How to Properly Join Two SQL Queries to Include Specific Columns

Learn effective methods to properly join two SQL queries and include specific columns using SQL and SQL Server techniques.
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Joining two SQL queries is a common task when working with databases, and it enables you to retrieve data from multiple tables in a single request. In SQL and SQL Server, there are various techniques to achieve this, each serving different purposes and scenarios.

Using INNER JOIN

An INNER JOIN retrieves records that have matching values in both tables. It’s useful when you only need the rows where there is a match in both tables.

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Using LEFT JOIN

A LEFT JOIN returns all records from the left table (table1), and the matched records from the right table (table2). If no match is found, the result is NULL on the side of the right table.

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Using RIGHT JOIN

A RIGHT JOIN is similar to a LEFT JOIN, but it returns all records from the right table and matched records from the left table.

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Using FULL OUTER JOIN

A FULL OUTER JOIN combines the results of both LEFT and RIGHT joins. It returns all records when there is a match in either left or right table.

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Using UNION

If you need to combine the results of two separate queries, you can use the UNION operator. Note that UNION removes duplicate records, whereas UNION ALL keeps all records, including duplicates.

Example using UNION:

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Example using UNION ALL:

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Conclusion

Joining two SQL queries to include specific columns requires understanding the type of join best suited for your requirement. Whether using INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL OUTER JOIN, or UNION, each method has its own set of rules and benefits. Select the most appropriate one based on the data you need to retrieve and the relationships between your tables.