arcgis desktop - Multi Value User Input as Variable in SQL Statement

I'm attempting to write a script that takes a user's selection as a parameter and uses that as part of the SelectByAttributes function. The parameter represents field attributes and is multi value, and the user can select one or multiple.

So what I have at this point is the list of all available attributes, and each selected as the defualt.

Also, Here is the tool validation code and Parameter Properties:

import arcpy
class ToolValidator(object):
  """Class for validating a tool's parameter values and controlling
  the behavior of the tool's dialog."""

  def __init__(self):
    """Setup arcpy and the list of tool parameters."""
    self.params = arcpy.GetParameterInfo()

  def initializeParameters(self):

    """Refine the properties of a tool's parameters.  This method is
    called when the tool is opened."""
    return

  def updateParameters(self):
    """Modify the values and properties of parameters before internal
    validation is performed.  This method is called whenever a parameter
    has been changed."""
    shape = "G:\_DPD\STEVEN\FIELD.shp"
    rows = arcpy.SearchCursor(shape)

    self.params[0].filter.list = sorted(list(set(r.getValue('DPD_FLD_TY') for r in rows)))
    all = self.params[0].filter.list
    self.params[0].value = all
    del rows
    return

  def updateMessages(self):
    """Modify the messages created by internal validation for each tool
    parameter.  This method is called after internal validation."""
    return

I want the user to have the option to run the tool with multiple attributes selected or with just one selected, and use that as part of the SQL statement within the SelectByAttribute function. For example, if the user selects "GAS" and "UNKNOWN", the SelectByAttributes function would look like

arcpy.SelectLayerByAttribute_management(lyr,"NEW_SELECTION",""""DPD_FLD_TY" = 'GAS' OR "DPD_FLD_TY" ='UNKNOWN'""")

So my main question is: what would the code be to check how many attributes are selected and based on that, have the SQL statement contain the proper amount of "OR" statements? Also, would the parameter be considered just one, or would it be multiple parameters i.e. GetParameterAsText(0) vs. GetParameterAsText(0)...GetParameterAsText(3)

I'm sorry if this is confusing, but I don't know exactly where to start with this problem.

This is how I am referencing the layer:

import arcpy


mxd = arcpy.mapping.MapDocument('CURRENT')
mm = arcpy.mapping.ListDataFrames(mxd, "Layers")[0]
lyr = arcpy.mapping.ListLayers(mxd, 'FIELD')[0]
field = arcpy.GetParameterAsText(0)