Table schema:
CREATE TABLE purchase (branch_id VARCHAR, year VARCHAR); CREATE TABLE branch (branch_id VARCHAR, city VARCHAR)

Question: What is the sum of total pounds of purchase in year 2018 for all branches in London?
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