CREATE TABLE member (member_id DECIMAL, card_number TEXT, name TEXT, hometown TEXT, level DECIMAL)
CREATE TABLE membership_register_branch (member_id DECIMAL, branch_id TEXT, register_year TEXT)
CREATE TABLE branch (branch_id DECIMAL, name TEXT, open_year TEXT, address_road TEXT, city TEXT, membership_amount TEXT)
CREATE TABLE purchase (member_id DECIMAL, branch_id TEXT, year TEXT, total_pounds DECIMAL)

What is the sum of total pounds of purchase in year 2018 for all branches in London?
SELECT SUM(total_pounds) FROM purchase AS T1 JOIN branch AS T2 ON T1.branch_id = T2.branch_id WHERE T2.city = 'London' AND T1.year = 2018
SELECT SUM("total_pounds") FROM "purchase" AS "T1" JOIN "branch" AS "T2" ON "T1"."branch_id" = "T2"."branch_id" AND "T2"."city" = 'London' WHERE "T1"."year" = 2018