SQL

Question 1

What is SQL?

Answer 1

A language to manage and query data in relational databases.

Question 2

What is a table in SQL?

Answer 2

A set of rows and columns that stores related data.

Question 3

What is a primary key?

Answer 3

A column that uniquely identifies each row in a table.

Question 4

What is a foreign key?

Answer 4

A column that links to the primary key of another table.

Question 5

How do you get data from a table?

Answer 5

Use SELECT.

Question 6

How do you insert data into a table?

Answer 6

Use INSERT INTO.

Question 7

How do you update data in a table?

Answer 7

Use UPDATE with SET and WHERE.

Question 8

How do you delete data from a table?

Answer 8

Use DELETE FROM with WHERE.

Question 9

How do you filter rows?

Answer 9

Use WHERE clause.

Question 10

How do you sort results?

Answer 10

Use ORDER BY.

Question 11

How do you limit the number of rows?

Answer 11

Use LIMIT.

Question 12

What is an INNER JOIN?

Answer 12

Returns matching rows from both tables.

Question 13

What is a LEFT JOIN?

Answer 13

Returns all rows from the left table, even if there’s no match in the right.

Question 14

What is a RIGHT JOIN?

Answer 14

Returns all rows from the right table, even if there’s no match in the left.

Question 15

What is a FULL OUTER JOIN?

Answer 15

Returns all rows from both tables, with NULLs where there’s no match.

Question 16

How do you count rows?

Answer 16

Use COUNT().

Question 17

How do you find the average value?

Answer 17

Use AVG().

Question 18

How do you find the total sum?

Answer 18

Use SUM().

Question 19

How do you group data?

Answer 19

Use GROUP BY.

Question 20

How do you filter grouped results?

Answer 20

Use HAVING.

Question 21

What is an index in SQL?

Answer 21

A structure that speeds up searches on a column.

Question 22

When should you create an index?

Answer 22

When a column is used often in WHERE, JOIN, or ORDER BY.

Question 23

How do you check query performance?

Answer 23

Use EXPLAIN before the query.

Question 24

How do you optimize a slow query?

Answer 24

Use EXPLAIN, add indexes, avoid SELECT *, and rewrite joins.

Question 25

What is a transaction?

Answer 25

A group of SQL operations that succeed or fail together.

Question 26

What does ACID stand for?

Answer 26

Atomicity, Consistency, Isolation, Durability.

Question 27

How do you start a transaction?

Answer 27

Use BEGIN or START TRANSACTION.

Question 28

How do you end a successful transaction?

Answer 28

Use COMMIT.

Question 29

How do you cancel a transaction?

Answer 29

Use ROLLBACK.

Question 30

What is a view?

Answer 30

A saved SQL query that you can query like a table.

Question 31

What is a subquery?

Answer 31

A query inside another query.

Question 32

What is a CTE?

Answer 32

A Common Table Expression that makes queries easier to read.

Question 33

How do you prevent SQL injection?

Answer 33

Use parameterized queries or prepared statements.

Question 34

Why avoid SELECT *?

Answer 34

It selects unnecessary data and can slow performance.

Question 35

Why use aliases in SQL?

Answer 35

To make column or table names shorter or clearer.