IF 條件=值1 THEN
　　　　RETURN(翻譯值1)
ELSIF 條件=值2 THEN
　　　　RETURN(翻譯值2)
　　　　......
ELSIF 條件=值n THEN
　　　　RETURN(翻譯值n)
ELSE
　　　　RETURN(缺省值)
END IF

select id,sign(id-2) from t_decode;

select lpad(id,2,0) from t_decode；

//創(chuàng)建表，插入數(shù)據(jù)，查詢
create table t_decode(
       id integer,
       name varchar2(10)
);

//插入數(shù)據(jù)
insert into t_decode values (1,'a');
insert into t_decode values (2,'b');
insert into t_decode values (3,'c');
insert into t_decode values (4,'a');

select id,name,
	decode(id,1,'第一個',2,'第二個',3,'第三個','沒有') new_id 
from t_decode;

select id,name,
   decode(sign(id - 4),1,'high id',0,'high id','-1',
                          decode(sign(id - 2),1,'mid id',0,'mid id',-1,'low id'))
from t_decode; 

-- 比較大小
select decode(sign(100-90),-1,100,90) from dual;

select id,name,decode(instr(name,'a'),0,'不含有a','含有a') as info from t_decode;

select 
	sum(decode(name,'a',id,0)) id_1,
	sum(decode(name,'b',id,0)) id_2,
	sum(decode(name,'c',id,0)) id_3 from t_decode;

select 
   decode(name,'a',id,0) id_1,
   decode(name,'b',id,0) id_2,
   decode(name,'c',id,0) id_3 
from t_decode;

-- 相等于：case when then else end
select 
	case name when 'a' then id else 0 end as id_1,
    case name when 'b' then id else 0 end as id_2,
    case name when 'c' then id else 0 end as id_3 
from t_decode;

結(jié)合Lpad函數(shù)，如何使主鍵的值自動加1并在前面補(bǔ)0
select lpad(decode(count(id),0,1,max(to_number(id)+1)),14,'0') new_id from t_decode;

select * from t_decode

select lpad(id,2,0) from t_decode

select to_number(id) from t_decode;

select max(to_number(id)+1) from t_decode;

select decode(count(id),0,1,max(to_number(id)+1)) from t_decode;

select lpad(decode(count(id),0,1,max(to_number(id)+1)),14,‘0') new_id from t_decode;