DECODE函数是ORACLE PL/SQL的功能强大的函数之一，目前还只有ORACLE公司的SQL提供了此函数，其它数据库厂商的SQL实现还没有此功能
select a.ok 合格情况,
sum(decode(cname, '计算机', countnum, 0)) 人数
from
(select
cname,
case when scgrade when scgrade >=60 and scgrade when scgrade >=70 and scgrade when scgrade >=85 and scgrade as ok,
count(scgrade) as countnum
from sc, c
where sc.cno = c.cno
group by
cname,
case when scgrade when scgrade >=60 and scgrade when scgrade >=70 and scgrade when scgrade >=85 and scgrade group by a.ok
order by a.ok desc
函数介绍
DECODE函数是Oracle PL/SQL的功能强大的函数之一，目前还只有ORACLE公司的SQL提供了此函数，其它数据库厂商的SQL实现还没有此功能。DECODE有什么用途呢？ 先构造一个例子，假设我们想给智星职员加工资，其标准是：工资在8000元以下的加20％；工资在8000元或以上的加15％，通常的做法是，先选出记录中的工资字段值? select salary into var-salary from employee，然后对变量var-salary用if-then-else或choose case之类的流控制语句进行判断。 如果用DECODE函数，那么我们就可以把这些流控制语句省略，通过SQL语句就可以直接完成。如下：select decode(sign(salary - 8000),>=0,salary*1.15,
关于DECODE
DECODE是Oracle公司独家提供的功能，它是一个功能很强的函数。它虽然不是SQL的标准，但对于性能非常有用。到目前，其他的数据库供应商还不能提供类似DECODE的功能，甚至有的数据库的供应商批评Oracle的SQL不标准。实际上，这种批评有些片面或不够水平。就象有些马车制造商抱怨亨利。福特的“马车”不标准一样。
1 DECODE 中的if-then-else逻辑
在逻辑编程中，经常用到If – Then –Else 进行逻辑判断。在DECODE的语法中，实际上就是这样的逻辑处理过程。它的语法如下：
DECODE(value, if1, then1, if2,then2, if3,then3, . . . else )
Value 代表某个表的任何类型的任意列或一个通过计算所得的任何结果。当每个value值被测试，如果value的值为if1，Decode 函数的结果是then1；如果value等于if2，Decode函数结果是then2；等等。事实上，可以给出多个if/then 配对。如果value结果不等于给出的任何配对时，Decode 结果就返回else 。
需要注意的是，这里的if、then及else 都可以是函数或计算表达式。
2 DECODE 的简单例子
Oracle系统中就有许多数据字典是使用decode 思想设计的，比如记录会话信息的V$SESSION数据字典视图就是这样。我们从《Oracle8i/9i Reference》资料中了解到，当用户登录成功后在V$SESSION中就有该用户的相应记录，但用户所进行的命令操作在该视图中只记录命令的代码（0—没有任何操作，2—Insert…），而不是具体的命令关键字。因此，我们需要了解当前各个用户的名字及他们所进行的操作时，要用下面命令才能得到详细的结果：
select sid,serial#,username,
DECODE(command,
0,’None’,
2,’Insert’,
3,’Select’,
6,’Update’,
7,’Delete’,
8,’Drop’,
‘Other’) cmmand
from v$session where username is not null;
3 DECODE实现表的转置
数据库中的表是由列和行构成的一个二维表。一般列在任何数据库中都是有限的数量，而行的变化较大，如果表很大，行的数量可能大上千万行。同一列的不同行可能有不同的值，而且不是预先定义的。
例:住房公积金报表置换实例：
1.各个单位在本地经办行进行开户，开户就是将单位的基本信息和职工信息的进行登记；
2.每月各个单位的会计到经办行交缴本单位的所有职工的住房公积金，系统记录有每个职工的交缴明细并在每条记录上记录有经办行的代码；
3.每月、季、半年及年终都要求将经办行 变为“列”给出个月的明细报表：
经办行：城西区 城东区
月份：
2001.01 xxxx1.xx xxxxx2.xx
2001.02 xxxx3.xx xxxxx4.xx
。 。 。 。 。 。
原来的数据顺序是：
城西区2001.01 xxxxx1.xx
城东区2001.01 xxxxx2.xx
城西区2001.02 xxxxx3.xx
城东区2001.02 xxxxx4.xx
住房公积金系统记录职工的每月交缴名细的pay_lst表结构是：
bank_code varchar2(6)NOT NULL, -- 经办行代码
acc_no varchar2(15) not null, -- 单位代码(单位帐号)
emp_acc_no varchar2(20) not null, -- 职工帐号
tran_date date not null, -- 交缴日期
tran_val Number(7,2) not null, -- 交缴额
sys_date date default sysdate, --系统日期
oper_id varchar2(10) --操作员代码
这样的表结构，一般按照将经办行作为行(row)进行统计是很容易的，但是如果希望将经办行变为列(column)这样的格式来输出就有困难。如果用DECODE函数来处理则变得很简单：
我们创建一个视图来对目前的pay_lst表进行查询。将经办行代码变为一些具体的经办行名称即可：
CREATE OR REPLACE VIEW bank_date_lst AS
Select to_char(tran_date,’yyyy.mm’),
SUM( DECODE ( bank_code,’001’, tran_val,0 )) 城西区，，
SUM( DECODE ( bank_code,’002’, tran_val,0 )) 城南区，
SUM( DECODE ( bank_code,’003’, tran_val,0 )) 城东区
FROM pay_lst
GROUP BY to_char(tran_date,’yyyy.mm’);
建立视图后，可直接对该视图进行查询就可按照列显示出结果。
Explain the ACID properties (Atomicity, Consistency, Isolation, Durability).Apr 16, 2025 am 12:20 AMACID 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: Database Management System vs. Programming LanguageApr 16, 2025 am 12:19 AMMySQL 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: Managing Data with SQL CommandsApr 16, 2025 am 12:19 AMMySQL 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's Purpose: Storing and Managing Data EffectivelyApr 16, 2025 am 12:16 AMMySQL 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.
SQL and MySQL: Understanding the RelationshipApr 16, 2025 am 12:14 AMThe 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.
Explain the role of InnoDB redo logs and undo logs.Apr 15, 2025 am 12:16 AMInnoDB 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 AMKey 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 AMUsingtemporary 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
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
AI Hentai Generator
Generate AI Hentai for free.
Hot Article
Hot Tools
ZendStudio 13.5.1 Mac
Powerful PHP integrated development environment
PhpStorm Mac version
The latest (2018.2.1) professional PHP integrated development tool
SecLists
SecLists is the ultimate security tester's companion. It is a collection of various types of lists that are frequently used during security assessments, all in one place. SecLists helps make security testing more efficient and productive by conveniently providing all the lists a security tester might need. List types include usernames, passwords, URLs, fuzzing payloads, sensitive data patterns, web shells, and more. The tester can simply pull this repository onto a new test machine and he will have access to every type of list he needs.
DVWA
Damn Vulnerable Web App (DVWA) is a PHP/MySQL web application that is very vulnerable. Its main goals are to be an aid for security professionals to test their skills and tools in a legal environment, to help web developers better understand the process of securing web applications, and to help teachers/students teach/learn in a classroom environment Web application security. The goal of DVWA is to practice some of the most common web vulnerabilities through a simple and straightforward interface, with varying degrees of difficulty. Please note that this software
VSCode Windows 64-bit Download
A free and powerful IDE editor launched by Microsoft
