Error 000358: Invalid Expression when trying to select layer by attribute

[TiffanieLaw]

I'm creating a script where I'm trying to select all the null values based on a specific field. The feature is joined with another feature, thus altering the title of fields to be FeatureName_FieldName. I want to keep these titles as I will be exporting the table and if I set qualified field names to False, the fields won't be able to be properly interpreted. 

            mayDevice = mayFeature+"_"+"DEVICE_ID"

            whereClause = mayDevice "IS NULL"

            arcpy.SelectLayerByAttribute_management(mayFeature, "NEW_SELECTION", whereClause)

I don't know how to include a variable in an "is null" expression. 

[DanPatterson_Retired]

Some suggestions Tiffanie:

print the query out to check what it looks like

do one manually and copy the result

check SQL reference for query expressions used in ArcGIS—ArcGIS Pro | ArcGIS Desktop there is an example there

[RandyBurton]

Your whereClause as shown is missing a + sign and space before " IS NULL".  When joining strings, I like to use .format as I find the code is a bit clearer.  You might try something like:

mayDevice = "{}_DEVICE_ID".format(mayFeature)

whereClause = "{} IS NULL".format(mayDevice)‍‍‍