IF Condition in an SQL query

Question

I\'m a newbie to SQL Server. Please help me to write the following Logic in a query.

If getnow() > today 4 PM
Then
    SELECT *
    FROM table
    WHERE M         


        

Answer 1

The idea here is to use the implication rewrite rule:

IF ( x ) THEN ( y )   is equivalent to  ( NOT ( x ) OR y )

In your case

IF ( DATEPART(HOUR, CURRENT_TIMESTAMP) >= 16 ) 
   THEN ( DATEDIFF(DAY, CURRENT_TIMESTAMP, MailDate) = 1 ) 

is equivalent to

( NOT ( DATEPART(HOUR, CURRENT_TIMESTAMP) >= 16 ) 
   OR ( DATEDIFF(DAY, CURRENT_TIMESTAMP, MailDate) = 1 ) ) 

and is itself equivalent to

( ( DATEPART(HOUR, CURRENT_TIMESTAMP) < 16 ) 
   OR ( DATEDIFF(DAY, CURRENT_TIMESTAMP, MailDate) = 1 ) )

Re-writing the original ELSE clause as an IF..THEN statement in its own right:

IF ( DATEPART(HOUR, CURRENT_TIMESTAMP) < 16 ) 
   THEN ( DATEDIFF(DAY, CURRENT_TIMESTAMP, MailDate) = 0 ) 

is equivalent to (this time omiting the intermediate step)

( ( DATEPART(HOUR, CURRENT_TIMESTAMP) >= 16 )
   OR ( DATEDIFF(DAY, CURRENT_TIMESTAMP, MailDate) = 0 ) )

The two expression can then be writting in conjunctive normal form ("a series of ANDs)

SELECT *
  FROM the_table
 WHERE ( ( DATEPART(HOUR, CURRENT_TIMESTAMP) < 16 ) 
          OR ( DATEDIFF(DAY, CURRENT_TIMESTAMP, MailDate) = 1 ) ) 
       AND 
       ( ( (DATEPART(HOUR, CURRENT_TIMESTAMP) >= 16
          OR ( DATEDIFF(DAY, CURRENT_TIMESTAMP, MailDate) = 0 ) )  ;