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DATE_FORMA T(date, format) 根据格式串format 格式化日期或日期和时间值date,返回结果串。
可用DATE_FORMAT( ) 来格式化DATE 或DATETIME 值,以便得到所希望的格式。根据format字符串格式化date值:
%S, %s 两位数字形式的秒( 00,01, . . ., 59)
%i 两位数字形式的分( 00,01, . . ., 59)
%H 两位数字形式的小时,24 小时(00,01, . . ., 23)
%h, %I 两位数字形式的小时,12 小时(01,02, . . ., 12)
%k 数字形式的小时,24 小时(0,1, . . ., 23)
%l 数字形式的小时,12 小时(1, 2, . . ., 12)
%T 24 小时的时间形式(h h : m m : s s)
%r 12 小时的时间形式(hh:mm:ss AM 或hh:mm:ss PM)
%p AM 或P M
%W 一周中每一天的名称( S u n d a y, Monday, . . ., Saturday)
%a 一周中每一天名称的缩写( Sun, Mon, . . ., Sat)
%d 两位数字表示月中的天数( 00, 01, . . ., 31)
%e 数字形式表示月中的天数( 1, 2, . . ., 31)
%D 英文后缀表示月中的天数( 1st, 2nd, 3rd, . . .)
%w 以数字形式表示周中的天数( 0 = S u n d a y, 1=Monday, . . ., 6=Saturday)
%j 以三位数字表示年中的天数( 001, 002, . . ., 366)
% U 周(0, 1, 52),其中Sunday 为周中的第一天
%u 周(0, 1, 52),其中Monday 为周中的第一天
%M 月名(J a n u a r y, February, . . ., December)
%b 缩写的月名( J a n u a r y, February, . . ., December)
%m 两位数字表示的月份( 01, 02, . . ., 12)
%c 数字表示的月份( 1, 2, . . ., 12)
%Y 四位数字表示的年份
%y 两位数字表示的年份
%% 直接值“%”
select date_format(日期字段,'%Y-%m-%d') as ‘日期' from testbitsCN.com

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.

SQL commands in MySQL can be divided into categories such as DDL, DML, DQL, DCL, etc., and are used to create, modify, delete databases and tables, insert, update, delete data, and perform complex query operations. 1. Basic usage includes CREATETABLE creation table, INSERTINTO insert data, and SELECT query data. 2. Advanced usage involves JOIN for table joins, subqueries and GROUPBY for data aggregation. 3. Common errors such as syntax errors, data type mismatch and permission problems can be debugged through syntax checking, data type conversion and permission management. 4. Performance optimization suggestions include using indexes, avoiding full table scanning, optimizing JOIN operations and using transactions to ensure data consistency.

InnoDB achieves atomicity through undolog, consistency and isolation through locking mechanism and MVCC, and persistence through redolog. 1) Atomicity: Use undolog to record the original data to ensure that the transaction can be rolled back. 2) Consistency: Ensure the data consistency through row-level locking and MVCC. 3) Isolation: Supports multiple isolation levels, and REPEATABLEREAD is used by default. 4) Persistence: Use redolog to record modifications to ensure that data is saved for a long time.

MySQL's position in databases and programming is very important. It is an open source relational database management system that is widely used in various application scenarios. 1) MySQL provides efficient data storage, organization and retrieval functions, supporting Web, mobile and enterprise-level systems. 2) It uses a client-server architecture, supports multiple storage engines and index optimization. 3) Basic usages include creating tables and inserting data, and advanced usages involve multi-table JOINs and complex queries. 4) Frequently asked questions such as SQL syntax errors and performance issues can be debugged through the EXPLAIN command and slow query log. 5) Performance optimization methods include rational use of indexes, optimized query and use of caches. Best practices include using transactions and PreparedStatemen

MySQL is suitable for small and large enterprises. 1) Small businesses can use MySQL for basic data management, such as storing customer information. 2) Large enterprises can use MySQL to process massive data and complex business logic to optimize query performance and transaction processing.

InnoDB effectively prevents phantom reading through Next-KeyLocking mechanism. 1) Next-KeyLocking combines row lock and gap lock to lock records and their gaps to prevent new records from being inserted. 2) In practical applications, by optimizing query and adjusting isolation levels, lock competition can be reduced and concurrency performance can be improved.


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