Subqueries
Problem setup
Search for candidates, whose grade in mathematics is over
average.
Solution:
Subquery
1. Find the average
2. Find the candidates, who
has grade over the average.
Main query
Step-by-step Solution
1. Find the average.
SELECT AVG(mathematics) FROM candidates;
AVG(MATEMAATIKA)
---------------61,4344828
2. Search for candidates having grade over the
average.
SELECT firstname, lastname, mathematics
FROM candidates
WHERE mathematics> 61,43;
FIRSTNAME LASTNAME MATHEMATICS
--------------- --------------- -------------Sulev
Kask
95
Ragnar
Lepp
100
Alo
Saar
94
Using a Subquery
SELECT firstname, lastname, mathematics FROM candidates
WHERE mathematics >
61,43
(SELECT AVG mathematics) FROM candidates);
FIRSTNAME LASTNAME MATHEMATICS
---------------------------------- --------------------------Sulev
Kask
95
Ragnar
Lepp
100
Alo
Saar
94
................
Types of Subqueries
•
Single-row subquery
Main query
returns
Subquery
•
Aaviksaar
Multiple-row subquery
Main query
returns
Subquery
•
Aaviksaar
Kivikas
Multiple-column subqueries
Main query
returns
Subquery
Aaviksaar 1,86
Kivikas
1,73
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 multiplerow subqueries
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
64
WHERE mathematics >
(SELECT mathematics FROM candidates
WHERE lastname='Link')
AND
45
mother_l <
(SELECT mother_l FROM candidates
WHERE lastname = 'Klade');
LASTNAME
MATHEMATICS MOTHER_L
--------------- ----------- --------------------------------Pappel
82
40
Saar
75
30
Group Functions in Subqueries
Search for worst result in math:
SQL> SELECT lastname, mathematics
FROM candidates
WHERE mathematics =
(SELECT MIN (mathematics)
candidates);
LASTNAME
MATHEMATICS
--------------- --------------------------Remmelgas
12
FROM
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‘);
Year of Birth Avg. math.
-- ----------------------79
67,75
81
59,8571429
84
62,8639053
58,48...
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<
(a < b1 ) OR (a < b2) OR (a < b3) ...
ANY {b1,b2,b3,...}
a<
(a < b1 ) AND (a < b2) AND (a < b3)
ALL {b1,b2,b3,...} ...
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
100
Remmelgas
Kati
96
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
95
3 63 60,9256198
Kuusik
Ragnar
100
3 72
60,9256198
.......
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
Operator "not exists"
Find the students, who has not sent any homework:
SQL> SELECT firstname, lastname from students s
2 WHERE NOT EXISTS
3
( SELECT * FROM homeworks h
4*
WHERE h.id=s.id)
FIRSTNAME LASTNAME
--------------- --------------Riina
Kaasik
Peeter
Tuulik
Siiri
Karu