SQL Query perguntas e respostas de entrevista
Pergunta 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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Pergunta 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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Pergunta 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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Pergunta 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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Pergunta 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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Mais uteis segundo os usuarios:
- What is SQL?
- What is the difference between SQL and MySQL?
- What is a primary key?
- What is an index in a database?
- What is a foreign key?