UNION operation is a more flexible option compared to UNION, which enables the concatenation of tuples from the participating relations. While UNION removes duplicates and only returns matched records, OUTER UNION goes the extra mile by including all unmatched rows as well.
This operation ensures that even entries without corresponding matches across relations are represented, substituting NULL values for missing columns.
Characteristics of OUTER UNION:

• All records from both relations are included, unmatched records are not omitted.
• NULL values are used for columns where there is no match in corresponding rows.
• It accommodates datasets with varying schema, creating a wider view that represents unmatched data.

OUTER UNION is especially helpful when you need to preserve all available information from the participating tables, regardless of the presence of corresponding records.