1. Subqueries

2. Problem setup
Search for candidates, whose grade in mathematics is over average.
Solution:
Subquery
Find the average
2. Find the candidates, who has grade over the average.
Main query

3. Step-by-step Solution
Find the average.
SELECT AVG(mathematics) FROM candidates;
AVG(MATEMAATIKA) ,
2. Search for candidates having grade over the average.
SELECT firstname, lastname, mathematics FROM candidates WHERE mathematics> 61,43; FIRSTNAME LASTNAME MATHEMATICS
Sulev Kask
Ragnar Lepp
Alo Saar

4. Using a Subquery
SELECT firstname, lastname, mathematics FROM candidates WHERE mathematics > (SELECT AVG mathematics) FROM candidates);
FIRSTNAME LASTNAME MATHEMATICS
Sulev Kask
Ragnar Lepp
Alo Saar
61,43

5. Types of Subqueries Single-row subquery Aaviksaar
Main query
returns
Subquery
Aaviksaar
Multiple-row subquery
Main query
returns
Aaviksaar Kivikas
Subquery
Multiple-column subqueries
Main query
Aaviksaar 1,86 Kivikas ,73
returns
Subquery

6. Guidelines for using subqueries
Enclose subqueries in parentheses
Place subqueries on the right side of the comparison operator (>, <=, IN,...) .
The ORDER-BY clause in the subquery is not needed unless you are performing the top-N analysis.
Use single-row operators with single-row subqueries and use multiple-row operators (IN, ANY, ALL) with multiple-row subqueries

7. Executing single-row subqueries
Search for candidates having result in mathematics greater than Link’s one, but result on mother language is less than Klade has.
SELECT lastname, mathematics, mother_l FROM candidates WHERE mathematics > (SELECT mathematics FROM candidates WHERE lastname='Link') AND mother_l < (SELECT mother_l FROM candidates WHERE lastname = 'Klade'); 64 45
LASTNAME MATHEMATICS MOTHER_L
Pappel
Saar

8. Group Functions in Subqueries
Search for worst result in math:
SQL> SELECT lastname, mathematics
FROM candidates
WHERE mathematics =
(SELECT MIN (mathematics) FROM candidates);
LASTNAME MATHEMATICS
Remmelgas

9. HAVING Clause in subqueries
Find the year of birth of candidates, the average math of whose exceeds the results of other years
SELECT SUBSTR(id,2,2) AS „Year of Birth“, AVG(mathematics) AS „Avg. math.“ FROM candidates GROUP BY SUBSTR(id,2,2) HAVING AVG(mathematics) > (SELECT AVG(mathematics) FROM candidates WHERE SUBSTR(id,2,2) =‘83‘);
58,48...
Year of Birth Avg. math.
,75 , ,

10. Multiple-row subqueries
Result has more than one row
You can use multiple-row operators
Operator
Meaning
a IN {b1,b2,b3,...}
(a = b1) OR (a = b2) OR (a = b3) ...
a < ANY {b1,b2,b3,...}
(a < b1 ) OR (a < b2) OR (a < b3) ...
a < ALL {b1,b2,b3,...}
(a < b1 ) AND (a < b2) AND (a < b3) ...

11. Multiple-row subqueries
Search for women, whose result in math is less than result of any male.
SELECT firstname, lastname, mathematics
FROM candidates
WHERE id LIKE '4%' AND mathematics < ALL
(SELECT mathematics FROM candidates
WHERE id LIKE '3%')
LASTNAME FIRSTNAME MATHEMATICS
Remmelgas Kairi

12. Multiple column subqueries
Find the best mathematicians from males/females
SELECT lastname, firstname, mathematics FROM candidates WHERE (SUBSTR(id,1,1) , mathematics) IN (SELECT SUBSTR(id,1,1) , MAX(mathematics) FROM candidates GROUP BY SUBSTR(id,1,1));
LASTNAME FIRSTNAME MATHEMATICS
Kuusk Rain
Remmelgas Kati

13. Subquery in FROM clause
Search for candidates, whose result in math exceeds the result of other candidates, having the same gender
SELECT a.lastnamei, a.firstname,a.mathematics, substr(a.id,1,1) AS gender, b.avg_math FROM candidates a, (SELECT SUBSTR(id,1,1) as gender, AVG(mathematics) AS avg_math FROM candidates GROUP BY SUBSTR(id,1,1)) b WHERE SUBSTR(a.id,1,1) = b.gender AND a.mathematics> b.avg_math;
LASTNAME FIRSTNAME MATHEMATICS G MATH AVG_MATH
Sarapuu Tõnu ,
Kuusik Ragnar ,

14. Operator "exists" Who has sent at least one homework in english:
SELECT firstname, lastname FROM lepikult.students s
WHERE EXISTS
( SELECT * FROM lepikult. homeworks h, lepikult. subjects o
WHERE h. id=s.id
and h.course_code = o.code
and instr(upper(o.name),'ENGLISH')>0);
FIRSTNAME LASTNAME
Anna Laasik
Tiina Kuusik

15. Operator "not exists"
Find the students, who has not sent any homework:
SQL> SELECT firstname, lastname from students s
2 WHERE NOT EXISTS
( SELECT * FROM homeworks h
4* WHERE h.id=s.id)
FIRSTNAME LASTNAME
Riina Kaasik
Peeter Tuulik
Siiri Karu