Tricky Sql Interview Questions
Challenging SQL Interview Questions
Tricky Sql Interview Questions
Tricky SQL interview questions often require a deep understanding of SQL concepts and the ability to think critically and logically. These questions may involve complex queries, joins, subqueries, aggregate functions, and data manipulation tasks, challenging candidates to demonstrate their problem-solving skills and expertise in SQL. Some common examples include queries that involve multiple tables with different relationships, finding duplicates in a table, handling null values, optimizing query performance, and writing queries to generate specific results. Candidates should be prepared to explain their thought process, communicate their reasoning clearly, and showcase their ability to handle challenging SQL scenarios under pressure.
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1 - What is the difference between WHERE and HAVING clause in SQL?
WHERE clause is used to filter rows before any grouping is performed, while HAVING clause is used to filter rows after the grouping is done. The WHERE clause is applied to individual rows before they are part of groups, whereas the HAVING clause is applied to the result of the group.
2) Explain SQL joins and their types.
SQL joins are used to combine rows from two or more tables based on a related column between them. Types of SQL joins include INNER JOIN, LEFT JOIN (or LEFT OUTER JOIN), RIGHT JOIN (or RIGHT OUTER JOIN), and FULL JOIN (or FULL OUTER JOIN). Inner join returns rows when there is at least one match in both tables. Left join returns all rows from the left table and the matched rows from the right table. Right join returns all rows from the right table and the matched rows from the left table. Full join returns rows when there is a match in one of the tables.
3) What is a subquery in SQL?
A subquery is a query within another query. It can be nested inside an outer query to retrieve data from the database. Subqueries can be used with SELECT, INSERT, UPDATE, and DELETE statements to handle complex conditions and perform operations on the result set returned by the subquery.
4) Explain the difference between UNION and UNION ALL in SQL.
UNION is used to combine the result sets of two or more SELECT statements and removes duplicate rows from the result set. UNION ALL, on the other hand, also combines the result sets of multiple SELECT statements but does not remove duplicate rows, resulting in a larger result set overall.
5) What is the difference between TRUNCATE and DELETE in SQL?
TRUNCATE is a DDL command used to delete all rows from a table, making it empty and cannot be rolled back. DELETE is a DML command used to remove specific rows based on conditions, and it can be rolled back if needed to restore the deleted data. TRUNCATE is faster than DELETE as it doesn't log individual row deletions.
6) What are SQL indexes and how do they improve performance?
Indexes are database objects that improve the speed of data retrieval operations on database tables. They enhance query performance by providing quick access to rows in a table, especially when searching or sorting data based on indexed columns. Indexes are created using one or more columns to speed up data retrieval operations and reduce query execution time.
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