联结
SQL最强大的功能之一就是能在数据检索查询的执行中联结表。而联结表是基于关系表。理解关系表如下,假如由同一供应商生产的多种物品,在数据库中需要存储产品信息,而在产品信息的这一行不存储供应商信息,因为这些产品的供应商是同一个,所以在产品信息这一行里记录一个唯一的供应商id就可以,供应商信息单独的存储一个表,并且也用为一个的id记录。这样,产品表与供应商表就通过 共同的供应商id连接起来了。如下表所示:
在vendors表中,vend_id称为主键,products表中的vend_id为外键。
为什么要用联结表呢,将数据分解为多个表能够更有效的存储,更方便的处理,并且具有更大的可伸缩性。但是这些好处是有代价的。如果数据存储在多个表中,怎么样用单条SELECT语句检索出数据?答案是使用联结。简单的说联结是一种机制,用来在一条SELECT语句中关联表,因此称之为联结。
下面的示例最主要的地方就是 vendors.vend_id = products.vend_id, 在比较时是 从 vendors表中逐一拿出每一行,这个每一行与products表中的所有行进行比较,筛选出products表中的满足条件的每一行。

此外联结表还可以用 INNER JOIN .. ON来表示,语句的作用与上面的示例是一样的:

联结多个表, 可以如下理解下例,WHERE的三个条件,第一个条件,产品表中的每一个产品都会有一个vend_id,所有这个条件不会过滤掉一些产品;第二个条件,处于订单上的产品,应该是所有产品的一部分分;第三个条件,订单编号为20005的产品;


ACID attributes include atomicity, consistency, isolation and durability, and are the cornerstone of database design. 1. Atomicity ensures that the transaction is either completely successful or completely failed. 2. Consistency ensures that the database remains consistent before and after a transaction. 3. Isolation ensures that transactions do not interfere with each other. 4. Persistence ensures that data is permanently saved after transaction submission.

MySQL is not only a database management system (DBMS) but also closely related to programming languages. 1) As a DBMS, MySQL is used to store, organize and retrieve data, and optimizing indexes can improve query performance. 2) Combining SQL with programming languages, embedded in Python, using ORM tools such as SQLAlchemy can simplify operations. 3) Performance optimization includes indexing, querying, caching, library and table division and transaction management.

MySQL uses SQL commands to manage data. 1. Basic commands include SELECT, INSERT, UPDATE and DELETE. 2. Advanced usage involves JOIN, subquery and aggregate functions. 3. Common errors include syntax, logic and performance issues. 4. Optimization tips include using indexes, avoiding SELECT* and using LIMIT.

MySQL is an efficient relational database management system suitable for storing and managing data. Its advantages include high-performance queries, flexible transaction processing and rich data types. In practical applications, MySQL is often used in e-commerce platforms, social networks and content management systems, but attention should be paid to performance optimization, data security and scalability.

The relationship between SQL and MySQL is the relationship between standard languages and specific implementations. 1.SQL is a standard language used to manage and operate relational databases, allowing data addition, deletion, modification and query. 2.MySQL is a specific database management system that uses SQL as its operating language and provides efficient data storage and management.

InnoDB uses redologs and undologs to ensure data consistency and reliability. 1.redologs record data page modification to ensure crash recovery and transaction persistence. 2.undologs records the original data value and supports transaction rollback and MVCC.

Key metrics for EXPLAIN commands include type, key, rows, and Extra. 1) The type reflects the access type of the query. The higher the value, the higher the efficiency, such as const is better than ALL. 2) The key displays the index used, and NULL indicates no index. 3) rows estimates the number of scanned rows, affecting query performance. 4) Extra provides additional information, such as Usingfilesort prompts that it needs to be optimized.

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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