How to Use IF...THEN Logic in SQL Server

Data Tutorial

SQL Server has a unique capability of allowing you to execute real-time programmatic logic on the values within your query. Based on those logical evaluations, you can generate various values as part of the returned data set.

Using the CASE Statement

This is most easily accomplished in all versions of SQL Server using the CASE statement, which acts as a logical IF...THEN...ELSE expression and returns various values depending on the result.

In this example below, we want to return an additional locale column that specifies whether our book takes place in Middle-earth or regular old Earth.

SELECT
  CASE
    WHEN
      books.title = 'The Hobbit'
        THEN
          'Middle-earth'
    WHEN
      books.primary_author = 'Tolkien'
        THEN
          'Earth'
    ELSE
      'Earth'
  END AS locale,
  books.*
FROM
  books

Before we examine the special CASE aspect of this statement, let’s temporarily remove the CASE to notice that this is an extremely simple SELECT statement on the surface:

SELECT
  books.*
FROM
  books

Therefore, let’s examine how the CASE section is structured and what logical behavior we’re performing.

CASE
  WHEN
    books.title = 'The Hobbit'
      THEN
        'Middle-earth'
  WHEN
    books.primary_author = 'Tolkien'
      THEN
        'Earth'
  ELSE
    'Earth'
END AS locale

To begin, we of initialize the CASE statement then specify under which conditions (WHEN) our CASE statement should evaluate a result. In this example, we’re examining the books.title and books.primary_author; if either fit our Tolkien-esque theme, THEN we return the value ‘Middle-earth.’ If neither fields match our search, we instead return the value of ‘Earth.’

To rearrange the logic as a psuedo-code IF...THEN...ELSE statement, we’re simply asking SQL to evaluate:

IF
  title == 'The Hobbit' OR
  primary_author == 'Tolkien'
THEN
  RETURN 'Middle-earth'
ELSE
  RETURN 'Earth'
END

Finally, it is critical to remember that a CASE statement must always be appended at the end with a matching END statement. In the above example, we’re also renaming the resulting value that is returned to locale, though that is certainly optional.

Using the IIF Function

If you are using a more modern version of SQL, it is useful to know that SQL Server 2012 introduced the very handy IIF function. IIF is a shorthand method for performing an IF...ELSE/CASE statement and returning one of two values, depending on the evaluation of the result.

Restructuring our above example to use IIF is quite simple.

SELECT
  IIF(
    books.title = 'The Hobbit' OR books.primary_author = 'Tolkien',
    'Middle-earth',
    'Earth')
  AS locale,
  books.*
FROM
  books

With an IIF function, we largely replace a lot of the syntactical sugar from the CASE statement with a few simple comma-seperators to differentiate our arguments.

All told, both CASE and IIF get the same job done, but if given the choice, IIF will generally be much simpler to use.