Question

Does SQL have both a short-circuit and a hierarchical multi-evaluation syntax?

Example of short-circuit assignment. Here decision gets the FIRST *** clause when … is true:

Short-circuit

   case  
     when (...) then (***) 
     when (...) then (***)
     when (...) then (***)
     else (...)
   end as decision

Example of hierarchical assignment. Here decision gets LAST *** expression when … is true.

Hierarchical

    if (...) then (decision = ***) end
    if (...) then (decision = ***) end
    if (...) then (decision = ***) end

Inter-Conversion

It is clear that reversing the ordering of the expressions switches between hierarchical and short-circuit. I’m wondering whether SQL also has a construct that would assign the LAST tr expression?

Test Case:

The following gives a very simple toy example:

select 
  flag1, flag2, flag3,
  case
     when flag1=1 and flag2=0 then 'LEFT'
     when flag1=0 and flag2=0 then 'NONE'
     when flag2=0 and flag3=1 then 'RIGHT'
  end as decision 
FROM 
( select 
  1 as flag1, 0 as flag2, 1 as flag3
  -- from dual                               -- if you use Oracle
) tmp ;

Short-circuit SQL would return ‘LEFT’

An hierarchical assignment would return ‘RIGHT’.

Edit: You can run this example in SQL-Fiddle by clicking on the link. (Thanks to ypercube for pointing out that useful site! (+1))

Answer 1

There is no expression that would do the Hierarchal assignment in one select but it can be achieved just like any other programming language through multiple if statements

DECLARE @flag1 BIT = 1, @flag2 BIT = 0, @flag3 BIT = 1, @decision VARCHAR(50)

IF @flag1 = 1 AND @flag2 = 0
    SET @decision = 'LEFT'

IF @flag1 = 0 AND @flag2 = 0
    SET @decision = 'NONE'

IF @flag2 = 0 AND @flag3 = 1
    SET @decision = 'RIGHT'

SELECT @decision