Differences between in and exists in MySQL

MySQL Medium in The statement is to make the appearance and the inner table as hash Connect , and exists A statement is an external statement loop loop , Every time loop Loop and query the inner table . Use scenarios ：

1. If the two tables in the query are the same size , Then use in and exists Not much difference .

2. If one of the two tables is smaller , One is a big watch , Then the sub query table is very useful exists, Small use of subquery table in.

3. not in and not exists： If the query statement uses not in, Then the internal and external tables are scanned , No index is used ; and not extsts The subquery of can still use the index on the table . So no matter which watch is big , use not exists All ratio not in Be quick .