Basic SQL Part 2

1. Trinity College
Basic SQL
Part 2
Timothy Richards
Trinity College, Hartford CT • Department of Computer Science • CPSC 372

2. Last Time
• SQL Language
• Declarative language
• Many Standards
• 1986-2008
• Data Definition Language (DDL)
• CREATE TABLE
• Data Manipulation Language (DML)
• SELECT-FROM-WHERE
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 2

3. Today
• Objectives
• Ambiguous attribute names, Aliasing, Renaming, and Tuple Variables
• Unspecified WHERE and the use of *
• Tables as sets
• Pattern matching and arithmetic operators
• Ordering query results
• INSERT, DELETE, UPDATE
• Goal
After today’s class you should be able to create a relational
database schema/instance using SQL DDL, manipulate data in the
tables you create, and define queries on that data.
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 3

4. Query Example
For every project located in ‘Stafford’, list the
project number, the controlling department number,
and the department manager’s last name, address,
and birth date.
SELECT Pnumber, Dnum,
Lname, Address,
Bdate
FROM PROJECT, DEPARTMENT,
EMPLOYEE
WHERE Dnum=Dnumber AND
Mgr_ssn=Ssn AND
Plocation=‘Stafford’;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 4

5. Attribute Ambiguity
In SQL, the same name can be used for two (or more)
attributes as long as the attributes are in different
relations.
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 5

6. Attribute Ambiguity
In SQL, the same name can be used for two (or more)
attributes as long as the attributes are in different
relations.
Ambiguous
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 6

7. Attribute Ambiguity
In SQL, the same name can be used for two (or more)
attributes as long as the attributes are in different
relations.
Ambiguous
Solution: qualify attributes names
with the relation names to avoid ambiguity
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 7

8. Attribute Ambiguity
Imagine the Dno and Lname attributes
of the EMPLOYEE relation were called
Dnumber and Name and the Dname attribute
of DEPARTMENT was called Name.
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 8

9. Attribute Ambiguity
Imagine the Dno and Lname attributes
of the EMPLOYEE relation were called
Dnumber and Name and the Dname attribute
of DEPARTMENT was called Name.
List the first name, last name, and address
of all employees in the Research department.
SELECT Fname, Name, Address
FROM EMPLOYEE, DEPARTMENT
WHERE Name=‘Research’ AND
Dnumber=Dnumber;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 9

10. Attribute Ambiguity
Imagine the Dno and Lname attributes
of the EMPLOYEE relation were called
Dnumber and Name and the Dname attribute
of DEPARTMENT was called Name.
List the first name, last name, and address
of all employees in the Research department.
SELECT Fname, Name, Address
FROM EMPLOYEE, DEPARTMENT
WHERE Name=‘Research’ AND Ambiguity
Dnumber=Dnumber;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 10

11. Attribute Ambiguity
Imagine the Dno and Lname attributes
of the EMPLOYEE relation were called
Dnumber and Name and the Dname attribute
of DEPARTMENT was called Name.
List the first name, last name, and address
of all employees in the Research department.
Qualify!
SELECT Fname, EMPLOYEE.Name, Address
FROM EMPLOYEE, DEPARTMENT
WHERE DEPARTMENT.Name=‘Research’ AND
DEPARTMENT.Dnumber=EMPLOYEE.Dnumber;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 11

12. Attribute Ambiguity
You can also qualify attribute names for clarity...
SELECT EMPLOYEE.Fname, EMPLOYEE.Name,
EMPLOYEE.Address
FROM EMPLOYEE, DEPARTMENT
WHERE DEPARTMENT.Name=‘Research’ AND
DEPARTMENT.Dnumber=EMPLOYEE.Dnumber;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 12

13. Attribute Ambiguity
You can also qualify attribute names for clarity...
This is quite verbose!
SELECT EMPLOYEE.Fname, EMPLOYEE.Name,
EMPLOYEE.Address
FROM EMPLOYEE, DEPARTMENT
WHERE DEPARTMENT.Name=‘Research’ AND
DEPARTMENT.Dnumber=EMPLOYEE.Dnumber;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 13

14. Attribute Ambiguity
You can also qualify attribute names for clarity...
This is quite verbose!
SELECT E.Fname, E.Name, E.Address
FROM EMPLOYEE AS E, DEPARTMENT AS D
WHERE D.Name=‘Research’ AND
D.Dnumber=E.Dnumber;
For this we can use table aliases!
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 14

15. Attribute Ambiguity
You can also qualify attribute names for clarity...
This is quite verbose!
SELECT E.Fname, E.Name, E.Address
FROM EMPLOYEE AS E, DEPARTMENT AS D
WHERE D.Name=‘Research’ AND
D.Dnumber=E.Dnumber;
Although this is nice for reducing typing...
it is required for queries that refer to the same relation twice!
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 15

16. Attribute Ambiguity
SELECT E.Fname, E.Lname, S.Fname, S.Lname
FROM EMPLOYEE AS E, EMPLOYEE AS S
WHERE E.Super_ssn=S.Ssn;
Although this is nice for reducing typing...
it is required for queries that refer to the same relation twice!
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 16

17. Attribute Ambiguity
What does this query mean in English?
SELECT E.Fname, E.Lname, S.Fname, S.Lname
FROM EMPLOYEE AS E, EMPLOYEE AS S
WHERE E.Super_ssn=S.Ssn;
Although this is nice for reducing typing...
it is required for queries that refer to the same relation twice!
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 17

18. Attribute Ambiguity
What does this query mean in English?
SELECT E.Fname, E.Lname, S.Fname, S.Lname
FROM EMPLOYEE AS E, EMPLOYEE AS S
WHERE E.Super_ssn=S.Ssn;
For each employee, retrieve the employee’s first
and last name and the first and last name
of his or her immediate supervisor.
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 18

19. Attribute Ambiguity
What does this query mean in English?
SELECT E.Fname, E.Lname, S.Fname, S.Lname
FROM EMPLOYEE AS E, EMPLOYEE AS S
WHERE E.Super_ssn=S.Ssn;
Tuple Variables
For each employee, retrieve the employee’s first
and last name and the first and last name
of his or her immediate supervisor.
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 19

20. Attribute Ambiguity
It is also possible to rename all attributes
EMPLOYEE AS E(Fn,Mi,Ln,Ssn,Bd,Addr,Sx,Sal,Sssn,Dno)
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 20

21. Missing WHERE
What if I do not want to impose a condition
on the tuples that are returned?
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 21

22. Missing WHERE
What if I do not want to impose a condition
on the tuples that are returned?
Turns out that you do not need a WHERE clause!
SELECT Ssn
FROM EMPLOYEE;
SELECT Ssn, Dname
FROM EMPLOYEE, DEPARTMENT;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 22

23. Missing WHERE
What if I do not want to impose a condition
on the tuples that are returned?
Turns out that you do not need a WHERE clause!
SELECT Ssn
FROM EMPLOYEE;
What tuples does SELECT Ssn, Dname
this return? FROM EMPLOYEE, DEPARTMENT;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 23

24. Missing WHERE
Because we do not specify a “join condition” between
the two tables, we get the cross product of
the specified attributes between the two tables!
What tuples does SELECT Ssn, Dname
this return? FROM EMPLOYEE, DEPARTMENT;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 24

25. Missing WHERE
SSN
E 02783465
02537596
Dname
D Research
Administration
Because we do not specify a “join condition” between
the two tables, we get the cross product of
the specified attributes between the two tables!
What tuples does SELECT Ssn, Dname
this return? FROM EMPLOYEE, DEPARTMENT;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 25

26. Missing WHERE
SSN
E 02783465 SSN Dname
02537596 02783465 Research
* 02537596 Research
Dname 02783465 Administration
D Research 02537596 Administration
Administration
Because we do not specify a “join condition” between
the two tables, we get the cross product of
the specified attributes between the two tables!
What tuples does SELECT Ssn, Dname
this return? FROM EMPLOYEE, DEPARTMENT;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 26

27. Missing WHERE
SSN
E 02783465 SSN Dname
02537596 02783465 Research
* 02537596 Research
Dname 02783465 Administration
D Research 02537596 Administration
Administration
We can get sensible results
if we include a join condition on
the SSN attribute.
What tuples does SELECT Ssn, Dname
this return? FROM EMPLOYEE, DEPARTMENT;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 27

28. Missing WHERE
SSN
E 02783465
SSN Dname
02537596
* 02783465 Research
Dname 02537596 Administration
D Research
Administration
We can get sensible results
if we include a join condition on
the SSN attribute.
What tuples does SELECT Ssn, Dname
this return? FROM EMPLOYEE, DEPARTMENT
WHERE Dno=Dnumber;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 28

29. Selecting *
Sometimes we want all the attributes
returned in a result.
SELECT *
FROM EMPLOYEE;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 29

30. Selecting *
Sometimes we want all the attributes
returned in a result.
SELECT *
FROM EMPLOYEE;
This will return each value
for every attribute in a tuple.
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 30

31. Tables as Sets
SQL typically treats a table as a set.
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 31

32. Tables as Sets
SQL typically treats a table as a set.
More accurately, as a multiset.
What is a multiset?
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 32

33. Tables as Sets
SQL typically treats a table as a set.
More accurately, as a multiset.
What is a multiset?
A set, where duplicate tuples
can appear more than once in a table.
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 33

34. Tables as Sets
SQL typically treats a table as a set.
More accurately, as a multiset.
What is a multiset?
A set, where duplicate tuples
can appear more than once in a table.
But, I thought that was bad?!
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 34

35. Tables as Sets
SQL typically treats a table as a set.
More accurately, as a multiset.
What is a multiset?
A set, where duplicate tuples
can appear more than once in a table.
But, I thought that was bad?!
Turns out, in practice this is useful...
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 35

36. Tables as Sets
• SQL allows duplicates because...
• Duplicate elimination is an expensive operation.
Can you think why?
• The user may want to see duplicate tuples in the result of a
query
• When an aggregate function is applied to tuples, in most
cases we do not want to eliminate duplicates.
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 36

37. Tables as Sets
Retrieve the salary of every employee
SELECT Salary
FROM EMPLOYEE;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 37

38. Tables as Sets
Retrieve the salary of every employee
SELECT Salary
FROM EMPLOYEE;
Ok, this returns tuples containing the salary
of each employee.
Is it useful?
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 38

39. Tables as Sets
Retrieve the salary of every employee
SELECT Salary
FROM EMPLOYEE;
Ok, this returns tuples containing the salary
of each employee.
Is it useful?
What if we only wanted to see the distinct salaries?
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 39

40. Tables as Sets
Retrieve the salary of every employee
SELECT DISTINCT Salary
FROM EMPLOYEE;
Ok, this returns tuples containing the salary
of each employee.
Is it useful?
What if we only wanted to see the distinct salaries?
This results in distinct salaries!
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 40

41. Tables as Sets
Retrieve the salary of every employee
We can use
SELECT ALL Salary
ALL to retrieve all
---same as default. FROM EMPLOYEE;
Ok, this returns tuples containing the salary
of each employee.
Is it useful?
What if we only wanted to see the distinct salaries?
This results in distinct salaries!
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 41

42. Tables as Sets
SQL has direct support for some set operations
Make a list of all project numbers for projects
that involve an employee whose last name is ‘Smith’,
either as a worker or as a manager of the department
that controls the project.
If SQL supports set union (it does), how might
we want to compose this query?
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 42

43. Tables as Sets
SQL has direct support for some set operations
Make a list of all project numbers for projects
that involve an employee whose last name is ‘Smith’,
either as a worker or as a manager of the department
that controls the project.
SELECT DISTINCT Pnumber
FROM PROJECT, DEPARTMENT, EMPLOYEE First write the query
WHERE Dnum=Dnumber AND Mgr_ssn=Ssn for the first part...
AND Lname=‘Smith’;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 43

44. Tables as Sets
SQL has direct support for some set operations
Make a list of all project numbers for projects
that involve an employee whose last name is ‘Smith’,
either as a worker or as a manager of the department
that controls the project.
SELECT DISTINCT Pnumber
FROM PROJECT, DEPARTMENT, EMPLOYEE
WHERE Dnum=Dnumber AND Mgr_ssn=Ssn
AND Lname=‘Smith’;
Then write a query for
SELECT DISTINCT Pnumber the second part...
FROM PROJECT, WORKS_ON, EMPLOYEE
WHERE Pnumber=Pno AND Essn=Ssn AND Lname=‘Smith’;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 44

45. Tables as Sets
SQL has direct support for some set operations
Make a list of all project numbers for projects
that involve an employee whose last name is ‘Smith’,
either as a worker or as a manager of the department
that controls the project.
(SELECT DISTINCT Pnumber
FROM PROJECT, DEPARTMENT, EMPLOYEE
WHERE Dnum=Dnumber AND Mgr_ssn=Ssn
AND Lname=‘Smith’) Then union the
UNION results together!
(SELECT DISTINCT Pnumber
FROM PROJECT, WORKS_ON, EMPLOYEE
WHERE Pnumber=Pno AND Essn=Ssn AND Lname=‘Smith’);
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 45

46. Tables as Sets
SQL has direct support for some set operations
In addition to UNION SQL also supports
set difference (EXCEPT) and
set intersection (INTERSECT)
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 46

47. Substring Pattern Matching
Retrieve all employees whose address is in Houston,Texas.
SELECT Fname, Lname
FROM EMPLOYEE
WHERE Address LIKE ‘%Houston,TX%’;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 47

48. Substring Pattern Matching
Find all employees who were born during the 1950s.
SELECT Fname, Lname
FROM EMPLOYEE
WHERE Bdate LIKE ‘__5_______’;
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 48

49. Arithmetic Operations
Show the resulting salaries if every employee working on
the ‘ProductX’ project is given a 10 percent raise.
SELECT E.Fname, E.Lname, E.Salary*1.1 AS incr
FROM EMPLOYEE AS E, WORKS_ON AS W, PROJECT AS P
WHERE E.Ssn=W.Essn AND W.Pno=P.Pnumber AND
P.Pname=‘ProductX’
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 49

50. Next time...
Ordering Query Results
AND
Insert, Deleting, and Updating, Oh My!
Trinity College, Hartford CT • Department of Computer Science • CPSC 372 50