How to conditionally select a value using the IF() function?

IF(cond, val1, val2) is a MySQL built-in function that returns the second argument if the first argument is TRUE, the third argument otherwise.

The first argument is treated as an integer and evaluated to TRUE if it is not 0. For example:

SELECT IF(2>1, 'yes', 'no'), IF(9, 'yes', 'no');
  -- +----------------------+--------------------+
  -- | IF(2>1, 'yes', 'no') | IF(9, 'yes', 'no') |
  -- +----------------------+--------------------+
  -- | yes                  | yes                |
  -- +----------------------+--------------------+

SELECT IF(TRUE, 'yes', 'no'), IF('TRUE', 'yes', 'no'), IF('100% TRUE', 'yes', 'no');
  -- +-----------------------+-------------------------+------------------------------+
  -- | IF(TRUE, 'yes', 'no') | IF('TRUE', 'yes', 'no') | IF('100% TRUE', 'yes', 'no') |
  -- +-----------------------+-------------------------+------------------------------+
  -- | yes                   | no                      | yes                          |
  -- +-----------------------+-------------------------+------------------------------+

SELECT IF(NULL, 'yes', 'no'), IF('NULL', 'yes', 'no');
  -- +-----------------------+-------------------------+
  -- | IF(TRUE, 'yes', 'no') | IF('True', 'yes', 'no') |
  -- +-----------------------+-------------------------+
  -- | yes                   | no                      |
  -- +-----------------------+-------------------------+

Reference information of the IF() function:

IF(cond, val1, val2): val
  Returns the second argument if the first argument is TRUE, 
  the third argument otherwise.

Arguments, return value and availability:
  cond: Required. The Boolean condition.
  val1, val2: Required. The values to be selected from.
  val: Return value. The selected value.
  Available since MySQL 4.0.

⇒ IFNULL() - Replacing NULL Value

⇐ GREATEST() - Finding the Greatest/Maximum Value

⇑ MySQL Functions for Miscellaneous Purposes

⇑⇑ MySQL Function References