Sql Server Query Interview Questions
Top SQL Server Query Interview Questions to Prepare For
Sql Server Query Interview Questions
During a SQL Server query interview, candidates may be asked a variety of questions that assess their knowledge and understanding of SQL Server database concepts and query writing skills. Common interview questions may include topics such as query optimization techniques, indexing strategies, joins, subqueries, stored procedures, functions, triggers, transaction management, data manipulation language (DML) statements, and troubleshooting common SQL Server-related issues. Candidates may also be asked to write SQL queries to retrieve specific information from a given database schema, demonstrate their ability to analyze and optimize query performance, and explain their thought process behind selecting certain query approaches. It is important for candidates to showcase their expertise in SQL Server querying and demonstrate proficiency in working with databases.
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1 - What is a subquery in SQL Server?
A subquery is a query within another SQL query and is enclosed in parentheses. It is used to retrieve data from one or more tables based on the result of the outer query. For example, to retrieve the names of employees who have a salary greater than the average salary, a subquery can be used to calculate the average salary first and then compare it with each employee's salary.
2) Differentiate between WHERE and HAVING clause in SQL Server.
The WHERE clause is used to filter rows based on a specified condition before the data is grouped, whereas the HAVING clause is used to filter groups based on a specified condition after the data is grouped. For example, when retrieving employees with a salary greater than 50000, the WHERE clause is used. If we want to retrieve departments with an average salary greater than 50000, the HAVING clause is used.
3) Explain the difference between UNION and UNION ALL in SQL Server.
UNION is used to combine the result sets of two or more SELECT statements and removes any duplicate rows, whereas UNION ALL combines the result sets of two or more SELECT statements but retains all rows, including duplicates. If you want to eliminate duplicates, use UNION; if you want to include duplicates, use UNION ALL.
4) What is normalization in SQL Server?
Normalization is the process of organizing data in a database to reduce redundancy and dependency by dividing large tables into smaller tables and defining relationships between them. There are different normal forms such as First Normal Form (1NF), Second Normal Form (2NF), and Third Normal Form (3NF) which help in achieving data integrity and efficiency.
5) Explain the difference between clustered and non clustered indexes in SQL Server.
A clustered index determines the physical order of data rows in a table and is organized based on the index key. It changes the way records are stored in the table. On the other hand, a non clustered index does not change the way records are stored and creates a separate structure that holds the index key and a pointer to the data row. Multiple non clustered indexes can be created on a table, but only one clustered index is allowed.
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