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今天,在连接数据库的时候,遇到了一个比较奇怪的问题,而且这个问题 现在出错的原理现在我还是搞不清,但是初步有了一个解决方案。 打开程序的时候,然后很习惯的想把数据打开,这个时候数据打不开 说是: 数据源连接已经被打开。 我就很纳闷了,然后想不管
今天,在连接数据库的时候,遇到了一个比较奇怪的问题,而且这个问题 现在出错的原理现在我还是搞不清,但是初步有了一个解决方案。
打开程序的时候,然后很习惯的想把数据打开,这个时候数据打不开 说是:数据源连接已经被打开。
我就很纳闷了,然后想不管它了,就直接运行,坑爹的,报错了。
没有访问权限?我是在我的机器上面呢。
行,没访问权限,那照着它的说法,那就改成有访问权限咯。于是找到放我数据库的文件夹,点击属性(properties),然后找到安全这项(Security)如下:
图一:打开 properties,点击“Edit” 按钮,会弹出图二的图片。
图二:接着点“Add”按钮。
图三:到了这一步 在点击“Advanced” 也就是中午 高级选项。
图四:点击“Find Now” 按钮,会在“Search result”列表中列出所有的用户。选择 “everyone” 然后点击“OK”。这一步是重中之重。
图五:然后可以在Group or user names里看到我们添加的用户。然后再下面的“Permissions for Everyone” 也就是设置权限里,选择“Full Control”,就OK了,这一步也是很关键的。
通过上面的设定,就可以访问我们的数据。另外有的时候,这样做还是不行?怎么办呢?那可能是你的数据库文件被设定成了只读,只需要再你的数据库文件上 右键,把
“只读”(read-only)去掉就可以了。
希望对你有点帮助,谢谢 晚安。

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Usingtemporary indicates that the need to create temporary tables in MySQL queries, which are commonly found in ORDERBY using DISTINCT, GROUPBY, or non-indexed columns. You can avoid the occurrence of indexes and rewrite queries and improve query performance. Specifically, when Usingtemporary appears in EXPLAIN output, it means that MySQL needs to create temporary tables to handle queries. This usually occurs when: 1) deduplication or grouping when using DISTINCT or GROUPBY; 2) sort when ORDERBY contains non-index columns; 3) use complex subquery or join operations. Optimization methods include: 1) ORDERBY and GROUPB

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MySQL is suitable for web applications and content management systems and is popular for its open source, high performance and ease of use. 1) Compared with PostgreSQL, MySQL performs better in simple queries and high concurrent read operations. 2) Compared with Oracle, MySQL is more popular among small and medium-sized enterprises because of its open source and low cost. 3) Compared with Microsoft SQL Server, MySQL is more suitable for cross-platform applications. 4) Unlike MongoDB, MySQL is more suitable for structured data and transaction processing.

MySQL index cardinality has a significant impact on query performance: 1. High cardinality index can more effectively narrow the data range and improve query efficiency; 2. Low cardinality index may lead to full table scanning and reduce query performance; 3. In joint index, high cardinality sequences should be placed in front to optimize query.

The MySQL learning path includes basic knowledge, core concepts, usage examples, and optimization techniques. 1) Understand basic concepts such as tables, rows, columns, and SQL queries. 2) Learn the definition, working principles and advantages of MySQL. 3) Master basic CRUD operations and advanced usage, such as indexes and stored procedures. 4) Familiar with common error debugging and performance optimization suggestions, such as rational use of indexes and optimization queries. Through these steps, you will have a full grasp of the use and optimization of MySQL.

MySQL's real-world applications include basic database design and complex query optimization. 1) Basic usage: used to store and manage user data, such as inserting, querying, updating and deleting user information. 2) Advanced usage: Handle complex business logic, such as order and inventory management of e-commerce platforms. 3) Performance optimization: Improve performance by rationally using indexes, partition tables and query caches.


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