iPhone does not support creating contact groups directly in the address book application, but similar functions can be achieved through the following methods: 1. Use the "Group" function in the address book application, click "Edit" > "Add Group" to create a custom group; 2. Add each contact to the corresponding group by editing it; 3. If you need to send a group message, you can create a new multi-person conversation in the information application; 4. Use iCloud or Gmail to synchronize tagged groups to facilitate the management of a large number of contacts on the computer. These methods are used in conjunction with efficient management of contact groups.

MySQL query performance optimization needs to start from the core points, including rational use of indexes, optimization of SQL statements, table structure design and partitioning strategies, and utilization of cache and monitoring tools. 1. Use indexes reasonably: Create indexes on commonly used query fields, avoid full table scanning, pay attention to the combined index order, do not add indexes in low selective fields, and avoid redundant indexes. 2. Optimize SQL queries: Avoid SELECT*, do not use functions in WHERE, reduce subquery nesting, and optimize paging query methods. 3. Table structure design and partitioning: select paradigm or anti-paradigm according to read and write scenarios, select appropriate field types, clean data regularly, and consider horizontal tables to divide tables or partition by time. 4. Utilize cache and monitoring: Use Redis cache to reduce database pressure and enable slow query

CTEs are a feature introduced by MySQL8.0 to improve the readability and maintenance of complex queries. 1. CTE is a temporary result set, which is only valid in the current query, has a clear structure, and supports duplicate references; 2. Compared with subqueries, CTE is more readable, reusable and supports recursion; 3. Recursive CTE can process hierarchical data, such as organizational structure, which needs to include initial query and recursion parts; 4. Use suggestions include avoiding abuse, naming specifications, paying attention to performance and debugging methods.

The aggregation function is used to perform calculations on a set of values ??and return a single value. Common ones include COUNT, SUM, AVG, MAX, and MIN; GROUPBY groups data by one or more columns and applies an aggregation function to each group. For example, GROUPBYuser_id is required to count the total order amount of each user; SELECTuser_id, SUM(amount)FROMordersGROUPBYuser_id; non-aggregated fields must appear in GROUPBY; multiple fields can be used for multi-condition grouping; HAVING is used instead of WHERE after grouping; application scenarios such as counting the number of classified products, maximum ordering users, monthly sales trends, etc. Mastering these can effectively solve the number

MySQL's EXPLAIN is a tool used to analyze query execution plans. You can view the execution process by adding EXPLAIN before the SELECT query. 1. The main fields include id, select_type, table, type, key, Extra, etc.; 2. Efficient query needs to pay attention to type (such as const, eq_ref is the best), key (whether to use the appropriate index) and Extra (avoid Usingfilesort and Usingtemporary); 3. Common optimization suggestions: avoid using functions or blurring the leading wildcards for fields, ensure the consistent field types, reasonably set the connection field index, optimize sorting and grouping operations to improve performance and reduce capital

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MySQLQueryCache is a built-in caching mechanism used to cache query statements and their results to improve the performance of duplicate queries. 1. It avoids repeated execution of the same query by directly returning cached results; 2. The cache is based on a complete SQL statement, and statement differences or table data changes will cause cache failure; 3. MySQL8.0 has completely removed this function due to poor concurrency performance, low hit rate and high maintenance costs; 4. Alternative solutions include using Redis/Memcached, database middleware ProxySQL, page cache and other more flexible and efficient caching strategies.

The security of remote access to MySQL can be guaranteed by restricting permissions, encrypting communications, and regular audits. 1. Set a strong password and enable SSL encryption. Force-ssl-mode=REQUIRED when connecting to the client; 2. Restrict access to IP and user rights, create a dedicated account and grant the minimum necessary permissions, and disable root remote login; 3. Configure firewall rules, close unnecessary ports, and use springboard machines or SSH tunnels to enhance access control; 4. Enable logging and regularly audit connection behavior, use monitoring tools to detect abnormal activities in a timely manner to ensure database security.