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15.6 Draw an entity-relationship diagram for a database with companies, people, and professionals (people who work for companies). 在公司Companies工作的人是专家 阅读全文
posted @ 2016-04-07 07:16
Grandyang
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15.5 What is denormalization? Explain the pros and cons. 逆规范化Denormalization是一种通过添加冗余数据的数据库优化技术，可以帮助我们减少关系数据库中耗时的交Join。在一般的规范化的数据库中，我们将数据存在不同的表中是为了减少冗 阅读全文
posted @ 2016-04-07 06:57
Grandyang
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Suppose that a website contains two tables, the Customers table and the Orders table. Write a SQL query to find all customers who never order anything 阅读全文
posted @ 2016-04-07 06:41
Grandyang
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Write a SQL query to find all duplicate emails in a table named Person. For example, your query should return the following for the above table: Note: 阅读全文
posted @ 2016-04-07 06:09
Grandyang
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