1. CS 405G: Introduction to Database Systems
SQL I
Instructor: Jinze Liu

2. DDL Sailors Boats Reserves sid sname rating age 1 Frodo 7 22 2 Bilbo39 3
Sam 8 27
CREATE TABLE Sailors
(sid INTEGER,
sname CHAR(20),
rating INTEGER,
age REAL,
PRIMARY KEY sid)
CREATE TABLE Boats
(bid INTEGER,
bname CHAR (20),
color CHAR(10)
PRIMARY KEY bid)
CREATE TABLE Reserves
bid INTEGER,
day DATE,
PRIMARY KEY (sid, bid, day),
FOREIGN KEY sid REFERENCES Sailors,
FOREIGN KEY bid REFERENCES Boats)
NOT NULL,
Boats
bid
bname
color
101
Nina
red
102
Pinta
blue
103
Santa Maria
NOT NULL,
NOT NULL,
Reserves
NOT NULL,
sid
bid
day 1
102
9/12 2
9/13
NOT NULL,

3. SQL DML DML includes 4 main statements:
SELECT (query), INSERT, UPDATE and DELETE
e.g: To find the names of all 19 year old students:
We’ll spend a lot of time on this one
PROJECT
sid
name
age
gpa
53666
Jones 18
3.4
53688
Smith
3.2
53650
login 19
3.8
SELECT S.name
FROM Students S
WHERE S.age=19
SELECT

4. Querying Multiple Relations
Can specify a join over two tables as follows:
SELECT S.name, E.cid
FROM Students S, Enrolled E
WHERE S.sid=E.sid AND E.grade=‘B'
PROJECT
SELECT
JOIN
S.name E.cid
Jones History105
result =

5. Basic SQL Query
DISTINCT: optional keyword indicating answer should not contain duplicates.
In SQL, default is that duplicates are not eliminated! (Result is called a “multiset”)
target-list : A list of attributes of tables in relation-list
SELECT [DISTINCT] target-list
FROM relation-list
WHERE qualification
qualification : Comparisons combined using AND, OR and NOT. Comparisons are Attr op const or Attr1 op Attr2, where op is one of ,,,, etc.
relation-list : A list of relation names, possibly with a range-variable after each name

6. Query Semantics
Semantics of an SQL query are defined in terms of the following conceptual evaluation strategy:
FROM clause: compute cross-product of all tables
WHERE clause: Check conditions, discard tuples that fail. (called “selection”).
3. SELECT clause: Delete unwanted fields. (called “projection”).
4. If DISTINCT specified, eliminate duplicate rows.
Probably the least efficient way to compute a query!
An optimizer will find more efficient strategies to get the same answer.

7. Query Semantics Example
SELECT sname
FROM Sailors, Reserves
WHERE Sailors.sid=Reserves.sid AND bid=103
Boats
bid
bname
color
101
Nina
red
102
Pinta
blue
103
Santa Maria X
Sailors
sid
sname
rating
age 1
Frodo 7 22 2
Bilbo 39 3
Sam 8 27
Reserves
sid
bid
day 1
102
9/12 2
103
9/13

8. Step 1: Compute the cross product
Sailors
Reserves
sid
sname
rating
age 1
Frodo 7 22 2
Bilbo 39 3
Sam 8 27
sid
bid
day 1
102
9/12 2
103
9/13
...
SailorsXReserves
sid
sname
rating
age
bid
day 1
Frodo 7 22
102
9/12 2
103
9/13
Bilbo 39 3
Sam 8 27

9. Step 1: How big? Question: If |S| is cardinality of Sailors, and
Reserves
sid
sname
rating
age 1
Frodo 7 22 2
Bilbo 39 3
Sam 8 27
sid
bid
day 1
102
9/12 2
103
9/13
Question:
If |S| is cardinality of Sailors, and
|R| is cardinality of Reserves,
What is the cardinality of Sailors X Reserves?
Answer: |S| * |R|
|Sailors X Reserves| = 3X2 = 6

10. Step 2: Check conditions in where clause
SELECT sname
FROM Sailors, Reserves
WHERE Sailors.sid=Reserves.sid AND bid=103
SailorsXReserves
sid
sname
rating
age
bid
day 1
Frodo 7 22
102
9/12 2
103
9/13
Bilbo 39 3
Sam 8 27

11. Step 3: Delete unwanted fields
SELECT sname
FROM Sailors, Reserves
WHERE Sailors.sid=Reserves.sid AND bid=103
SailorsXReserves
sid
sname
rating
age
bid
day 1
Frodo 7 22
102
9/12 2
103
9/13
Bilbo 39 3
Sam 8 27

12. Range Variables Used for short hand Needed when ambiguity could arise
e.g two tables with the same column name:
SELECT sname
FROM Sailors, Reserves
WHERE Sailors.sid=Reserves.sid AND Reserves.bid=103
FROM Sailors S, Reserves R
WHERE S.sid=R.sid AND R.bid=103
Question: do range variables remind you of anything?
Variables in relational calculus

13. Sometimes you need a range variable
e.g a Self-join:
SELECT R1.bid, R1.date
FROM Reserves R1, Reserves R2
WHERE R1.bid = R2.bid and
R1.date = R2.date and
R1.sid != R2.sid
bid
day
103
9/12
bid
day
103
9/12
Reserves
Reserves
sid
bid
day 1
102
9/12 3
103 4
9/13 2
sid
bid
day 1
102
9/12 3
103 4
9/13 2 R1 R2 R1 R2 R2 R2 R1

14. Sometimes you need a range variable
SELECT R1.bid, R1.day
FROM Reserves R1, Reserves R2
WHERE R1.bid = R2.bid and
R1.day = R2.day and
R1.sid != R2.sid
bid
day
103
9/12
bid
day
103
9/12
What are we computing?
Boats reserved on the same day
by different sailors

15. SELECT Clause Expressions
Can use “*” if you want all columns:
SELECT *
FROM Sailors x
WHERE x.age > 20
Can use arithmetic expressions (add other operations we’ll discuss later)
SELECT S.age, S.age-5 AS age1, 2*S.age AS age2
FROM Sailors S
WHERE S.sname = ‘Dustin’
Can use AS to provide column names
SELECT S1.sname AS name1, S2.sname AS name2
FROM Sailors S1, Sailors S2
WHERE 2*S1.rating = S2.rating - 1

16. WHERE Clause Expressions
Can also have expressions in WHERE clause:
SELECT S1.sname AS name1, S2.sname AS name2
FROM Sailors S1, Sailors S2
WHERE 2*S1.rating = S2.rating - 1
“LIKE” is used for string matching.
SELECT S.age, S.age-5 AS age1, 2*S.age AS age2
FROM Sailors S
WHERE S.sname LIKE ‘B_l%o’
`_’ stands for any one character and `%’ stands for 0 or more arbitrary characters.

17. Set versus bag semantics
No duplicates
Relational model and algebra use set semantics
Bag
Duplicates allowed
Number of duplicates is significant
SQL uses bag semantics by default

18. Set versus bag example πSID Enroll Enroll SELECT SID FROM Enroll; sid
1234
1124
1123
Enroll
sid
cid
grade
1234
647 A
108 B
1124
1123
SELECT SID FROM Enroll;
sid
1234
1124
1123

19. A case for bag semantics
Efficiency
Saves time of eliminating duplicates
Which one is more useful?
πGPA Student
SELECT GPA FROM Student;
The first query just returns all possible GPA’s
The second query returns the actual GPA distribution
Besides, SQL provides the option of set semantics with DISTINCT keyword

20. SELECT DISTINCT SELECT DISTINCT S.sid FROM Sailors S, Reserves R
bid
day 1
102
9/12 2
103
9/13
sid
sname
rating
age 1
Frodo 7 22 2
Bilbo 39 3
Sam 8 27
SELECT DISTINCT S.sid
FROM Sailors S, Reserves R
WHERE S.sid=R.sid
sid 1 2
Find sailors that have reserved at least one boat

21. SELECT DISTINCT How about: SELECT S.sid FROM Sailors S, Reserves R
WHERE S.sid=R.sid
sid 1 2

22. SELECT DISTINCT How about: SELECT S.sname FROM Sailors S, Reserves R
sid
sname
rating
age 1
Frodo 7 22 2
Bilbo 39 3
Sam 8 27 4 5 32
How about:
sname
Frodo
Bilbo
SELECT S.sname
FROM Sailors S,
Reserves R
WHERE S.sid=R.sid
Reserves
sid
bid
day 1
102
9/12 2
103
9/13 4
105
vs:
SELECT DISTINCT S.sname
FROM Sailors S,
Reserves R
WHERE S.sid=R.sid
sname
Frodo
Bilbo
Do we find all sailors that reserved at least one boat?

23. Find sids of sailors who’ve reserved a red or a green boat
ANDs, ORs, UNIONs and INTERSECTs
Find sids of sailors who’ve reserved a red or a green boat
Boats
SELECT R.sid
FROM Boats B,Reserves R
WHERE(B.color=‘red’ OR
B.color=‘green’) AND R.bid=B.bid
bid
bname
color
101
Nina
red
102
Pinta
blue
103
Santa Maria
105
Titanic
green
sid 2 4
Sailors
Reserves
sid
sname
rating
age 1
Frodo 7 22 2
Bilbo 39 3
Sam 8 27 X
sid
bid
day 1
102
9/12 2
103
9/13 4
105

24. Find sids of sailors who’ve reserved a red and a green boat
ANDs and ORs
Find sids of sailors who’ve reserved a red and a green boat
Boats
SELECT R.sid
FROM Boats B,Reserves R
WHERE(B.color=‘red’ AND
B.color=‘green’) AND R.bid=B.bid X
bid
bname
color
101
Nina
red
102
Pinta
blue
103
Santa Maria
105
Titanic
green
Sailors
Reserves
sid
sname
rating
age 1
Frodo 7 22 2
Bilbo 39 3
Sam 8 27
sid
bid
day 1
101
9/12 2
103
9/13
105

25.  = Use INTERSECT instead of AND SELECT R.sid FROM Boats B,Reserves R
Exercise: try to rewrite this query using a self join instead of INTERSECT!
SELECT R.sid
FROM Boats B,Reserves R
WHERE B.color = ‘red’
AND R.bid=B.bid
INTERSECT
WHERE B.color = ‘green’ AND R.bid=B.bid
Boats
bid
bname
color
101
Nina
red
102
Pinta
blue
103
Santa Maria
105
Titanic
green
Reserves
sid
bid
day 1
101
9/12 2
103
9/13
105
sid 1 2
sid 1
sid 1  =

26. =  Could also use UNION for the OR query SELECT R.sid
FROM Boats B, Reserves R
WHERE B.color = ‘red’
AND R.bid=B.bid
UNION
SELECT R.sid FROM Boats B, Reserves R
WHERE B.color = ‘green’
Boats
bid
bname
color
101
Nina
red
102
Pinta
blue
103
Santa Maria
105
Titanic
green
Reserves
sid
bid
day 1
102
9/12 2
103
9/13 4
105
sid 2 4
sid 2
sid 4 = 

27. EXCEPT: Set Difference
Sailors
sid
sname
rating
age 1
Frodo 7 22 2
Bilbo 39 3
Sam 8 27
Find sids of sailors who have not reserved a boat
SELECT S.sid
FROM Sailors S
EXCEPT
FROM Sailors S, Reserves R
WHERE S.sid=R.sid
Reserves
sid
bid
day 1
102
9/12 2
103
9/13
105
First find the set of sailors who have reserved a boat…
sid 3
and then compare it with the rest of the sailors