This presentation was used for the study meeting in YNS.

1. Practice on
Practical SQL
Hideshi Ogoshi
YNS Philippines Inc.
SEP-22-2016

2. Misunderstanding of
Object-Relational Mapping
• We use ORM. That does not mean we can totally
rely on it, since it is not totally black boxed to the
developers.

3. ORM Pros & Cons
• Pros
• Transforms result set into a data structure, which can
be easily handled in the programs
• Handles many kinds of RDBMS transparently with same
code
• Cons
• Hard to control generating SQL
• Hard to use for the complicated requirements

4. Relational Database
• MySQL, PostgreSQL, Oracle Database, DB2,
MSSQL, SQLite, etc.
• Relational database is based on set theory.

5. Set Theory
• Set theory is the branch of mathematical logic
that studies sets, which are collections of objects.

6. Terms of Set Theory
• Member
• Subset
• Union
• Intersection
• Difference
• Tuple
• Selection
• Projection

7. Member
• A fundamental binary relation between an object o
and a set A.
• o is a member of A, denoted o ∈ A.
• Example: 2 ∈ {1, 2, 3}
1
2
3

8. Subset
• If all the members of set A are also members of set B.
• A is a subset of B, denoted A ⊆ B.
• Example: {1, 2} ⊆ {1, 2, 3, 4}
1 2
3
4

9. Union
• Union creates a new set, which contains all the members
of 2 sets.
• Union of the sets A and B, denoted A ∪ B.
• Example: {1, 2, 3} ∪ {2, 3, 4} = {1, 2, 3, 4}
2
31 4

10. Intersection
• Intersection creates a new set, which contains common
members between 2 sets.
• Intersection of the sets A and B, denoted A ∩ B.
• Example: {1, 2, 3} ∩ {2, 3, 4} = {2, 3}
2
31 4

11. Difference
• Difference creates a new set, which contains remaining
members in a set after subtracting the set from another set.
• Difference of the sets A and B, denoted A ー B.
• Example: {1, 2, 3} ー {2, 3, 4} = {1}
2
31 4

12. Tuple
• is a kind of data type, which represents finite
ordered list of elements
• Example: {(a, 1), (b,2), (c, 3)}
(b,2)
(c,3)
(a,1)

13. Selection
• Selecting all tuples from set T1 if a tuple matches
the conditions. Then creates T2 as new set.
• σ Age >= 34 ^ Weight <= 50 (Person)
(b,2)
(c,3)
(a,1)
(c,3)
(a,1)
T1 T2

14. Projection
• Picking a sub-set of all available columns
• Modifying an attribute’s value
• Π Age,Weight (Person)
(b,2)
(c,3)
(a,1)
f(x)
f(x)
f(x)
4
6
2

15. Correspondence
between set theory and RDB
• Table is a set of objects
• Record is a tuple
• SELECT is projection
• WHERE is selection

16. INNER JOIN
• is an expression, which joins two tables with conditions.
• returns records, which exactly match the conditions.

17. OUTER JOIN
• LEFT OUTER JOIN
• RIGHT OUTER JOIN
• FULL OUTER JOIN

18. LEFT OUTER JOIN
• is an expression, which joins two tables with conditions.
• If conditions are matched, it returns values on both side of the tables
• If not, it returns values in the left side of the table with NULL on the right side

19. RIGHT OUTER JOIN
• is an expression, which joins two tables with conditions.
• If conditions are matched, it returns values on both side of the tables
• If not, it returns values in the right side of the table with NULL on the left side

20. FULL OUTER JOIN
• is an expression, which joins two tables with conditions.
• If conditions are matched, it returns values on both side of the tables
• If not, it returns values in the either side of the table with NULL on the other side

21. UNION (ALL)
• is a kind of operator, which combines result of
two or more SELECT statements.

22. Question 1
• Try to write SQL, which realises following Venn
diagram.
• Use setA and setB tables.

23. Question 2
• Try to write SQL, which realises following Venn
diagram.
• Use setA and setB tables.

24. Question 3
• Try to fetch students, who belong to as same course as
Mike.
• Make sure to exclude Mike from the result.

25. CASE
SELECT
product_name
, amount
, CASE
WHEN amount = 0
THEN 'Nothing. You should order ASAP!'
WHEN amount > 0 AND amount <= 10
THEN 'You have enough stock.'
WHEN amount > 10
THEN 'Too much. It’ll be dead stock.'
WHEN amount IS NULL
THEN 'Something went wrong on the system..'
ELSE 'Amount is minus. Check the stock.'
END AS suggestion
FROM stock;

26. COUNT
SELECT COUNT(name) FROM students;

27. Question 4
• Try to count number of male and number of
female respectively.
gender number
male 4
female 2
number_of_male number_of_female
4 2
1)
2)

28. Question 5
• Try to show ranking of the students by their
scores ordered by score descending.
name score rank
Beth 95 1
Mike 90 2
Jack 80 3
John 80 3
Tina 80 3
Nick 75 6

29. Question 6
• Try to show ranking of the students by their
scores ordered by score descending.
name score rank
Beth 95 1
Mike 90 2
Jack 80 3
John 80 3
Tina 80 3
Nick 75 4