Oracle IN EXISTS 区别

The EXISTS condition is considered "to be met" if the subquery returns at least one row.

SELECT *
FROM suppliers
WHERE EXISTS
  (select *
    from orders
    where suppliers.supplier_id = orders.supplier_id);

This select statement will return all records from the suppliers table where there is at least one record in the orders table with the same supplier_id.

The IN function helps reduce the need to use multiple OR conditions.

SELECT *
FROM suppliers
WHERE supplier_name in ( 'IBM', 'Hewlett Packard', 'Microsoft');

This would return all rows where the supplier_name is either IBM, Hewlett Packard, or Microsoft. Because the * is used in the select, all fields from the suppliers table would appear in the result set.

It is equivalent to the following statement:

SELECT *
FROM suppliers
WHERE supplier_name = 'IBM'
OR supplier_name = 'Hewlett Packard'
OR supplier_name = 'Microsoft';

As you can see, using the IN function makes the statement easier to read and more efficient.