在实际开发过程中,特别是在做数据查询的时候,能够根据动态生成的sql语句将查询的结果转化并返回到业务处理逻辑(或页面展示出来)能大大的减轻业务逻辑的处理复杂度。
大部分情况下,这种动态生成的sql查询语句写法如下:代码如下:
select A表.字段1,A表.字段2,B表.字段返回,C表.字段返回 from A表 ,B表,C表 [where A表,B表,C表关联及各自的条件语句]
但是这个方法有一个缺点,那就是在动态的生成这个查询语句的业务逻辑程序仍然很复杂。这里就介绍一个降低业务逻辑复杂度的查询sql生成方式。其语法结构如下:
代码如下:
select A表.字段1,A表.字段2,B表.字段,C表.字段 from A表 [where A表的条件语句]
业务逻辑程序通过这种方式生成的sql语句时只需修改select的字段,而不需像通用方法那样需要同时动态修改select字段,from的表,以及where 语句。这样真个业务逻辑就能将生成sql语句的关注点由3+个减少为1个。下面就该方式实现举例如下:
首先,建立三个表,一个反应学生基本情况的信息表——student表,两个存放学生相关信息的代码表——sexCode表(性别代码表),gradeCode(年纪代码表),建表语句如下:
代码如下:
-- Create table STUDENT
create table STUDENT
(
ID number,
name nvarchar2(10),
sex char(1),
grade char(1),
age number(2)
)
tablespace SDMP
storage
(
initial 64K
minextents 1
maxextents unlimited
);
-- Add comments to the columns
comment on column STUDENT.name
is '学生姓名';
comment on column STUDENT.sex
is '学生性别';
comment on column STUDENT.grade
is '年级';
comment on column STUDENT.age
is '年龄';
代码如下:
-- Create table SEXCODE
create table SEXCODE
(
DM char(1),
MC nvarchar2(5)
)
tablespace SDMP
storage
(
initial 64K
minextents 1
maxextents unlimited
);
-- Add comments to the columns
comment on column SEXCODE.DM
is '代码';
comment on column SEXCODE.MC
is '名称';
代码如下:
-- Create table GRADECODE
create table GRADECODE
(
DM CHAR(1),
MC NVARCHAR2(5)
)
tablespace SDMP
pctfree 10
initrans 1
maxtrans 255
storage
(
initial 64K
minextents 1
maxextents unlimited
);
-- Add comments to the columns
comment on column GRADECODE.DM
is '代码';
comment on column GRADECODE.MC
is '名称';
然后,执行以下insert语句,分别在每个表中填入信息。
代码如下:
--insert into student
insert into student(id,name,sex,grade,age) values(1,'张三','1','2',8);
insert into student(id,name,sex,grade,age) values(2,'李四','0','1',11);
insert into student(id,name,sex,grade,age) values(3,'王五','1','2',9);
insert into student(id,name,sex,grade,age) values(4,'刘二','0','4',8);
insert into student(id,name,sex,grade,age) values(5,'韩六','0','3',6);
--insert into sexcode
insert into sexcode(dm,mc) values('1','男');
insert into sexcode(dm,mc) values('0','女');
--insert into gradecode
insert into gradecode(dm,mc) values('1','一年级');
insert into gradecode(dm,mc) values('2','二年级');
insert into gradecode(dm,mc) values('3','三年级');
最后,给出常用sql查询方式和本文倡导的查询方式及其查询结果比较:
通用查询方式及其查询结果如下:
代码如下:
select s.id,s.name,sc.mc sex,gc.mc grade,s.age
from student s,sexcode sc,gradecode gc
where sc.dm=s.sex(+) and s.grade=gc.dm(+)
本问题出查询方法及其查询结果如下
代码如下:
select s.id,s.name,s.age,
(select mc from sexcode where dm=s.sex) sex,
(select mc from gradecode where dm=s.grade) grade
from student s
注:1.对于二者的性能,这里只是做了个简单测试,1000条数据查询耗时二者相当,而且本文提到方法甚至略优于普通方法。
2.此方法目前只在oracle数据库中实现并测试,其他数据库请自行测试。

InnoDBBufferPool reduces disk I/O by caching data and indexing pages, improving database performance. Its working principle includes: 1. Data reading: Read data from BufferPool; 2. Data writing: After modifying the data, write to BufferPool and refresh it to disk regularly; 3. Cache management: Use the LRU algorithm to manage cache pages; 4. Reading mechanism: Load adjacent data pages in advance. By sizing the BufferPool and using multiple instances, database performance can be optimized.

Compared with other programming languages, MySQL is mainly used to store and manage data, while other languages such as Python, Java, and C are used for logical processing and application development. MySQL is known for its high performance, scalability and cross-platform support, suitable for data management needs, while other languages have advantages in their respective fields such as data analytics, enterprise applications, and system programming.

MySQL is worth learning because it is a powerful open source database management system suitable for data storage, management and analysis. 1) MySQL is a relational database that uses SQL to operate data and is suitable for structured data management. 2) The SQL language is the key to interacting with MySQL and supports CRUD operations. 3) The working principle of MySQL includes client/server architecture, storage engine and query optimizer. 4) Basic usage includes creating databases and tables, and advanced usage involves joining tables using JOIN. 5) Common errors include syntax errors and permission issues, and debugging skills include checking syntax and using EXPLAIN commands. 6) Performance optimization involves the use of indexes, optimization of SQL statements and regular maintenance of databases.

MySQL is suitable for beginners to learn database skills. 1. Install MySQL server and client tools. 2. Understand basic SQL queries, such as SELECT. 3. Master data operations: create tables, insert, update, and delete data. 4. Learn advanced skills: subquery and window functions. 5. Debugging and optimization: Check syntax, use indexes, avoid SELECT*, and use LIMIT.

MySQL efficiently manages structured data through table structure and SQL query, and implements inter-table relationships through foreign keys. 1. Define the data format and type when creating a table. 2. Use foreign keys to establish relationships between tables. 3. Improve performance through indexing and query optimization. 4. Regularly backup and monitor databases to ensure data security and performance optimization.

MySQL is an open source relational database management system that is widely used in Web development. Its key features include: 1. Supports multiple storage engines, such as InnoDB and MyISAM, suitable for different scenarios; 2. Provides master-slave replication functions to facilitate load balancing and data backup; 3. Improve query efficiency through query optimization and index use.

SQL is used to interact with MySQL database to realize data addition, deletion, modification, inspection and database design. 1) SQL performs data operations through SELECT, INSERT, UPDATE, DELETE statements; 2) Use CREATE, ALTER, DROP statements for database design and management; 3) Complex queries and data analysis are implemented through SQL to improve business decision-making efficiency.

The basic operations of MySQL include creating databases, tables, and using SQL to perform CRUD operations on data. 1. Create a database: CREATEDATABASEmy_first_db; 2. Create a table: CREATETABLEbooks(idINTAUTO_INCREMENTPRIMARYKEY, titleVARCHAR(100)NOTNULL, authorVARCHAR(100)NOTNULL, published_yearINT); 3. Insert data: INSERTINTObooks(title, author, published_year)VA


Hot AI Tools

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Undress AI Tool
Undress images for free

Clothoff.io
AI clothes remover

Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Hot Tools

Atom editor mac version download
The most popular open source editor

SublimeText3 Linux new version
SublimeText3 Linux latest version

SublimeText3 Mac version
God-level code editing software (SublimeText3)

SublimeText3 English version
Recommended: Win version, supports code prompts!

SAP NetWeaver Server Adapter for Eclipse
Integrate Eclipse with SAP NetWeaver application server.