I am fairly new to SQL query tuning. I have been trying to understand how to write equivalent queries. While going through Prof. J. Widom's Stanford online course, I noticed that there are some subqueries that cannot be written by joins. For example, see this:

Sub queries in where clause: Two examples at the following time positions in the video:

I am fairly new to SQL query tuning. I have been trying to understand how to write equivalent queries. While going through Prof. J. Widom's Stanford online video lectures, she makes mention that some subqueries that cannot be written by JOINs without the use of DISTINCT. For example, see this:

Schema and table creation are as follows,

Schema and table creation (for PostgreSQL) are as follows,

1. 4:45 / 20:13 - GPA example
2. 6:39 / 20:13 - students applied to CS but not to EE

1. 4:45 / 20:13 - GPA example

   SELECT GPA
   FROM Student
   WHERE sID in (select sID from Apply where major = 'CS');
   

2. 6:39 / 20:13 - students applied to CS but not to EE

   SELECT sID, sName
   FROM Student
   WHERE sID IN (select sID from Apply where major = 'CS')
     AND sID NOT IN (select sID from Apply where major = 'EE');
   

Sample Data

Schema and table creation are as follows,

CREATE TEMP TABLE college AS
SELECT cname::text, state::text, enrollment::int
FROM ( VALUES
  ('Stanford', 'CA', 15000),
  ('Berkeley', 'CA', 36000),
  ('MIT', 'MA', 10000),
  ('Cornell', 'NY', 21000)
) AS College(cname, state, enrollment);

CREATE TEMP TABLE student AS
SELECT sid::int, sname::text, gpa::real, sizehs::int
FROM ( VALUES
  (123, 'Amy', 3.9, 1000),
  (234, 'Bob', 3.6, 1500),
  (345, 'Craig', 3.5, 500),
  (456, 'Doris', 3.9, 1000),
  (567, 'Edward', 2.9, 2000),
  (678, 'Fay', 3.8, 200),
  (789, 'Gary', 3.4, 800),
  (987, 'Helen', 3.7, 800),
  (876, 'Irene', 3.9, 400),
  (765, 'Jay', 2.9, 1500),
  (654, 'Amy', 3.9, 1000),
  (543, 'Craig', 3.4, 2000)
) AS Student(sid, sname, gpa, sizehs);

CREATE TEMP TABLE apply AS
SELECT sid::int, cname::text, major::text, decision::text
FROM ( VALUES
  (123, 'Stanford', 'CS', 'Y'),
  (123, 'Stanford', 'EE', 'N'),
  (123, 'Berkeley', 'CS', 'Y'),
  (123, 'Cornell', 'EE', 'Y'),
  (234, 'Berkeley', 'biology', 'N'),
  (345, 'MIT', 'bioengineering', 'Y'),
  (345, 'Cornell', 'bioengineering', 'N'),
  (345, 'Cornell', 'CS', 'Y'),
  (345, 'Cornell', 'EE', 'N'),
  (678, 'Stanford', 'history', 'Y'),
  (987, 'Stanford', 'CS', 'Y'),
  (987, 'Berkeley', 'CS', 'Y'),
  (876, 'Stanford', 'CS', 'N'),
  (876, 'MIT', 'biology', 'Y'),
  (876, 'MIT', 'marine biology', 'N'),
  (765, 'Stanford', 'history', 'Y'),
  (765, 'Cornell', 'history', 'N'),
  (765, 'Cornell', 'psychology', 'Y'),
  (543, 'MIT', 'CS', 'N')
) AS apply(sid, cname, major, decision);