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SQL Query 面试题与答案

问题 21. Explain the difference between a primary key and a unique key.

A primary key is used to uniquely identify each record in a table and cannot contain NULL values. A unique key enforces uniqueness but can contain NULL values.

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问题 22. Write a SQL query to find the nth highest salary from an Employee table.

SELECT salary FROM Employee ORDER BY salary DESC LIMIT 1 OFFSET n-1;

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问题 23. Explain the difference between UNION and JOIN.

UNION is used to combine the results of two or more SELECT statements, while JOIN is used to retrieve data from multiple tables based on a related column between them.

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问题 24. What is a trigger in SQL?

A trigger is a set of instructions that are automatically executed (or 'triggered') in response to certain events on a particular table or view.

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问题 25. Write a SQL query to update multiple rows in a table.

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

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