SQL Query 面试题与答案

问题 41. Explain the concept of SQL injection.

SQL injection is a security vulnerability where an attacker can insert malicious SQL code into a query, potentially gaining unauthorized access to a database.

问题 42. What is the purpose of the SQL DISTINCT keyword?

The DISTINCT keyword is used to return unique values in the result set of a SELECT query by eliminating duplicate rows.

Example:

SELECT DISTINCT column FROM table;

问题 43. Write a SQL query to find the employees who do not belong to any department.

SELECT * FROM Employee WHERE department_id IS NULL;

问题 44. Explain the difference between the INNER JOIN and OUTER JOIN.

INNER JOIN returns rows when there is a match in both tables, while OUTER JOIN returns all rows from one table and the matching rows from the other table, filling in the missing values with NULLs.

Example:

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

问题 45. What is the purpose of the SQL UPDATE statement?

The UPDATE statement is used to modify the existing records in a table by changing the values of one or more columns.

Example:

UPDATE table SET column1 = value1, column2 = value2 WHERE condition;