SQL, or Structured Query Language, is a standard programming language used to manage and manipulate relational databases. It is a fundamental skill for data scientists to have as it is commonly used to extract and analyze data.
1. What is SQL?
SQL is a programming language used to manage and manipulate relational databases. It is used to create, modify, and delete data in a database, as well as query data from a database.
2. What are the different types of SQL statements?
There are several types of SQL statements, including SELECT, INSERT, UPDATE, DELETE, CREATE, ALTER, and DROP. The SELECT statement is used to retrieve data from a database, while the INSERT statement is used to insert new data into a table. The UPDATE statement is used to modify existing data in a table, and the DELETE statement is used to delete data from a table. The CREATE statement is used to create new tables, while the ALTER statement is used to modify existing tables. The DROP statement is used to delete tables.
3. What is a primary key?
A primary key is a unique identifier for each record in a table. It is a column or a combination of columns that uniquely identify each record in a table. A primary key cannot contain null values and must be unique for each record.
4. What is a foreign key?
A foreign key is a column or a combination of columns that is used to establish a relationship between two tables. It is a reference to the primary key in another table. The foreign key ensures referential integrity between the two tables.
5. What is the difference between the WHERE and HAVING clauses?
The WHERE clause is used to filter records based on a specific condition. It is used to retrieve records that meet a certain criteria. The HAVING clause is used to filter records based on a specific condition, but it is used with aggregate functions such as SUM, AVG, MIN, MAX, and COUNT.
6. What is a JOIN?
A JOIN is used to combine rows from two or more tables based on a related column between them. It is used to retrieve data that is spread across multiple tables.
7. What are the different types of JOINs?
There are several types of JOINs, including INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN. The INNER JOIN returns only the records that have matching values in both tables, while the LEFT JOIN returns all records from the left table and matching records from the right table. The RIGHT JOIN returns all records from the right table and matching records from the left table, while the FULL OUTER JOIN returns all records from both tables.
8. What is a subquery?
A subquery is a query that is nested within another query. It is used to retrieve data from one or more tables based on a specific condition.
9. What is the difference between a UNION and UNION ALL?
A UNION is used to combine the results of two or more SELECT statements into a single result set, while removing duplicates. UNION ALL, on the other hand, is used to combine the results of two or more SELECT statements into a single result set, including duplicates.
10. What is normalization?
Normalization is the process of organizing data in a database to reduce redundancy and improve data integrity. It involves breaking down a large table into smaller tables to reduce data duplication and ensure data consistency.
11. What is denormalization?
Denormalization is the process of adding redundant data to a database to improve performance. It involves combining related data into a single table to reduce the number of JOINs required to retrieve data.
12. What is an index?
An index is a data structure that is used to improve the performance of database queries. It is created on one or more columns in a table and allows the database to quickly locate the data that matches a specific query.
13. What is a stored procedure?
A stored procedure is a precompiled set of SQL statements that are stored in the database and can be called by other programs. It is used to simplify complex SQL queries and improve database performance.
14. What is a trigger?
A trigger is a set of SQL statements that are automatically executed by the database when a certain event occurs. It is used to enforce business rules, perform data validation, and maintain data integrity.
15. What are the different types of triggers?
There are two types of triggers in SQL: BEFORE triggers and AFTER triggers. BEFORE triggers are executed before the event that triggered them, while AFTER triggers are executed after the event that triggered them.
In conclusion, SQL is a critical skill for data scientists to have, and understanding these top technical interview questions and answers can help prepare for an interview. By knowing the basics of SQL, data scientists can effectively extract and analyze data, improving their ability to make data-driven decisions.
FAQs (Frequently Asked Questions)
Q: What is SQL?
A: SQL is a programming language used to manage and manipulate relational databases.
Q: What are the different types of SQL statements?
A: There are several types of SQL statements, including SELECT, INSERT, UPDATE, DELETE, CREATE, ALTER, and DROP.
Q: What is a primary key?
A: A primary key is a unique identifier for each record in a table.
Q: What is a foreign key?
A: A foreign key is a column or a combination of columns that is used to establish a relationship between two tables.
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