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Exist select book_key from book where exists (select book_key from sales); In select book_key from book where book_key in (select book_key from sales); The EXISTS clause is much faster than IN when the subquery results is very large.

这里写目录标题题目思路 题目 Write an SQL query to find for each user, the join date and the number of orders they made as a buyer in 2019. The query result format is in the following example: Users table: ±--------±-----------±---------------+ | user_id | join_date |

这里写目录标题题目思路思路2 题目 Several friends at a cinema ticket office would like to reserve consecutive available seats. Can you help to query all the consecutive available seats order by the seat_id using the following cinema table? seat_id free 1 1 2 0

目录题目：思路： 题目： In social network like Facebook or Twitter, people send friend requests and accept others’ requests as well. Now given two tables as below: Table: friend_request sender_id send_to_id request_date 1 2 2016_06-01 1 3 2016_06-01 1 4 2