system/Database/BaseBuilder.php
public function decrement(string $column, int $value = 1)
	{
		$column = $this->db->protectIdentifiers($column);

		$sql = $this->_update($this->QBFrom[0], [$column => "{$column}-{$value}"]);

		return $this->db->query($sql, $this->binds, false);
	}
public function decrement(string $column, int $value = 1)
	{
		$column = $this->db->protectIdentifiers($column);

		$sql = $this->_update($this->QBFrom[0], [$column => "{$column}-{$value}"]);

		return $this->db->query($sql, $this->binds, false);
	}
Decrements a numeric column by the specified value.

@param string  $column
@param integer $value

@return boolean