5. Oracle case when 以及decode

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两种方式:

        1. case x(字段) when xx1 then .... when xx2 then....   else.......end case;

       2.  case   when xx1=yy1  then ..... when xx2=yy2  then...... when  xx3=yy3  then....else........end case;

使用例子；

            case X when XX1 then ..when xx2 then ....else ....end case ;

declare 
nowDate varchar2(100):='2218';

begin

case nowDate   
when  '2017'  then  
dbms_output.put_line('年份为:'|| nowDate);

when '2018' then
dbms_output.put_line('年份为:'|| nowDate);

when '2019' then
dbms_output.put_line('年份为:'|| nowDate);
else
dbms_output.put_line('你们猜错了');
dbms_output.put_line('年份为:'|| nowDate);
end case;

end;

     case   when xx1 then ..... when xx2 then...... when xx3 then....else........end case;

declare 
nowDate varchar2(100):='2218';

begin

case    
when nowDate='2017'  then  
dbms_output.put_line('年份为:'|| nowDate);

when nowDate='2018' then
dbms_output.put_line('年份为:'|| nowDate);

when nowDate='2019' then
dbms_output.put_line('年份为:'|| nowDate);
else
dbms_output.put_line('你们猜错了');
dbms_output.put_line('年份为:'|| nowDate);
end case;

end;

  SQL查询语句的写法:

   

 select T.DUMMY,
          case
            when DUMMY = 'X' then
             'dual defualt is X'
            else
             'dual defualt value is other'
          
          END DUMMY_DEFAULT
   
 from dual T

  select T.DUMMY,
          case  DUMMY
            when  'X' then
             'dual defualt is X'
            when  'XY' then
             'dual defualt is XY'
            else
             'dual defualt value is other'
          
          END DUMMY_DEFAULT
from dual T

where 条件子句中

select * from dual T where  DUMMY =
      case  DUMMY
          when  'X' then 'X'
          when  'XY' then 'XY' 
          else  ''
         END    

decode（条件A，A1，条件B，B1，条件C C3）智能用户select 中，不能用户where条件后面

select decode(count(*) ,1, 100000, 2,1000 )   from dual;