SQL Interview Questions - ISNULL vs COALESCE

SQL Interview Questions - ISNULL vs COALESCE Function

Often, we need to handle NULL values in our data, or compare several data elements and return a non-null value, or evaluate a value for nullability, and if the value is NULL return an alternative non-null value. SQL provides functions and methodologies to evaluate and handle Null values including COALESCE and ISNULL functions and CASE statements. In this article, we will see some important Interview Questions based on this topic.

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Below are some common Interview Questions based on the SQL NULL value and methods to handle the NULL values:

Q01. What is the difference between ISNULL() and COALESCE()?

Q02. What is the difference between ISNULL() and IS NULL?

Q03. What is the difference between ISNULL() and NULLIF()?

Q04. What is the difference between NULLIF() and IIF()?

Q05. How ISNULL() differs from NVL() and IFNULL()?

Q06. What is the difference between COALESCE() and CASE?

Q07. Can you use COALESCE() in place of ISNULL()?

Q08. Can you use user-defined type in ISNULL() or COALESCE()?

Q09. Can you use mixed data types arguments in ISNULL() and COALESCE()?

Q10. Does input order matters in COALESCE() and ISNULL()?

Q11. Can a COALESCE() expression ever return the NULL value?

Q12. Can a ISNULL() function ever returns NULL value?

Q13. What is the use of the dm_exec_describe_first_result_set command?

Q14. What data type is used by the ISNULL() function to return the result?

Q15. What data type is used by the COALESCE() function to return the result?

Q16. What will be the result data type if you pass different data type operands in the ISNULL() function?

Q17. What will be the result data type if you pass different data type operands in COALESCE() function?

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