mSQL database in and exists usage comparison

Exists: Usage: SELECT * FROM table1 where exists (SELECT * from table2 where table1.id = table2.id)
Returns records with the same ID in Table1 and table2 (double loop) result and in syntax, thinking differently,
In:select * FROM table1 where ID in (select ID from table2)

2 ideas are different: exists: first get each table1 data, and then get the ID field, go to table2 table to find the corresponding value, find the description matches the condition
In: The ID probability is obtained first, and then the table2 data is retrieved to determine whether the current ID is within the ID collection

For in: If the amount of data in the table2 is too large to use, because he needs to find out and store so much data first, it is expensive and can be used exists

mSQL database in and exists usage comparison