1. EECS 647: Introduction to Database Systems
Instructor: Luke Huan
Spring 2007

2. Luke Huan Univ. of Kansas
Administrative
Final project is posted at the class website.
Go through it and we will discuss the final project at next class meeting
9/19/2018
Luke Huan Univ. of Kansas

3. Luke Huan Univ. of Kansas
Review
SFW statement:
Sort the outputs:
USE bag operation:
Enforce set operation:
Output average:
Partition rows in a table:
Select certain groups:
ORDER BY
Default choice
DISTINCT
AVG (aggregation function)
GROUP BY
HAVING
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Luke Huan Univ. of Kansas

4. Luke Huan Univ. of Kansas
Review
Nested queries
Treat as a table
Treat as a scalar IN
EXIST
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Luke Huan Univ. of Kansas

5. Luke Huan Univ. of Kansas
Today’s Topic
NULL value
Outer join
Nested queries
Table creation and constraints
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Luke Huan Univ. of Kansas

6. Incomplete information
Example: Student (SID, name, age, GPA)
Value unknown
We do not know Nelson’s age
Value not applicable
Nelson has not taken any classes yet; what is his GPA?
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Luke Huan Univ. of Kansas

7. Luke Huan Univ. of Kansas
Solution 1
A dedicated special value for each domain (type)
GPA cannot be –1, so use –1 as a special value to indicate a missing or invalid GPA
Leads to incorrect answers if not careful
SELECT AVG(GPA) FROM Student;
Complicates applications
SELECT AVG(GPA) FROM Student WHERE GPA <> -1;
Remember the Y2K bug?
“00” was used as a missing or invalid year value
9/19/2018
Luke Huan Univ. of Kansas

8. Luke Huan Univ. of Kansas
Solution 2
A valid-bit for every column
Student (SID, name, name_is_valid, age, age_is_valid, GPA, GPA_is_valid)
Complicates schema and queries
SELECT AVG(GPA) FROM Student WHERE GPA_is_valid;
9/19/2018
Luke Huan Univ. of Kansas

9. Luke Huan Univ. of Kansas
Solution 3?
Decompose the table; missing row = missing value
StudentName (SID, name) StudentAge (SID, age) StudentGPA (SID, GPA)
StudentID (SID)
Conceptually the cleanest solution
Still complicates schema and queries
How to get all information about a student in a table?
Would natural join work?
9/19/2018
Luke Huan Univ. of Kansas

10. Luke Huan Univ. of Kansas
SQL’s solution
A special value NULL
For every domain
Special rules for dealing with NULL’s
Example: Student (SID, name, age, GPA)
h 789, “Nelson”, NULL, NULL i
9/19/2018
Luke Huan Univ. of Kansas

11. Luke Huan Univ. of Kansas
Three-valued logic
TRUE = 1, FALSE = 0, UNKNOWN = 0.5
x AND y = min(x, y)
x OR y = max(x, y)
NOT x = 1 – x
WHERE and HAVING clauses only select rows for output if the condition evaluates to TRUE
UNKNOWN is not enough
AND
True
False
NULL
UNK OR
True
False
NULL
TRUE
UNK
AND OR
9/19/2018
Luke Huan Univ. of Kansas

12. Luke Huan Univ. of Kansas
Computing with NULL’s
(Arithmetic operation) when we operate on a NULL and another value (including another NULL) using +, –, etc., the result is NULL
Aggregate functions ignore NULL, except COUNT(*) (since it counts rows)
When we compare a NULL with another value (including another NULL) using =, >, etc., the result is UNKNOWN
9/19/2018
Luke Huan Univ. of Kansas

13. Unfortunate consequences
SELECT AVG(GPA) FROM Student
3.4
SELECT SUM(GPA)/COUNT(*) FROM Student;
2.72
SELECT * FROM Student;
SELECT * FROM Student WHERE GPA = GPA
Not equivalent
sid
name
age
gpa
1234
John Smith 21
3.5
1123
Mary Carter 19
3.8
1011
Bob Lee 22
NULL
1204
Susan Wong
3.4
1306
Kevin Kim 18
2.9
WHY DO YOU CARE WHEN EQUIVALENCES ARE BROKEN?
Q.O. is harder.
9/19/2018
Luke Huan Univ. of Kansas

14. Luke Huan Univ. of Kansas
Another problem
Example: Who has NULL GPA values?
SELECT * FROM Student WHERE GPA = NULL;
Does not work; never returns anything
(SELECT * FROM Student) EXCEPT ALL (SELECT * FROM Student WHERE GPA = GPA)
Works, but ugly
Introduced built-in predicates IS NULL and IS NOT NULL
SELECT * FROM Student WHERE GPA IS NULL;
9/19/2018
Luke Huan Univ. of Kansas

15. Luke Huan Univ. of Kansas
Outerjoin motivation
Example: a master class list
SELECT c.CID, s.SID FROM Enroll e, Student s, Course c
WHERE e.SID = s.SID and c.CID = e.CID;
What if a student take no classes
For these students, CID column should be NULL
What if a course with no student enrolled yet?
For these courses, SID should be NULL
Very useful in information integration as well
9/19/2018
Luke Huan Univ. of Kansas

16. Luke Huan Univ. of Kansas
Outerjoin
SELECT * FROM R FULL OUTER JOIN S ON p;
A full outer join between R and S (denoted R ! S) includes all rows in the result of R !pS, plus
“Dangling” R rows (those that do not join with any S rows) padded with NULL’s for S’s columns
“Dangling” S rows (those that do not join with any R rows) padded with NULL’s for R’s columns
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Luke Huan Univ. of Kansas

17. Luke Huan Univ. of Kansas
Outerjoin (II)
SELECT * FROM R LEFT OUTER JOIN S ON p;;
SELECT * FROM R RIGHT OUTER JOIN S ON p;
A left outer join (R ! S) includes rows in R ! S plus dangling R rows padded with NULL’s
A right outer join (R ! S) includes rows in R ! S plus dangling S rows padded with NULL’s
9/19/2018
Luke Huan Univ. of Kansas

18. Luke Huan Univ. of Kansas
Outerjoin examples
SELECT * FROM Employee LEFT OUTER JOIN Department ON Eid = Mid
Employee
Eid
Name
1123
John Smith
1234
Mary Carter
1311
Bob Lee
Eid
Name
Did
Mid
Dname
1123
John Smith 4
Research
1234
Mary Carter 5
Finance
1311
Bob Lee
NULL
SELECT * FROM Employee FULL OUTER JOIN Department ON Eid = Mid
SELECT * FROM Employee RIGHT OUTER JOIN Department ON Eid = Mid
Department
Eid
Name
Did
Mid
Dname
1123
John Smith 4
Research
1234
Mary Carter 5
Finance
1311
Bob Lee
NULL 6
1312 HR
Eid
Name
Did
Mid
Dname
1123
John Smith 4
Research
1234
Mary Carter 5
Finance
NULL 6
1312 HR
Did
Mid
Dname 4
1123
Research 5
1234
Finance 6
1312 HR
9/19/2018
Luke Huan Univ. of Kansas

19. Luke Huan Univ. of Kansas
Summary
Query
SELECT-FROM-WHERE statements
Ordering
Set and bag operations
Aggregation and grouping
Table expressions, subqueries
NULL
Outerjoins
9/19/2018
Luke Huan Univ. of Kansas

20. Luke Huan Univ. of Kansas
Exercise
Goal: Get the Name of one’s supervisor
EMPLOYEE (Eid, Mid, Name)
SQL Statement:
SELECT EID, Name FROM EMPLOYEE WHERE MId =Eid;
SELECT E1.EID, E2.Name FROM EMPLOYEE E1, EMPLOYEE E2 WHERE E1.MId = E2.EId;
Eid
Mid
Name
1123
1234
John Smith
1311
Mary Carter
1611
Bob Lee
1455
Lisa Wang
Jack Snow
Eid
Name
1123
Mary Carter
1234
Bob Lee
1311
Jack Snow
1455
1611
9/19/2018
Luke Huan Univ. of Kansas

21. Luke Huan Univ. of Kansas
Exercise
Goal: group employees according to their department, for each department, list EIds of its employees and list the head count
EMPLOYEE (Eid, Name), DEPARTMENT(Eid, Did)
SQL Statement:
SELECT Did, Eid, COUNT(Eid)
FROM EMPLOYEE e, DEPARTMENT d
WHERE e.Eid = d.Eid
GROUP BY Did
There is something wrong!
Wishful thinking (list a group of Eid after grouping) won’t work
Solution: produce (1) a summary table listing Did and head count, and (2) sort Department according to Did.
9/19/2018
Luke Huan Univ. of Kansas

22. More on Nested Queries: Scoping
To find out which table a column belongs to
Start with the immediately surrounding query
If not found, look in the one surrounding that; repeat if necessary
Use table_name.column_name notation and AS (renaming) to avoid confusion
9/19/2018
Luke Huan Univ. of Kansas

23. Luke Huan Univ. of Kansas
An Example
SELECT * FROM Student s WHERE EXISTS (SELECT * FROM Enroll e WHERE SID = s.SID AND EXISTS (SELECT * FROM Enroll WHERE SID = s.SID AND CID <> e.CID));
Students who are taking at least two courses
9/19/2018
Luke Huan Univ. of Kansas

24. Quantified subqueries
A quantified subquery can be used as a value in a WHERE condition
Universal quantification (for all): … WHERE x op ALL (subquery) …
True iff for all t in the result of subquery, x op t
Existential quantification (exists): … WHERE x op ANY (subquery) …
True iff there exists some t in the result of subquery such that x op t
Beware
In common parlance, “any” and “all” seem to be synonyms
In SQL, ANY really means “some”
9/19/2018
Luke Huan Univ. of Kansas

25. Examples of quantified subqueries
Which employees have the highest salary?
Employee (Sid, Name, Salary)
SELECT * FROM Employee WHERE Salary >= ALL
(SELECT Salary FROM Employee);
How about the lowest salary?
SELECT * FROM Employee WHERE Salary <= ALL
(SELECT salary FROM Employee);
9/19/2018
Luke Huan Univ. of Kansas

26. More ways of getting the highest Salary
Who has the highest Salary?
SELECT * FROM Employee e WHERE NOT EXISTS (SELECT * FROM Employee WHERE Salary > e.Salary);
SELECT * FROM Employee WHERE Eid NOT IN (SELECT e1.SID FROM Employee e1, Employee e2
WHERE e1.Salary < e2.Salary);
9/19/2018
Luke Huan Univ. of Kansas

27. Luke Huan Univ. of Kansas
Nested Queries
Nested queries do not add expression power to SQL
For convenient
Write intuitive SQL queries
Can always use SQL queries without nesting to complete the same task (though sometime it is hard)
9/19/2018
Luke Huan Univ. of Kansas

28. Luke Huan Univ. of Kansas
More Exercise
Sailors (sid: INTEGER, sname: string, rating: INTEGER, age: REAL)
Boats (bid: INTEGER, bname: string, color: string)
Reserves (sid: INTEGER, bid: INTEGER)
Sailors
Boats
sid
sname
rating
age 1
Fred 7 22 2
Jim 39 3
Nancy 8 27
bid
bname
color
101
Nina
red
102
Pinta
blue
103
Santa Maria
sid
bid 1
102 2
Reserves
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Luke Huan Univ. of Kansas

29. Luke Huan Univ. of Kansas
Exercise I
Find sid’s of sailors who’ve reserved a red AND a green boat
SELECT R1.sid
FROM Boats B1, Reserves R1, Boats B2, Reserves R2
WHERE B1.color=‘red’ AND B2.color=‘green’
AND R1.bid=B1.bid AND R2.bid=B2.bid
AND R1.sid=R2.sid
SELECT sid
FROM Boats, Reserves
Where B.color = ‘red’
INTERSECT
(SELECT sid
Where B.color = ‘green’)
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Luke Huan Univ. of Kansas

30. Luke Huan Univ. of Kansas
Exercise II
Find sid’s of sailors who have not reserved a boat
SELECT sid
FROM Sailors
EXCEPT
(SELECT S.sid
FROM Sailors S, Reserves R
WHERE S.sid=R.sid)
Non-monotonic operation!
9/19/2018
Luke Huan Univ. of Kansas

31. Exercise III (a tough one)
Find sailors who’ve reserved all boats
SELECT Sid
FROM Sailor
EXCEPT
FROM
(SELECT bid, sid
FROM Boat, Sailor
Reserves)
Non-monotonic operation!
#All sailors
#Those who do not reserve all boats
#All possible combinations between sid and bid
#Existing reservations
9/19/2018
Luke Huan Univ. of Kansas

32. Summary of SQL features covered so far
SELECT-FROM-WHERE statements
Set and bag operations
Ordering
Aggregation and grouping
Table expressions, subqueries
NULL’s and outerjoins
Next: data modification statements, constraints
9/19/2018
Luke Huan Univ. of Kansas

33. Relational Query Languages
Two sublanguages:
DDL – Data Definition Language
Define and modify schema
DML – Data Manipulation Language
Queries can be written intuitively.
Insert, delete, update rows in a table
DBMS is responsible for efficient evaluation.
The key: precise semantics for relational queries.
Optimizer can re-order operations, without affecting query answer.
Choices driven by “cost model”

34. Constraints in Table Declaration
Restrictions on allowable data in a database
In addition to the simple structure and type restrictions imposed by the table definitions
Declared as part of the schema
Enforced by the DBMS
Why use constraints?
Protect data integrity (catch errors)
Tell the DBMS about the data (so it can optimize better)
9/19/2018
Luke Huan Univ. of Kansas

35. Types of SQL constraints
Key constraint
NOT NULL
Referential integrity (foreign key)
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Luke Huan Univ. of Kansas

36. Luke Huan Univ. of Kansas
Key declaration
At most one PRIMARY KEY per table
Typically implies a primary index
Rows are stored inside the index, typically sorted by the primary key value ) best speedup for queries
Any number of UNIQUE keys per table
Typically implies a secondary index
To speedup queries
9/19/2018
Luke Huan Univ. of Kansas

37. Luke Huan Univ. of Kansas
NOT NULL
Indicates that the value of an attribute can not be NULL
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Luke Huan Univ. of Kansas

38. Luke Huan Univ. of Kansas
Examples
CREATE TABLE Student (SID INTEGER NOT NULL PRIMARY KEY, name VARCHAR(30) NOT NULL, VARCHAR(30) UNIQUE, age INTEGER, GPA FLOAT);
CREATE TABLE Course (CID CHAR(10) NOT NULL PRIMARY KEY, title VARCHAR(100) NOT NULL);
CREATE TABLE Enroll (SID INTEGER NOT NULL, CID CHAR(10) NOT NULL, PRIMARY KEY(SID, CID));
Used for query
processing
This form is required for multi-attribute keys
9/19/2018
Luke Huan Univ. of Kansas

39. Referential integrity example
Enroll.SID references Student.SID
If an SID appears in Enroll, it must appear in Student
Enroll.CID references Course.CID
If a CID appears in Enroll, it must appear in Course
That is, no “dangling pointers”
How do you translate a relationship set in E/R?
Referential integrity!
9/19/2018
Luke Huan Univ. of Kansas

40. Referential integrity in SQL
Referenced column(s) must be PRIMARY KEY
Referencing column(s) form a FOREIGN KEY
Example
CREATE TABLE Enroll (SID INTEGER NOT NULL, CID CHAR(10) NOT NULL, PRIMARY KEY(SID, CID), FOREIGN KEY CID REFERENCES Course(CID)
FOREIGN KEY SID REFERENCES Student(SID));
9/19/2018
Luke Huan Univ. of Kansas

41. Enforcing referential integrity
Example: Enroll.SID references Student.SID
Insert or update an Enroll row so it refers to a non-existent SID
Reject
Delete or update a Student row whose SID is referenced by some Enroll row
Cascade: ripple changes to all referring rows
Set NULL: set all references to NULL
Set DEFAULT: use the default value of the attribute
All four options can be specified in SQL
9/19/2018
Luke Huan Univ. of Kansas

42. Luke Huan Univ. of Kansas
SQL Example
CREATE TABLE Enroll (SID INTEGER NOT NULL, CID CHAR(10) NOT NULL, PRIMARY KEY(SID, CID), FOREIGN KEY CID REFERENCES Course(CID) CASCADE ON DELETE
FOREIGN KEY SID REFERENCES Student(SID) CASCADE ON DELETE);
Delete all student enrollments for a canceled class
Delete all student enrollments for a dropped student
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Luke Huan Univ. of Kansas

43. Database Modification Operations
INSERT
DELETE
UPDATE
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Luke Huan Univ. of Kansas

44. Luke Huan Univ. of Kansas
INSERT
Insert one row
INSERT INTO Enroll VALUES (456, ’EECS647’);
Student 456 takes EECS647
Insert the result of a query
INSERT INTO Enroll (SELECT SID, ’EECS647’ FROM Student WHERE SID NOT IN (SELECT SID FROM Enroll
WHERE CID = ’EECS647’));
Force everybody to take EECS647!
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Luke Huan Univ. of Kansas

45. Luke Huan Univ. of Kansas
DELETE
Delete everything
DELETE FROM Enroll;
Delete according to a WHERE condition
Example: Student 456 drops CPS116
DELETE FROM Enroll WHERE SID = 456 AND CID = ’EECS647’;
Example: Drop students from all EECS classes with GPA lower than 1.0
DELETE FROM Enroll WHERE SID IN (SELECT SID FROM Student WHERE GPA < 1.0) AND CID LIKE ’EECS%’;
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Luke Huan Univ. of Kansas

46. Luke Huan Univ. of Kansas
UPDATE
Example: Student 142 changes name to “Barney”
UPDATE Student SET name = ’Barney’ WHERE SID = 142;
9/19/2018
Luke Huan Univ. of Kansas

47. Summary of SQL features covered so far
Query
SELECT-FROM-WHERE statements
Set and bag operations
Table expressions, subqueries
Aggregation and grouping
Ordering
Outerjoins
Constraints
Modification
INSERT/DELETE/UPDATE
Next: triggers, views, indexes
9/19/2018
Luke Huan Univ. of Kansas