T-SQL: Related "IF"/"ELSE IF" statements and "WHEN" clauses in a "CASE" should not have the same condition

A CASE and a chain of IF/ELSE IF statements is evaluated from top to bottom. At most, only one branch will be executed: the first one with a condition that evaluates to true.

Therefore, duplicating a condition automatically leads to dead code. Usually, this is due to a copy/paste error. At best, it's simply dead code and at worst, it's a bug that is likely to induce further bugs as the code is maintained, and obviously it could lead to unexpected behavior.

Noncompliant Code Example

IF @x = 1
  PRINT 'A'
ELSE IF @x = 2
  PRINT 'B'
ELSE IF @x = 1 -- Noncompliant
  PRINT 'C'

SELECT
  CASE col1
    WHEN 1 
      THEN 'A'
    WHEN 2 
      THEN 'B'
    WHEN 1  -- Noncompliant
      THEN 'C'
    ELSE 'D'
  END
FROM table1

Compliant Solution

IF @x = 1
  PRINT 'A'
ELSE IF @x = 2
  PRINT 'B'
ELSE IF @x = 3
  PRINT 'C'

SELECT
  CASE col1
    WHEN 1 
      THEN 'A'
    WHEN 2 
      THEN 'B'
    WHEN 3 
      THEN 'C'
    ELSE 'D'
  END
FROM table1