chat gpt 에게 물어본 내용 정확한 듯

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what is different "IN" and "EXISTS" in ORACLE?

In Oracle, "IN" and "EXISTS" are two different operators that are used to determine if a certain value exists within a set of values or a subquery.

The "IN" operator is used to check if a value exists in a list of values or a subquery. The syntax for using "IN" is as followsvalue IN (list_of_values)

 value IN (list_of_values)

For example, the following query will return all the rows from the "employees" table where the department id is in the list of values (10, 20, 30):

sqlCopy code
SELECT *
FROM employees
WHERE department_id IN (10, 20, 30);

The "EXISTS" operator is used to check if a subquery returns any rows. The syntax for using "EXISTS" is as follows:

EXISTS (subquery)

For example, the following query will return all the departments from the "departments" table that have at least one employee:

SELECT *
FROM departments
WHERE EXISTS (
  SELECT 1
  FROM employees
  WHERE departments.department_id = employees.department_id
);

In general, the "EXISTS" operator is more efficient than the "IN" operator when the subquery returns a large number of rows. The "IN" operator can become slow and consume a lot of resources if the list of values is large.

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so only difference is efficiency?

Efficiency is one of the main differences between the "IN" and "EXISTS" operators, but there are other differences as well:

1. Purpose: The main purpose of the "IN" operator is to check if a value exists in a list of values or a subquery, whereas the main purpose of the "EXISTS" operator is to check if a subquery returns any rows.
2. Result: The "IN" operator returns a Boolean value indicating whether the value exists in the list of values or the subquery, whereas the "EXISTS" operator returns a Boolean value indicating whether the subquery returns any rows.
3. Null values: The "IN" operator can return unexpected results when dealing with null values, because a null value is not equal to any value, including itself. The "EXISTS" operator does not have this issue, as it only checks if the subquery returns any rows, not the specific values in those rows.

So, while efficiency is an important difference between the "IN" and "EXISTS" operators, it's not the only difference. The choice between "IN" and "EXISTS" should be based on the specific requirements of the query and the data being queried.