SQL Query 面试题与答案

问题 26. Explain the purpose of the ORDER BY clause.

The ORDER BY clause is used to sort the result set of a query in ascending or descending order based on one or more columns.

Example:

SELECT column1, column2 FROM table ORDER BY column1 DESC;

问题 27. What is a self-join?

A self-join is a regular join, but the table is joined with itself. It is useful when you want to combine rows with related data in the same table.

Example:

SELECT employee1.name, employee2.name FROM Employee employee1, Employee employee2 WHERE employee1.manager_id = employee2.employee_id;

问题 28. Write a SQL query to find the most recent record in a table.

SELECT * FROM table ORDER BY timestamp_column DESC LIMIT 1;

问题 29. Explain the concept of a composite key.

A composite key consists of more than one column and is used to uniquely identify a record in a table when a single column is not sufficient.

问题 30. What is the difference between a stored procedure and a function?

A stored procedure does not necessarily return a value, while a function must return a value. Functions can be used in SQL statements, whereas procedures cannot.