Many novice developers are not aware what amazing things can be archived with SQL very efficiently saving you from long writing long blocks of lengthly classic code. The case statement is one of the magical statements.


 CASE [ expression ]
   WHEN condition_1 THEN result_1
   WHEN condition_2 THEN result_2
   WHEN condition_3 THEN result_3
   WHEN condition_n THEN result_n
   ELSE result

The case statement evaluates each condition from to to button. When the condition matches it then the value given by then and terminates (short circut). If no condition matches the else clause is evaluated.
 There are two forms one where the a columns is compared to values in the when clause

SELECT table_name, CASE owner
  WHEN 'SYS' THEN 'The owner is SYS'
  WHEN 'SYSTEM' THEN 'The owner is SYSTEM'
  ELSE 'The owner is another value'
END FROM all_tables;
and another when an expression is used in the when clause.

 select a, 
  when  a >= 3 then 'Three'
  when  a >= 2 then 'Two'
  when  a >= 1 then 'One'
 select 1 a from dual union all
 select 2 a from dual union all
 select 3 a from dual 

results in
 1	One
 2	Two
 3	Three


  with exp as
       when expiration_date  <  to_date('01.10.2014', '') then 'New Contracts before vor 01.10.2014'
       when expiration_date  <  to_date('01.01.2015', '') then 'New Contracts from ab 01.10.2014'
       when expiration_date  <  to_date('01.04.2015', '') then 'New Contracts from ab 01.01.2015'       
       when expiration_date  <  to_date('01.07.2015', '') then 'New Contracts from ab 01.04.2015' 
       when expiration_date  >= to_date('01.07.2015', '') then 'New Contracts from ab 01.07.2015'
  end as period  
  , count(*) as Num
 from contracts
 select period, count(*) 
 from exp
 group by period
 order by period ;

Build English correct output of a list connected with and

    option || case rownum <> 1 then ' and ' end as Options; 
 from option;

  Pepperoni and
  Cheese and

Status: Published Date: 2014-11-30 12:48:31 +0100 (So, 30 Nov 2014) Revision: 32

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