Database language exercises 5 - SELECT (nested query EXISTS, collection, query a derived table based)

EXISTS representatives existential quantifier ∃.
With the predicate EXISTS subquery does not return any data, produced only a logical true value "true" or logically false value "false".
If the inner query result is not empty, then the outer layer of the WHERE clause returns true, otherwise returns false.
EXISTS subquery by the leads, with the goal column expression often used *, because the subquery with EXISTS only return true or false value, given the column name moot.
After using the existential quantifier NOT EXISTS, if the inner query result is empty, the outer layer of the WHERE clause returns a true value, otherwise return value.
[3.60] to query all patients enrolled in the 1st course student names.

SELECT Sname
FROM   Student
WHERE  EXISTS
         (SELECT *
		 FROM SC
		 WHERE Sno=Student.Sno AND Cno= '1');

Find Principle: Sno value taken by one of the Student, the Sno value SC if present, and which Cno = '1', the result is not empty article, is TRUE, this entry Student.Sname display.

[3.61] Example query student's name is not No. 1 elective courses.

SELECT Sname
FROM   Student
WHERE  NOT EXISTS(
		SELECT *
		FROM SC
		WHERE Sno = Student.Sno AND Cno='1');

All sub-query with IN predicate, comparison operators, ANY and ALL predicates EXISTS predicate can be used with sub-queries equivalent replacement.
[3.62] patients enrolled in a full course of query names of the students.
SQL language is not universal quantifier, predicate can be converted with the universal quantifier is equivalent with the existential quantifier predicate:
(∀x) P≡¬ (∃x (to p))
because there is no full quantifiers, may be subject meaning converted into an equivalent form of existential quantifiers: query such students, no one elective course he does not. Its SQL statement is as follows:

SELECT Sname
FROM   Student
WHERE  NOT EXISTS
    (SELECT *
	FROM Course
	WHERE NOT EXISTS(
			SELECT *
			FROM SC
			WHERE Sno= Student.Sno 
			AND Cno= Course.Cno) );

Thus using EXIST / NOT EXIST to implement the query with a measure word.
[3.63] Example query least number of students enrolled in a full course of 201,215,122 student elective.
SQL language contains no logical operation
can be used to convert the predicate calculus predicate logic implication equivalent to:
P → q≡¬p∨q

SELECT DISTINCT Sno
FROM   SC SCX
WHERE  NOT EXISTS(
		SELECT *
        FROM SC SCY
        WHERE SCY.Sno = '201215122'  AND  NOT EXISTS(SELECT *                                    	FROM SC SCZ                                   		WHERE SCZ.Sno=SCX.Sno AND SCZ.Cno=SCY.Cno));

[3.64] Example queries Department of Computer Science students and age not more than 19-year-old student.

SELECT *
FROM  Student
WHERE Sdept= 'CS'
UNION
SELECT *
FROM  Student
WHERE Sage<=19;

[3.65] Example 1 query elective courses or elective courses students 2.

SELECT Sno
FROM   SC
WHERE  Cno='1'
UNION
SELECT Sno
FROM   SC
WHERE  Cno='2';

[3.66] Example queries Department of Computer Science students age is not more than 19-year-old student intersection.

SELECT *
FROM  Student
WHERE Sdept='CS' 
INTERSECT
SELECT *
FROM  Student
WHERE Sage<=19 

[3.67] query both patients enrolled in courses 1 and 2 of elective courses students.

SELECT Sno
FROM   SC
WHERE  Cno='1'
INTERSECT
SELECT Sno
FROM   SC
WHERE  Cno='2';

Of course, the solution is more than one nested query can be achieved:

SELECT Sno
FROM   SC
WHERE  Cno='1' AND Sno IN
                    (SELECT Sno
                    FROM   SC
                    WHERE  Cno='2');

[3.68] Example queries Department of Computer Science students age is not more than 19-year-old student set difference.

SELECT *
FROM  Student
WHERE Sdept='CS'
EXCEPT
SELECT  *
FROM  Student
WHERE Sage <=19;