Database schema:

CREATE TABLE customers (
	"CustomerID" INTEGER NOT NULL -- example: ['5', '3'],
	"Segment" TEXT -- client segment, example: ['LAM', 'SME'],
	"Currency" TEXT -- example: ['CZK', 'EUR'],
	PRIMARY KEY ("CustomerID"), 
	UNIQUE ("CustomerID")
)


CREATE TABLE gasstations (
	"GasStationID" INTEGER NOT NULL -- Gas Station ID, example: ['45', '44'],
	"ChainID" INTEGER -- Chain ID, example: ['6', '13'],
	"Country" TEXT -- example: ['CZE', 'SVK'],
	"Segment" TEXT -- chain segment, example: ['Value for money', 'Premium'],
	PRIMARY KEY ("GasStationID"), 
	UNIQUE ("GasStationID")
)


CREATE TABLE products (
	"ProductID" INTEGER NOT NULL -- Product ID, example: ['2', '1'],
	"Description" TEXT -- example: ['Rucní zadání', 'Nafta'],
	PRIMARY KEY ("ProductID"), 
	UNIQUE ("ProductID")
)


CREATE TABLE transactions_1k (
	"TransactionID" INTEGER -- Transaction ID, example: ['2', '1'],
	"Date" DATE -- example: ['2012-08-24', '2012-08-23'],
	"Time" TEXT -- example: ['09:41:00', '10:03:00'],
	"CustomerID" INTEGER -- Customer ID, example: ['31543', '46707'],
	"CardID" INTEGER -- Card ID, example: ['486621', '550134'],
	"GasStationID" INTEGER -- Gas Station ID, example: ['3704', '656'],
	"ProductID" INTEGER -- Product ID, example: ['2', '23'],
	"Amount" INTEGER -- example: ['28', '18'],
	"Price" REAL -- example: ['672.64', '430.72'],
	PRIMARY KEY ("TransactionID")
)


CREATE TABLE yearmonth (
	"CustomerID" INTEGER NOT NULL -- Customer ID, example: ['39', '63'],
	"Date" TEXT NOT NULL -- example: ['201201', '201112'],
	"Consumption" REAL -- example: ['1598.28', '528.3'],
	PRIMARY KEY ("Date", "CustomerID"), 
	CONSTRAINT fk_yearmonth_customers FOREIGN KEY("CustomerID") REFERENCES customers ("CustomerID")
)

Question: Which client ID consumed the most in September 2013?
Evidence: September 2013 refers to yearmonth.date = '201309'

SQL:
SELECT T1.CustomerID FROM customers AS T1 INNER JOIN yearmonth AS T2 ON T1.CustomerID = T2.CustomerID WHERE T2.Date = '201309' GROUP BY T1.CustomerID ORDER BY SUM(T2.Consumption) DESC LIMIT 1