Redis
is an open source log-type Key-Value database written in ANSI C language, supports network, can be memory-based and persistent, and provides API in multiple languages. Since March 15, 2010, the development of Redis has been hosted by VMware. Since May 2013, the development of Redis has been sponsored by Pivotal.
Why use redis?
For example, if there are 20 million user information in the system, the user information is basically fixed and rarely changes once entered. Then every time you load all user information, you have to request the database. The database compiles and executes your query statement, which will be much less efficient because this information does not change frequently and the amount of data is large.
In larger cases, the usual approach is to add it to the cache and make a judgment before each fetch. If the cache is not empty, then get the value from the cache. If it is empty, then request the database. , and add data to the cache, which greatly improves system access efficiency.
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SQL is a standard language for managing relational databases, while MySQL is a specific database management system. SQL provides a unified syntax and is suitable for a variety of databases; MySQL is lightweight and open source, with stable performance but has bottlenecks in big data processing.

The SQL learning curve is steep, but it can be mastered through practice and understanding the core concepts. 1. Basic operations include SELECT, INSERT, UPDATE, DELETE. 2. Query execution is divided into three steps: analysis, optimization and execution. 3. Basic usage is such as querying employee information, and advanced usage is such as using JOIN connection table. 4. Common errors include not using alias and SQL injection, and parameterized query is required to prevent it. 5. Performance optimization is achieved by selecting necessary columns and maintaining code readability.

SQL commands are divided into five categories in MySQL: DQL, DDL, DML, DCL and TCL, and are used to define, operate and control database data. MySQL processes SQL commands through lexical analysis, syntax analysis, optimization and execution, and uses index and query optimizers to improve performance. Examples of usage include SELECT for data queries and JOIN for multi-table operations. Common errors include syntax, logic, and performance issues, and optimization strategies include using indexes, optimizing queries, and choosing the right storage engine.

Advanced query skills in SQL include subqueries, window functions, CTEs and complex JOINs, which can handle complex data analysis requirements. 1) Subquery is used to find the employees with the highest salary in each department. 2) Window functions and CTE are used to analyze employee salary growth trends. 3) Performance optimization strategies include index optimization, query rewriting and using partition tables.

MySQL is an open source relational database management system that provides standard SQL functions and extensions. 1) MySQL supports standard SQL operations such as CREATE, INSERT, UPDATE, DELETE, and extends the LIMIT clause. 2) It uses storage engines such as InnoDB and MyISAM, which are suitable for different scenarios. 3) Users can efficiently use MySQL through advanced functions such as creating tables, inserting data, and using stored procedures.

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SQL indexes can significantly improve query performance through clever design. 1. Select the appropriate index type, such as B-tree, hash or full text index. 2. Use composite index to optimize multi-field query. 3. Avoid over-index to reduce data maintenance overhead. 4. Maintain indexes regularly, including rebuilding and removing unnecessary indexes.

To delete a constraint in SQL, perform the following steps: Identify the constraint name to be deleted; use the ALTER TABLE statement: ALTER TABLE table name DROP CONSTRAINT constraint name; confirm deletion.


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