DDL、DML和DCL的理解_MySQL-Mysql Tutorial-php.cn

Home

Database

Mysql Tutorial

DDL、DML和DCL的理解_MySQL

WBOYWBOYWBOYWBOYWBOYWBOYWBOYWBOYWBOYWBOYWBOYWBOYWB

Jun 01, 2016 pm 01:31 PM

database

bitsCN.com DL、DML和DCL的理解 1、DDL 1-1、DDL的概述 DDL（Data Definition Language 数据定义语言）用于操作对象和对象的属性，这种对象包括数据库本身，以及数据库对象，像：表、视图等等，DDL对这些对象和属性的管理和定义具体表现在Create、Drop和Alter上。特别注意：DDL操作的“对象”的概念，”对象“包括对象及对象的属性，而且对象最小也比记录大个层次。以表举例：Create创建数据表，Alter可以更改该表的字段，Drop可以删除这个表，从这里我们可以看到，DDL所站的高度，他不会对具体的数据进行操作。 1-2、DDL的主要语句（操作） Create语句：可以创建数据库和数据库的一些对象。 Drop语句：可以删除数据表、索引、触发程序、条件约束以及数据表的权限等。 Alter语句：修改数据表定义及属性。 1-3、DDL的操作对象（表） 1-3-1、表的概念表的创建就是用来存放数据用的，由于我们存放的数据的不通，所以我们需要定义些数据类型，以方便管理。 1-3-2、表的属性 www.bitsCN.com 主键属性：主键就是主键约束，只不过起的名字不同了，主键的起名偏向于虚的（就是描述描述这件事），主键约束起名偏向于实得（就是描述操作的实施），描述的都是同一件事，主键约束就是表中的一个属性；在一个表中最多可以有一个主键；一个主键可以定义在一个或多个字段；主键使一个或多个字段的值必须唯一且不为空，这样做可以通过该字段或该组字段中的值唯一的代表一条记录。唯一属性：一个表中只能有一个主键属性，为了方表用户，提出唯一约束；唯一约束可以定义在一个或多个字段上；唯一约束使该字段或该组字段中的值唯一，可以为空，但是，不能重复。外键属性：又叫外键，又叫外键约束，跟主键和主键约束的关系是一样的；外键约束针对的两个表，如果表A的主关键字是表B中的字段，则该字段称为表B的外键，表A称为主表，表B称为从表，但要注意，必须要计算机要知道你是这种关系。核查、Null和缺省属性：核查属性又叫核查约束，Null属性又叫Null约束，缺省属性又叫缺省约束；这些名称是描述一件事，描述一种情况，这件事或这张情况我们当然可以人为的那样特意做（输入数据是注意就行），但是，他们的本意是实现自动化，也就是让计算机做这件事。（你知道为什么建立主键和唯一约束的时候，会自动的创建索引吗？而且是唯一索引，想一想索引大多在那些字段上用，以及索引的作用就会知道了。像主键约束、唯一约束、非空约束、外键约束、核查约束和缺省约束这些操作都是使表具有某些特性，所以在这里我认为他们都是表的属性。） 2、DML 2-1、DML的概述 DML（Data Manipulation Language 数据操控语言）用于操作数据库对象中包含的数据，也就是说操作的单位是记录。 2-2、DML的主要语句（操作） Insert语句：向数据表张插入一条记录。 Delete语句：删除数据表中的一条或多条记录，也可以删除数据表中的所有记录，但是，它的操作对象仍是记录。 Update语句：用于修改已存在表中的记录的内容。 2-3、DML的操作对象——记录 2-3-1、注意当我们对记录进行Insert、Delete和Update操作的时候，一定要注意，一定要清楚DDL对其的一些操作。 www.bitsCN.com 3、DCL 3-1、DCL的概述 DCL（Data Control Language 数据控制语句）的操作是数据库对象的权限，这些操作的确定使数据更加的安全。 3-2、DCL的主要语句（操作） Grant语句：允许对象的创建者给某用户或某组或所有用户（PUBLIC）某些特定的权限。 Revoke语句：可以废除某用户或某组或所有用户访问权限 3-3、DCL的操作对象（用户）此时的用户指的是数据库用户。 bitsCN.com

Statement

The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn

Explain the role of InnoDB redo logs and undo logs.Apr 15, 2025 am 12:16 AM

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.

What are the key metrics to look for in an EXPLAIN output (type, key, rows, Extra)?Apr 15, 2025 am 12:15 AM

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.

What is the Using temporary status in EXPLAIN and how to avoid it?Apr 15, 2025 am 12:14 AM

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

Describe the different SQL transaction isolation levels (Read Uncommitted, Read Committed, Repeatable Read, Serializable) and their implications in MySQL/InnoDB.Apr 15, 2025 am 12:11 AM

MySQL/InnoDB supports four transaction isolation levels: ReadUncommitted, ReadCommitted, RepeatableRead and Serializable. 1.ReadUncommitted allows reading of uncommitted data, which may cause dirty reading. 2. ReadCommitted avoids dirty reading, but non-repeatable reading may occur. 3.RepeatableRead is the default level, avoiding dirty reading and non-repeatable reading, but phantom reading may occur. 4. Serializable avoids all concurrency problems but reduces concurrency. Choosing the appropriate isolation level requires balancing data consistency and performance requirements.

MySQL vs. Other Databases: Comparing the OptionsApr 15, 2025 am 12:08 AM

MySQL is suitable for web applications and content management systems and is popular for its open source, high performance and ease of use. 1) Compared with PostgreSQL, MySQL performs better in simple queries and high concurrent read operations. 2) Compared with Oracle, MySQL is more popular among small and medium-sized enterprises because of its open source and low cost. 3) Compared with Microsoft SQL Server, MySQL is more suitable for cross-platform applications. 4) Unlike MongoDB, MySQL is more suitable for structured data and transaction processing.

How does MySQL index cardinality affect query performance?Apr 14, 2025 am 12:18 AM

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.

MySQL: Resources and Tutorials for New UsersApr 14, 2025 am 12:16 AM

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.

Real-World MySQL: Examples and Use CasesApr 14, 2025 am 12:15 AM

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.

See all articles