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using join by in arcpy.da.searchcursor() - what is the right syntax?

yonatanrubinstein

Hi, I'm trying to run a code that uses a "join by statement" inside a searchcursor (it was done to imitate the join by inside the iterate features tool in the modelbuilder). I've tried this code (as a trial before implementing it on the real script), but I can't seem to find the right syntax or any documentation.

import arcpy  fc = "D:\\Projects\\proj1\\SumsOfRoutes.gdb\\all_routes_1stBatch" field = "r_Identity" field2 = "Shape_Length" ##sql = "(None, 'GROUP BY \"r_Identity\"')" sql = "(None,'Group BY r_Identity')"   fields=["*"] cursor = arcpy.da.SearchCursor(fc,fields,sql_clause=sql)   # print r_Identity and Shape_Length of each feature in feature class for row in cursor:     print(str(int(row.getValue(field))) + ":  " + str(row.getValue(field2)) + " meters")

Does anybody know how it should be written?

EDIT: for the script above, I get the following error:

TypeError: 'sql_clause' should be sequense of 2 strings(prefix, postfix)
Thanks.

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Accepted answers

JamesCrandall

Update:

This must be a bug or something because I don't see how this can be correct. According to Shaun's reply in this thread: http://forums.arcgis.com/threads/77876-Group-By-functionality-of-da.cursors-sql_clause the suggestion is to include the "ObjectID" field in the group by clause to overcome the error you are experiencing. Here's my working code:

 fc = r'H:\Documents\ArcGIS\Default.gdb\RainGauges' fields = ('Data_Type')  sql=[None, "GROUP BY Data_Type, ObjectID"] cursor = arcpy.da.SearchCursor(fc, fields,sql_clause=sql)  for row in cursor:     print row[0]

HOWEVER, this completely defeats the ability to actually group rows because the ObjectID is a unique value!! lol... Maybe I am simply misunderstanding here, but that is exactly what is happening in my test data. That is, it doesn't matter if real/actual groups exist in my select fields, they will NEVER group correctly because I must include the ObjectID in the group by clause (it is unique values).

I am happy to be corrected on this one!

All comments

JamesCrandall

The OBJECTID should not be required, if it ever was it is a bug and is fixed in 10.2.1.

It is required/an issue at 10.1 SP1

MathewCoyle

The OBJECTID should not be required, if it ever was it is a bug and is fixed in 10.2.1.

yonatanrubinstein

You cannot use '*' when using sql_clause. You must specify each field explicitly.

OK, I GOT IT (I THINK). The same fields should be in the "group-by" part and in the "fields" part.

THANK YOU JAMES, THANK YOU MATHEW

it is very confusing, and the ObjectID part is weird, I wouldn't have got it on my own in a million years. Thanks.

JamesCrandall

This is truly a strange and confusing method available for the da.SearchCursor. I mean the GROUP BY clause is normally applied because you are attempting to summarize things (say SUM or AVERAGE, or other some such metric). But it is entirely weird to require ObjectID which would negate any ability to correctly perform such aggragetion, let alone not even be able to include such a thing as COUNT, MIN, MAX, AVERGE, etc...

I hope I am simply missing the obvious here.

MathewCoyle

OK, THANKS I GET AN ANSWER BY RUNNING THE BELOW TEXT:
import arcpy

fc = "D:\\Projects\\proj1\\SumsOfRoutes.gdb\\all_routes_1stBatch"
field1 = "r_Identity"
##field2 = "Shape_Length"

sql = [None,"Group BY r_Identity, ObjectID"]


fields=[field1]
cursor = arcpy.da.SearchCursor(fc,fields,sql_clause=sql)


# print r_Identity and Shape_Length of each feature in feature class
for row in cursor:
    print str(row)
Only I don't understand the thing with the OBJECTID,

and more so, this constellation DOES NOT HELP ME, AS I CAN'T REACH THE OTHER FIELDS OF THE FEATURES.

substitute field1 with "*" brings back the runtime error

You cannot use '*' when using sql_clause. You must specify each field explicitly.

yonatanrubinstein

You cannot include Shape_Length (see bold above) in your select because it is not included in your GROUP BY portion. Remove it and see if it at least completes without error.

OK, THANKS I GET AN ANSWER BY RUNNING THE BELOW TEXT:

import arcpy

fc = "D:\\Projects\\proj1\\SumsOfRoutes.gdb\\all_routes_1stBatch"
field1 = "r_Identity"
##field2 = "Shape_Length"

sql = [None,"Group BY r_Identity, ObjectID"]


fields=[field1]
cursor = arcpy.da.SearchCursor(fc,fields,sql_clause=sql)


# print r_Identity and Shape_Length of each feature in feature class
for row in cursor:
    print str(row)

Only I don't understand the thing with the OBJECTID,

and more so, this constellation DOES NOT HELP ME, AS I CAN'T REACH THE OTHER FIELDS OF THE FEATURES.

substitute field1 with "*" brings back the runtime error

MathewCoyle

You are right I was using ORDER BY.

JamesCrandall

You must list all fields that will be included in the sql_clause parameter in the fields parameter.
fc = r'H:\Documents\ArcGIS\Default.gdb\RainGauges'
fields = ('Data_Type', 'ObjectID')
sql=[None, "GROUP BY Data_Type, ObjectID"]
cursor = arcpy.da.SearchCursor(fc, fields,sql_clause=sql)

for row in cursor:
    print row[0]

Matthew,

I am still not understanding how this will correctly perform any GROUP BY because of the requirement include the unique values in ObjectID field. How is this ever going to group on the target fields????

MathewCoyle

You must list all fields that will be included in the sql_clause parameter in the fields parameter.

fc = r'H:\Documents\ArcGIS\Default.gdb\RainGauges'
fields = ('Data_Type', 'ObjectID')
sql=[None, "GROUP BY Data_Type, ObjectID"]
cursor = arcpy.da.SearchCursor(fc, fields,sql_clause=sql)

for row in cursor:
    print row[0]

JamesCrandall

I get the same runtimeerror as before... 😞

Is "r_Identity" the actual field name?

Make sure the spellng is correct. I get the same error in my working example if it is incorrectly spelled.

yonatanrubinstein

Update:

This must be a bug or something because I don't see how this can be correct. According to Shaun's reply in this thread: http://forums.arcgis.com/threads/77876-Group-By-functionality-of-da.cursors-sql_clause the suggestion is to include the "ObjectID" field in the group by clause to overcome the error you are experiencing. Here's my working code:
fc = r'H:\Documents\ArcGIS\Default.gdb\RainGauges'
fields = ('Data_Type') 
sql=[None, "GROUP BY Data_Type, ObjectID"]
cursor = arcpy.da.SearchCursor(fc, fields,sql_clause=sql)

for row in cursor:
    print row[0]
HOWEVER, this completely defeats the ability to actually group rows because the ObjectID is a unique value!! lol... Maybe I am simply misunderstanding here, but that is exactly what is happening in my test data. That is, it doesn't matter if real/actual groups exist in my select fields, they will NEVER group correctly because I must include the ObjectID in the group by clause (it is unique values).

I am happy to be corrected on this one!

I get the same runtimeerror as before... 😞

JamesCrandall

OK, James,

I tried to rewrite the script by your 2 suggestions:
import arcpy

fc = "D:\\Projects\\proj1\\SumsOfRoutes.gdb\\all_routes_1stBatch"
field1 = "r_Identity"
field2 = "Shape_Length"

sql = [None,'Group BY r_Identity']


fields=[field1,field2]
cursor = arcpy.da.SearchCursor(fc,fields,sql_clause=sql)


# print r_Identity and Shape_Length of each feature in feature class
for row in cursor:
    print str(row) 
I get the following error:
RuntimeError: An invalid SQL stament was used. [SELECT r_Identity, Shape_Length, OBJECTID FROM all_routes_1stBatch GROUP BY r_Identity] 
Any Ideas as to what's wrong?

and the question I asked Joshua: I know listing only 2 fields will increase the efficiency of the cursor, but will it enable me to use other fields in the layer later on?

You cannot include Shape_Length (see bold above) in your select because it is not included in your GROUP BY portion. Remove it and see if it at least completes without error.

yonatanrubinstein

OK, James,

I tried to rewrite the script by your 2 suggestions:

import arcpy

fc = "D:\\Projects\\proj1\\SumsOfRoutes.gdb\\all_routes_1stBatch"
field1 = "r_Identity"
field2 = "Shape_Length"

sql = [None,'Group BY r_Identity']


fields=[field1,field2]
cursor = arcpy.da.SearchCursor(fc,fields,sql_clause=sql)


# print r_Identity and Shape_Length of each feature in feature class
for row in cursor:
    print str(row)

I get the following error:

RuntimeError: An invalid SQL stament was used. [SELECT r_Identity, Shape_Length, OBJECTID FROM all_routes_1stBatch GROUP BY r_Identity]

Any Ideas as to what's wrong?

and the question I asked Joshua: I know listing only 2 fields will increase the efficiency of the cursor, but will it enable me to use other fields in the layer later on?

JamesCrandall

 fc = r'H:\Documents\ArcGIS\Default.gdb\RainGauges' fields = ('Data_Type')  sql=[None, "GROUP BY Data_Type, ObjectID"] cursor = arcpy.da.SearchCursor(fc, fields,sql_clause=sql)  for row in cursor:     print row[0]

JamesCrandall

I just noticed you have this:

fields=["*"]

This is not allowable in a GROUP BY statement. That is, you could not write it as:

SELECT *
FROM table1
GROUP BY table1.field1

You must specify the fields you wish to include in the group by too!!! Like this:

SELECT table1.field1, table1.field2
FROM table1
GROUP BY table1.field1, table1.field2

yonatanrubinstein

Hi Joshua,

Thanks for the quick response.

1. As far as I Know, the sql_clause is a string. As evidence, when I do as you suggested and write

sql = (None,'GROUP BY r_Identity')

I get the following error:

TypeError: 'where_clause' is not a string

, so that is why I have the quotation marks. Any Ideas?

2. I know listing only 2 fields will increase the efficiency of the cursor, but will it enable me to use other field in the layer, later on?

JamesCrandall

Here's my guess...

Change:

sql = (None,'GROUP BY r_Identity')

To:

sql = [None,"GROUP BY r_Identity"]

JoshuaChisholm

Hello Yonatan,

I think sql_clause needs to be passed as a tuple. Try removing the quotes:

sql = (None,'GROUP BY r_Identity')

On a side note, you could probably increase the efficiency of your script by only pulling the fields you need for the da.SearchCursor. Like this:

fields=[field,field2]

Let me know how it goes!

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