What is the sum of total pounds of purchase in year 2018 for all branches in London?
CREATE TABLE purchase (branch_id VARCHAR, year VARCHAR); CREATE TABLE branch (branch_id VARCHAR, city VARCHAR)
[SQL] 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 [/SQL]