How to count the number of '1' in a binary string using the BIT_COUNT() function?

BIT_COUNT(int) is a MySQL built-in function that converts an unsigned integer to binary string and counts the number of '1's in the string. For example:

SELECT 64, BIN(64), BIT_COUNT(64);
  -- +----+---------+---------------+
  -- | 64 | BIN(64) | BIT_COUNT(64) |
  -- +----+---------+---------------+
  -- | 64 | 1000000 |             1 |
  -- +----+---------+---------------+

SELECT 1279, BIN(1279), BIT_COUNT(1279);
  -- +------+-------------+-----------------+
  -- | 1279 | BIN(1279)   | BIT_COUNT(1279) |
  -- +------+-------------+-----------------+
  -- | 1279 | 10011111111 |               9 |
  -- +------+-------------+-----------------+

SELECT x'4142', BIN(x'4142'), BIT_COUNT(x'4142');
  -- +---------+-----------------+--------------------+
  -- | x'4142' | BIN(x'4142')    | BIT_COUNT(x'4142') |
  -- +---------+-----------------+--------------------+
  -- | AB      | 100000101000010 |                  4 |
  -- +---------+-----------------+--------------------+

Reference information of the BIT_COUNT() function:

BIT_COUNT(int): count
  Converts an unsigned integer to binary string and counts 
  the number of '1's in the string. 

Arguments, return value and availability:
  int: Required. The unsigned integer to be examined.
  count: Return value. The number of '1's in the binary string representation.
  Available since MySQL 4.0.

⇒ CEIL() - Synonym for CEILING()

⇐ BIN() - Converting Integer to Binary String

⇑ MySQL Functions on Numeric Values

⇑⇑ MySQL Function References