1. More SQL Nested and Union queries, and more
iSQLplus:
More SQL Nested and Union queries, and more
For most queries, we use the same sample data as before:
Personnel, Branch, Transport tables.
Reading:
For example: Connolly/Begg (4th Ed) : / / / 5.3.9
Any SQL book!!!
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2. XY XY XY YX X Y X Y X Y X Y
SQL2
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Match up the set operations, diagrams and SQL words
XY
XY
XY
YX a. b. c. d.
i. UNION
ii. MINUS
iii. INTERSECT
iv. EXCEPT Y X X Y X Y
Note: EXCEPT and MINUS do the same thing. EXCEPT is standard SQL, MINUS is Oracle dialect. Access has only UNION implemented. X Y

3. Set operations in SQL
Set operations are union (), intersect (), set difference ()
Oracle SQL has corresponding operators UNION, INTERSECT, MINUS (standard SQL: EXCEPT)
use these to “glue together” two SELECT statements with union-compatible results
Can combine several using brackets like in maths
by default deletes duplicates - use UNION ALL to keep them in
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4. Example Reader Staff Student Library readers
{optional, and}
Staff
Student
Library readers
each reader can be staff, student, both, or neither
Stored in three relations:
Reader(readerNo, name, address) contains all readers. staff also have a record in Staff, students in Student
Staff(readerNo*, department, )
Student(readerNo*,matricNo, course, )
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5. Example: INTERSECT Find all readers who are both staff and student
Reader(readerNo, name, address) Staff(readerNo, department, )
Student(readerNo,matricNo, course, )
Example: INTERSECT
Find all readers who are both staff and student
SQL:
No semi-colon at end of first SELECT. Why?
SELECT ReaderNo FROM Staff INTERSECT
SELECT ReaderNo FROM Student;
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6. Example: MINUS SELECT ReaderNo FROM Reader
SQL2
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Reader(readerNo, name, address) Staff(readerNo, department, )
Student(readerNo,matricNo, course, )
Example: MINUS
Find all readers who are not students
SQL:
SELECT ReaderNo FROM Reader
MINUS SELECT ReaderNo FROM Student;
To get names is difficult if not impossible with this query structure.
You might try SELECT ReaderNo, name FROM Reader MINUS SELECT ReaderNo FROM Student; but this would not work because not the same columns, so not union-compatible. But student does not have a name field, so could not add it there. Would be much better to use NOT IN instead.
How could you change this to get the names as well?
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7. Reminder: Basic SQL SELECT <attributes> FROM <Tables>
May include aliases and aggregate functions
FROM <Tables>
List Tables and may use Join operations
WHERE <condition>
Use logical expressions and may Join tables
GROUP BY <aggregate statistics>
HAVING <aggregate condition>
ORDER BY <sorting attributes>
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8. Using IS NULL To find missing/empty values use IS NULL
To exclude them use IS NOT NULL
Useful in LEFT / RIGHT / FULL JOINS to find values that don't have a match
SELECT *
FROM personnel
WHERE bonus IS NULL;
SELECT *
FROM personnel
WHERE bonus IS NOT NULL;
Query 2-1.sql
Query 2-2.sql
Note: NULL is different from 0!!!!!
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9. Reminder Example
“Find staff outside London who don't have a company car”
SELECT surname, city
FROM personnel p LEFT JOIN transport t ON p.snum=t.snum, branch b
WHERE city <> 'LONDON'
AND t.snum IS NULL
AND p.div = b.div;
Query 2-3.sql
Hints:
How many tables are required by the query?
Remember to join the tables
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10. Nested Queries Queries can be used within other queries
usually in WHERE clause
can also be in
HAVING clause
or used in calculated columns in SELECT
expression in WHERE compares value with a query result
uses math operators =, >, <, >=, <=, <> and keywords ALL, ANY, IN, NOT IN, EXISTS, NOT EXISTS
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11. Example 1 SELECT Regno, make FROM transport WHERE mileage <
SQL2
November 18
Example 1
Find cars whose mileage is less than the Jaguar's.
Two steps:
Find mileage of the Jaguar
Compare other mileages with that one
Nested query (subquery) does all of this.
Query 2-4.sql
SELECT Regno, make FROM transport
WHERE mileage <
(SELECT mileage FROM transport WHERE make='JAGUAR');
Danger! This only works if the subquery returns a single value!
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12. SQL2
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Example 2
Find staff with lower than average salaries, and show how much lower they are
SELECT snum, surname,
salary-(SELECT AVG(salary)FROM personnel)
AS difference
FROM Personnel p
WHERE salary < (SELECT AVG(salary)
FROM personnel);
Query 2-5.sql
Note: need the subqueries because aggregate functions are not allowed in WHERE and cannot be mixed with non-aggregates in SELECT
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13. Nested Queries : Operators
If sub-query can return more than one result use an operator in the comparison (IN, NOT IN, ANY, ALL, EXISTS, NOT EXISTS) IN
compares a value with a set and evaluates to True if the value is in the set and False otherwise
set can be a query result or a simple set, e.g. WHERE town IN ('Dundee', 'Perth', 'Aberdeen')
NOT IN
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14. Nested Queries: more operators
ANY
at least one result of the subquery must fulfil the condition
ALL
all results of the subquery must fulfil the comparison
EXISTS
returns True if there is at least one tuple in the subquery and False otherwise
NOT EXISTS
checks for empty sub-query
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15. Nested Examples: IN, ANY
Find all staff working in London
SELECT snum, surname FROM personnel
WHERE div IN
(SELECT div FROM branch WHERE city='LONDON'); or
WHERE div = ANY
Query 2-6.sql
Query 2-7.sql
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16. Find staff earning more than those in branch 30
SQL2
November 18
Find staff earning more than those in branch 30 A
SELECT * FROM personnel
WHERE div <> 30 and salary >
(SELECT salary FROM personnel WHERE div=30); B
SELECT * FROM personnel
WHERE div != 30 and salary > ANY
(SELECT salary FROM personnel WHERE div=30); C
SELECT * FROM personnel
WHERE div <> 30 and salary > ALL
(SELECT salary FROM personnel WHERE div=30);
A will not run, because the subquery returns more than one value, so > is not a valid comparison operator.
B will run, and returns staff not in branch 30 who earn more than someone (anyone) in branch 30; i.e. they earn more than AT LEAST ONE PERSON FROM BRANCH 30.
C will run, and returns staff not in branch 30 who earn more than EVERYONE in branch 30; i.e. they earn more than EACH AND EVERY PERSON FROM BRANCH 30.
So, I would regard C as correct (and this is script 2-8.sql). However, the question is actually not phrased very precisely, so B could also be regarded as correct. D
Both A and C are correct
Query 2-8.sql

17. Nested Examples: ALL SELECT * FROM branch WHERE div <> ALL
branches which don’t have a director
SELECT *
FROM branch
WHERE div <> ALL
(SELECT div
FROM personnel
WHERE jobtitle = 'DIRECTOR');
Query 2-9.sql
Note: no space between < and >
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18. How did this work? Subquery produces the result
Outer Query takes this result
and produces a list of branches which are none of these i.e. 20 10 30 40 50
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19. Remember to specify join criterion
EXISTS / NOT EXISTS
branches which have no staff
SELECT *
FROM branch
WHERE NOT EXISTS
(SELECT *
FROM personnel p
WHERE p.div = branch.div);
Query sql
Remember to specify join criterion
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20. How did it work?
DBMS evaluates sub-query separately for each value of p.div
If sub-query result is empty, NOT EXISTS is true, so this branch is in the result of the main query
If p.div=branch.div is omitted, sub-query is evaluated for all branches at once, sub-result not empty, so main query returns nothing
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21. Which of these queries also find branches with no staff?
SQL2
November 18
Which of these queries also find branches with no staff? A
SELECT * FROM branch
MINUS
SELECT div FROM personnel;
Query 2-11a.sql B
SELECT b.*
FROM personnel p RIGHT JOIN branch b
ON p.div=b.div
WHERE b.div IS NULL;
Query 2-11b.sql C
SELECT * FROM branch
WHERE div NOT IN
(SELECT div FROM personnel);
The ideas used for all queries are all fine and are all equivalent, but:
A gives error as columns not union-compatible. Would be ok if * is replaced by div.
B returns no results as b.div is the wrong column to apply NULL criterion to. Replace b.div IS NULL with p.div IS NULL.
C is correct.
Query 2-11c.sql D
Both B and C are correct but A is not E
All queries are correct

22. Back to the start SELECT ReaderNo FROM Reader
SQL2
November 18
Reader(readerNo, name, address) Staff(readerNo, department, )
Student(readerNo,matricNo, course, )
Back to the start
Find all readers who are not students
SQL:
SELECT ReaderNo FROM Reader
MINUS SELECT ReaderNo FROM Student;
To get names is difficult if not impossible with this query structure.
You might try SELECT ReaderNo, name FROM Reader MINUS SELECT ReaderNo FROM Student; but this would not work because not the same columns, so not union-compatible. But student does not have a name field, so could not add it there. Would be much better to use NOT IN instead.
So now, write another query that will find the readerno and name of all readers who are not students.
November 18

23. Summary Basic SQL SELECT Advanced SQL SELECT Basic query facilities
SELECT FROM WHERE GROUP BY HAVING ORDER BY
IS NULL / IS NOT NULL
Advanced SQL SELECT
Nested Queries
ALL, ANY, EXISTS, NOT EXISTS, IN, NOT IN
Set operators
UNION, INTERSECT, MINUS
November 18