SQL Query 面试题与答案

问题 6. Explain INNER JOIN and LEFT JOIN.

INNER JOIN returns rows when there is a match in both tables. LEFT JOIN returns all rows from the left table and matching rows from the right table.

Example:

SELECT * FROM table1 INNER JOIN table2 ON table1.id = table2.id;

问题 7. What is a primary key?

A primary key is a unique identifier for a record in a table. It must contain unique values and cannot be NULL.

问题 8. Write a SQL query to calculate the average salary for each department.

SELECT department, AVG(salary) FROM Employee GROUP BY department;

问题 9. Explain the difference between UNION and UNION ALL.

UNION combines and returns unique rows from multiple SELECT statements, while UNION ALL returns all rows including duplicates.

Example:

SELECT column1 FROM table1 UNION SELECT column1 FROM table2;

问题 10. What is an index in a database?

An index is a data structure that improves the speed of data retrieval operations on a database table at the cost of additional writes and storage space.