Top Sql Queries Asked In Interview

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Top Sql Queries Asked In Interview

Suggested: "Commonly Asked SQL Queries in Interviews"

Top Sql Queries Asked In Interview

In interviews, common SQL queries that are often asked include queries to retrieve specific data using SELECT statements with WHERE clauses, filtering and sorting data using GROUP BY and ORDER BY clauses, joining tables using INNER JOIN, LEFT JOIN, or RIGHT JOIN clauses, aggregating data using functions such as COUNT, SUM, AVG, MIN, and MAX, and subqueries to perform nested queries or obtain data from multiple tables. Additionally, queries to update or delete data using UPDATE and DELETE statements, as well as creating and modifying tables using CREATE TABLE and ALTER TABLE commands may also be commonly tested. It is essential to have a strong understanding of these fundamental SQL queries to demonstrate proficiency in querying databases during interviews.

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1 - What is a subquery in SQL?

A subquery is a query nested within another SQL query. It can be used to retrieve data from multiple tables in a single query or to perform calculations on the result set of the outer query. Subqueries can be used in SELECT, INSERT, UPDATE, and DELETE statements and are often used to filter or aggregate data.

2) Explain the difference between WHERE and HAVING clause.

The WHERE clause is used to filter rows based on a specific condition in a SELECT, UPDATE, or DELETE statement. It operates on individual rows before the result set is grouped and sorted. On the other hand, the HAVING clause is used to filter groups based on aggregate conditions in a SELECT statement that contains a GROUP BY clause. It is applied after the rows are grouped and allows filtering on aggregated values.

3) What is a join in SQL?

A join is used to combine rows from two or more tables based on a related column between them. There are different types of joins in SQL, including INNER JOIN (returns rows that have matching values in both tables), LEFT JOIN (returns all rows from the left table and matching rows from the right table), RIGHT JOIN (returns all rows from the right table and matching rows from the left table), and FULL JOIN (returns rows when there is a match in either table).

4) How do you optimize SQL queries for better performance?

SQL query optimization involves various techniques such as using indexes to speed up data retrieval, writing efficient queries by avoiding unnecessary calculations and joins, reducing the amount of data being retrieved, and properly structuring the database schema with normalization. Additionally, analyzing query execution plans, using appropriate data types, and regularly monitoring and tuning queries can help improve performance.

5) Explain the difference between UNION and UNION ALL in SQL.

UNION and UNION ALL are used to combine the result sets of two or more SELECT statements. The main difference is that UNION removes duplicate rows from the combined result set, while UNION ALL includes all rows, including duplicates. UNION ALL is generally faster than UNION, but if you want to eliminate duplicate rows, you should use UNION.

 

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