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1.两结构相同的表数据间移植
Inset into 表一
Select 字段1,字段2,....字段n from表二
建立数据库时设置数据库编码
create database 数据库名 charset uft8
模糊查询时
%(任意字符任意个数)
_(单个字符)
2.聚合函数使用时必须有group by给定分组条件
比如有一个表
求平均成绩是用聚合函数avg()时必须制定此函数作用的范围依据
3.分组,排序,再次筛选,取几条执行顺序
Where>group by >having>order by >limit
4.如上表要求查询2门及2门以上同学的平均分
方法一:分步查询
先查出2门及2门以上不及格学生是谁
然后再查这些同学的平均分,需要2个select语句
select name from stu where score=2
便得到表
name |
张三 |
李四 |
注意sql的执行顺序
先执行select name from stu where score
会得到表2:
name |
张三 |
张三 |
李四 |
李四 |
王五 |
之后再分组和再筛选
但要注意虽然group by在having前,但group by是作为having的筛选分组条件的
然后在表2中having筛选即得
第二步:
找到这些学生后,在把学生名最为查询条件再次筛选,因为第一步筛选掉了及格的了的成绩,结果破坏了数据的原始性,所以还得进行一次筛选
select name,avg(score) from stu where name in (select name from stu where
score=2) group by name;
方法二:利用select后可以直接是判断式及本题特点
分析:首先运行sql查询
select name ,score
得到下表3:
name |
Score |
张三 |
0 |
张三 |
1 |
张三 |
1 |
李四 |
1 |
李四 |
1 |
王五 |
1 |
在select后的字段或条件mysql会依次与表中的数据对比,如score
比如字段name没有要比较的值就显示它在表中的值
从表三发现:sum(score
于是
select name ,avg(score),sum(score
gk>=2;
聚合函数的执行时同时进行
得到结果表:
显然方法二比一更简洁高效
未完待续....!
bitsCN.com

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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