Oracle:decode function

One 、 sketch

DECODE The function is ORACLE PL/SQL One of the powerful functions of , At present, only ORACLE The company's SQL Provides this function , Of other database manufacturers SQL The implementation doesn't have this feature yet .DECODE What's the use of it ？
Let's start with an example , Suppose you want to increase the salary of the staff , The standard is ： Wages in 8000 If it's less than yuan, add 20%; Wages in 8000 Yuan or above plus 15%, The usual way is , First, select the salary field value in the record select salary into var-salary from employee And then for variables var-salary use if-then-else or case when then else end And other flow control statements . If you use DECODE function , Then you can omit these flow control statements , adopt SQL Statement can be completed directly . as follows ：

select 
decode(sign(salary-8000),
        -1,salary*1.2,
        1,salary*1.15,
        salary*1.15) 
from employee

Isn't that neat ？ DECODE The grammar of ：DECODE(value,if1,then1,if2,then2,if3,then3,…,else), Said if value be equal to if1 when ,DECODE The result of the function returns then1,……, If it's not equal to any one if value , Then return to else. For the first time ,DECODE You can only do the equivalent test , But I just saw , Replace by some function or calculation value, Yes, it can make DECODE The function has a value greater than 、 Less than or equal to the function .

1、 Compare the size
select decode(sign( Variable 1- Variable 2),-1, Variable 1, Variable 2) from dual; -- Take the smaller value
sign() The function is... Based on a value 0、 Positive or negative , Return respectively 0、1、-1
　　 for example :
　　 Variable 1=10, Variable 2=20
　　 be sign( Variable 1- Variable 2) return -1,decode The decoding result is “ Variable 1”, The purpose of taking a smaller value is achieved .
　　
2、 surface 、 View structure transformation
There's a sales list sale, The structure of the table is :
　　month　　　 char(6)　　　　　 -- month
　　sell　　　　number(10,2)　　　-- Monthly sales amount

The available data are :
　　200001　　1000
　　200002　　1100
　　200003　　1200
　　200004　　1300
　　200005　　1400
　　200006　　1500
　　200007　　1600
　　200101　　1100
　　200202　　1200
　　200301　　1300

You want to convert data into the following structure ：

year　　　 char(4)　　　　　 -- year
　　month1　　 number(10,2)　　　--1 Monthly sales amount
　　month2　　 number(10,2)　　　--2 Monthly sales amount
　　month3　　 number(10,2)　　　--3 Monthly sales amount
　　month4　　 number(10,2)　　　--4 Monthly sales amount
　　month5　　 number(10,2)　　　--5 Monthly sales amount
　　month6　　 number(10,2)　　　--6 Monthly sales amount
　　month7　 　number(10,2)　　　--7 Monthly sales amount
　　month8　 　number(10,2)　　　--8 Monthly sales amount
　　month9　　 number(10,2)　　　--9 Monthly sales amount
　　month10　 number(10,2)　　 --10 Monthly sales amount
　　month11　 number(10,2)　　 --11 Monthly sales amount
　　month12　　number(10,2)　　 --12 Monthly sales amount

Structural transformation SQL Statement for ：

create or replace view
v_sale(year,month1,month2,month3,month4,month5,month6,month7,month8,month9,month10,month11,month12)
as
select
　　substrb(month,1,4),
　　sum(decode(substrb(month,5,2),'01',sell,0)),
　　sum(decode(substrb(month,5,2),'02',sell,0)),
　　sum(decode(substrb(month,5,2),'03',sell,0)),
　　sum(decode(substrb(month,5,2),'04',sell,0)),

Add 1：
Student transcript student, Now use decode The function implements the following functions ： achievement >85, Show excellence ;>70 Show good ;>60 pass ; Otherwise, I will fail .
hypothesis student The number of is id, The result is score, that ：

select id,
decode(sign(score-85),  
1,' good ',  
0,' good ',
-1, decode(sign(score-70),
1,' good ',
0,' good ',
-1, decode(sign(score-60),
1,' pass ',
0,' pass ',
-1,' fail, '))) 
from student;

Add 2：
Decode The syntax structure of the function is as follows :

decode (expression, search_1, result_1)
decode (expression, search_1, result_1, search_2, result_2)
decode (expression, search_1, result_1, search_2, result_2, ...., search_n, result_n)
decode (expression, search_1, result_1, default)
decode (expression, search_1, result_1, search_2, result_2, default)
decode (expression, search_1, result_1, search_2, result_2, ...., search_n, result_n, default)

decode Function to compare an expression with a search word , If the match returns a result ; If it doesn't match , return default value ; If not defined default value , Return null value .