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20 SQL Union Interview Questions and Answers

Prepare for the types of questions you are likely to be asked when interviewing for a position where SQL Union will be used.

SQL Union is a powerful tool that allows developers to combine the results of two or more SQL queries into a single result set. This can be useful when you need to retrieve data from multiple tables or when you want to combine data from multiple queries for further analysis. If you’re interviewing for a position that involves working with SQL Union, it’s important to be prepared to answer questions about how it works and how you would use it to solve various problems. In this article, we’ll discuss some common SQL Union interview questions and how you can answer them.

SQL Union Interview Questions and Answers

Here are 20 commonly asked SQL Union interview questions and answers to prepare you for your interview:

1. What is a Union in SQL?

A Union in SQL is used to combine the results of two or more queries into a single result set. This is useful when you want to combine data from multiple tables or when you want to combine data from multiple queries.

2. Can you explain the differences between UNION, UNION ALL and INTERSECT commands in Oracle SQL?

The UNION command in SQL is used to combine the results of two or more queries into a single result set. The UNION ALL command is similar to UNION, but it includes duplicate rows in the results. The INTERSECT command is used to find the intersection of two or more queries, meaning the rows that are returned by both queries.

3. How can you use union to find common records between two tables?

Union is a SQL keyword that is used to combine the results of two or more SQL queries into a single result set. When using union, the columns of the resulting table will be the union of the columns of the input tables. The number of columns and their data types must be the same in all of the input tables, and the order of the columns must be the same in all of the input tables.

4. What’s the difference between a JOIN and a UNION operation in MySQL?

A UNION operation in MySQL is used to combine the results of two or more SQL queries into a single result set. A JOIN operation, on the other hand, is used to combine the data from two or more tables into a single result set.

5. What are some different uses of UNION operators in SQL?

UNION operators are commonly used to combine the results of two or more SQL queries into a single result set. This can be useful when you want to combine data from multiple tables into a single report, or when you want to find all the rows that match any of a number of conditions. UNION operators can also be used to eliminate duplicate rows from a result set.

6. Is it possible to have a column with duplicate values when using a UNION command in Oracle SQL? If yes, then how?

Yes, it is possible to have a column with duplicate values when using a UNION command in Oracle SQL. This can happen if the columns being selected for the UNION have different data types. For example, if one column is an INT and the other is a VARCHAR, then the UNION will treat them as two separate columns and will not remove any duplicates.

7. Can you give me an example of a case where a full outer join would be useful?

A full outer join would be useful in a case where you want to combine the results of two tables, but you don’t want to miss any data from either table. A full outer join will include all rows from both tables, even if there is no matching data in the other table.

8. Why do we need to use DISTINCT keyword when using a UNION operator in Oracle SQL?

The DISTINCT keyword is used with the UNION operator to remove any duplicate rows that might be returned from the query. Without the DISTINCT keyword, the UNION operator would simply return all rows, including any duplicates.

9. When should I use a RIGHT OUTER JOIN instead of a LEFT OUTER JOIN?

If you want all of the rows from the right table, even if there are no matches in the left table, then you should use a RIGHT OUTER JOIN.

10. Why are unions useful for data scientists?

Unions are useful for data scientists because they allow for the combination of data from multiple tables into a single table. This can be helpful when trying to analyze data from multiple sources at once.

11. Can you explain what type conversion errors are in the context of SQL statements that include joins or unions?

A type conversion error can occur when two columns in different tables being joined or unioned are of different data types. For example, if one column is an INT and the other is a VARCHAR, a type conversion error will occur if you try to join or union the two columns. In order to avoid this error, you will need to make sure that the data types of the columns being joined or unioned are the same.

12. What is your opinion on whether or not it’s important to keep the number of columns in a select statement consistent when using unions?

I believe that it is important to keep the number of columns consistent when using unions, as it makes the resulting data much easier to read and understand. If the number of columns is not consistent, then it can be very confusing to try and figure out what the data is actually telling you.

13. What is the best way to ensure data integrity while performing a union operation in Oracle SQL?

The best way to ensure data integrity while performing a union operation in Oracle SQL is to use the “union all” operator instead of the “union” operator. The “union all” operator will return all rows from both tables, even if there are duplicates, whereas the “union” operator will only return unique rows.

14. What happens if there are many NULLs in one table when using a UNION statement?

When using a UNION statement, the resulting table will only contain one row for each distinct combination of values. This means that if there are many NULL values in one table, they will all be grouped together in the resulting table.

15. What is the difference between a FULL OUTER JOIN and a CROSS JOIN?

A FULL OUTER JOIN returns all rows from both tables, even if there are no matches between the two tables. A CROSS JOIN returns all rows from both tables, but only includes columns from the first table.

16. What’s the difference between a UNION and a VIEW?

A UNION is a query that combines the results of two or more other queries. A VIEW is a saved query that can be used like a table.

17. What are the advantages and disadvantages of using a UNION vs. a JOIN?

The main advantage of using a UNION is that it allows you to combine the results of two or more SQL queries into a single result set. This can be useful if you want to combine data from multiple tables into a single report, for example. The main disadvantage of using a UNION is that it can be slower and more resource-intensive than using a JOIN, since the database has to process the results of each query separately before combining them.

18. Are UNION operations faster than MERGE operations in PostgreSQL?

There is no definitive answer to this question, as the performance of UNION and MERGE operations can vary depending on a number of factors. In general, however, UNION operations tend to be faster than MERGE operations, since they require less work to perform.

19. Under which circumstances does a UNION operation work better than a JOIN operation?

A UNION operation is used to combine the results of two or more SQL queries into a single result set. A JOIN operation is used to combine the data from two or more tables into a single result set. A UNION operation is typically used when the data from the two queries are not in the same format, while a JOIN operation is used when the data from the two queries are in the same format.

20. What’s the difference between a UNION and EXCEPT clause in SQL Server?

A UNION clause is used to combine the results of two or more SQL queries into a single result set. An EXCEPT clause is used to return all of the rows from the first query that are not also returned by the second query.

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