1. SQL

2. Introduction
Structured Query Language (SQL) is the most widely used commercial relational database language.
It was originally developed at IBM in the SEQUEL-XRM and System-R projects ( ).
Almost immediately, other vendors introduced DBMS products based on SQL, and it is now a de facto standard.
The SQL continues to evolve in response to the changing need.

3. The SQL language has several aspects to it:
The Data Manipulation Language (DML)
The subset of SQL allows users to pose queries and to insert, delete, and modify rows.
The Data Definition Language (DDL)
The subset of SQL supports the creation, deletion, and modification to definitions for tables and views.
Triggers and Advanced Integrity Constraints
The feature was introduced in SQL:1999. The standard includes support for triggers, which are action executed by the DBMS whenever changes to the database meet conditions specified in the trigger

4. Embedded and Dynamic SQL
Embedded SQL features allow SQL code to be called from a host language such as C or COBOL.
Dynamic SQL features allow a query to be constructed and executed at run-time.
Client-Server Execution and Remote Database Access
Client application can connect to an SQL server.
Access data from a database over a network.
Transaction Management
Control how transaction are executed.
Security
Provides mechanisms to control users’ access to data.

5. Basic SQL Query The basic form of SQL relation-list target-list
A list of relation names (possibly with a range-variable after each name).
target-list
A list of attributes of relations in relation-list.
SELECT [DISTINCT] target-list
FROM relation-list
WHERE qualification

6. qualification DISTINCT
Comparisons (Attr op const or Attr1 op Attr2, where op is one of ) combined using AND, OR and NOT.
DISTINCT
an optional keyword indicating that the answer should not contain duplicates. Default is that duplicates are not eliminated.
SELECT [DISTINCT] target-list
FROM relation-list
WHERE qualification

7. WHERE clause (optional)
SELECT clause
specifies columns to be retained in the result.
FROM clause
specifies a cross-product of tables.
WHERE clause (optional)
specifies selection conditions on the tables mentioned in the FROM clause.
An SQL query intuitively corresponds to a relational algebra expression involving selections, projections, and cross-products.

8. SELECT DISTINCT a1, a2, …, an FROM R1, R2, …, Rm WHERE P
 a1, a2, …, an (  P ( R1 x R2 x … x Rm ) )

9. Example: Find the names of all branches in the loan relation.
SELECT branch-name
FROM Loan
Loan
Result

10. To remove duplications
SELECT DISTINCT branch-name
FROM Loan
Loan
Result

11. Conceptual Evaluation Strategy
Compute the cross-product of relation-list.
Discard resulting tuples if they fail qualifications.
Delete attributes that are not in target-list.
If DISTINCT is specified, eliminate duplicate rows.
(This strategy is probably the least efficient way to compute a query! An optimizer will find more efficient strategies to compute the same answers.)

12. Example (Q1, p.137): Find the names of sailors who have reserved boat number 103.
sid
bid
day 22
101
10/10/96 58
103
11/12/96
sid
sname
rating
age 22
dustin 7
45.0 31
lubber 8
55.5 58
rusty 10
35.0
Instance R3 of Reserves
Instance S4 of Sailors

13. FROM Sailors S, Reserves R WHERE S.sid=R.sid AND R.bid=103
sname
rating
age 22
dustin 7
45.0 31
lubber 8
55.5 58
rusty 10
35.0
sid
bid
day 22
101
10/10/96 58
103
11/12/96
SELECT S.sname
FROM Sailors S, Reserves R
WHERE S.sid=R.sid AND R.bid=103
sid
sname
rating
age
bid
day 22
dustin 7
45.0
101
10/10/96 58
103
11/12/96 31
lubber 8
55.5
rusty 10
35.0
Row remains after
selection.
S4 X R3
Result

14. A Note on Range Variables
Really needed only if the same relation appears twice in the FROM clause. The previous query can also be written as: or
SELECT S.sname
FROM Sailors S, Reserves R
WHERE S.sid=R.sid AND bid=103
It is good style,
however, to use
range variables
always!
SELECT sname
FROM Sailors, Reserves
WHERE Sailors.sid=Reserves.sid
AND bid=103

15. More Examples Given the following schema:
Sailors(sid: integer, sname: string, rating:integer, age: real)
Boats(bid: integer, bname; string, color: string)
Reserves(sid: integer, bid: integer, day: date)
sailors
sid
sname
rating
Boats
bid
bname
age
color
Reserves
day

16. Example: Find the sids of sailors who have reserved a red boat.
sname
rating
Boats
bid
bname
age
color
Reserves
day
Example: Find the sids of sailors who have reserved a red boat.
Example: Find the names of sailors who have reserved a red boat.
SELECT R.sid
FROM Boat B, Reserves R
WHERE B.bid = R.bid AND B.color = ‘red’
SELECT S.sname
FROM Sailors S, Reserves R, Boat B
WHERE S.sid = R.sid AND R.bid = B.bid AND
B.color = ‘red’

17. Example: Find the colors of boats reserved by Lubber.
sailors
sid
sname
rating
Boats
bid
bname
age
color
Reserves
day
Example: Find the colors of boats reserved by Lubber.
(In general, there may be more than one sailor called Lubber. In this case, it will return the colors of boats reserved by some Lubber).
SELECT B.color
FROM Sailors S, Reserves R, Boat B
WHERE S.sid = R.sid AND R.bid = B.bid AND
S.name = ‘Lubber’.

18. sailors
sid
sname
rating
Boats
bid
bname
age
color
Reserves
day
Example: Find the names of sailors who have reserved at least one boat.
(If a sailor has not made a reservation, the second step in the conceptual evaluation strategy would eliminate all rows in the cross-product that involve this sailor).
SELECT S.name
FROM Sailors S, Reserves R
WHERE S.sid = R.sid

19. Expressions and Strings
SELECT S.age, age1=S.age-5, 2*S.age AS age2
FROM Sailors S
WHERE S.sname LIKE ‘B_%B’
Illustrates use of arithmetic expressions and string pattern matching: Find triples (of ages of sailors and two fields defined by expressions) for sailors whose names begin and end with B and contain at least three characters.
AS and = are two ways to name fields in result.
LIKE is used for string matching. `_’ stands for any one character and `%’ stands for 0 or more arbitrary characters.

20. Union, Intersect, and Except
SQL provides three set-manipulation constructs that extend the basic query form presented earlier.
Union ()
Intersection ()
Except ()
(many systems recognize the keyword MINUS for EXCEPT)

21. Example: Find sid’s of sailors who’ve reserved a red or a green boat
sname
rating
Boats
bid
bname
age
color
Reserves
day
Example: Find sid’s of sailors who’ve reserved a red or a green boat
SELECT S.sid
FROM Sailors S, Boats B, Reserves R
WHERE S.sid=R.sid AND R.bid=B.bid
AND (B.color=‘red’ OR B.color=‘green’)
UNION: Can be used to compute the union of any two union-compatible sets of tuples (which are themselves the result of SQL queries).
If we replace OR by AND in the first version, what do we get?
Also available: EXCEPT (What do we get if we replace UNION by EXCEPT?)
SELECT S.sid
FROM Sailors S, Boats B, Reserves R
WHERE S.sid=R.sid AND R.bid=B.bid
AND B.color=‘red’
UNION
AND B.color=‘green’

22. Example: Find sid’s of sailors who’ve reserved a red and a green boat
sname
rating
Boats
bid
bname
age
color
Reserves
day
Example: Find sid’s of sailors who’ve reserved a red and a green boat
SELECT S.sid
FROM Sailors S, Boats B1, Reserves R1,
Boats B2, Reserves R2
WHERE S.sid=R1.sid AND R1.bid=B1.bid
AND S.sid=R2.sid AND R2.bid=B2.bid
AND (B1.color=‘red’ AND B2.color=‘green’)
INTERSECT: Can be used to compute the intersection of any two union-compatible sets of tuples.
Included in the SQL/92 standard, but some systems don’t support it.
SELECT S.sid
FROM Sailors S, Boats B, Reserves R
WHERE S.sid=R.sid AND R.bid=B.bid
AND B.color=‘red’
INTERSECT
AND B.color=‘green’

23. sailors
sid
sname
rating
Boats
bid
bname
age
color
Reserves
day
Example: Find sid’s of all sailors who’ve reserved red boat but not green boat.
SELECT S.sid
FROM Sailors S, Boats B, Reserves R
WHERE S.sid=R.sid AND R.bid=B.bid
AND B.color=‘red’
EXCEPT
AND B.color=‘green’
Indeed, since the Reserves relation contains sid information, there is no need to look at the Sailors relation.
SELECT R.sid
FROM Boats B, Reserves R
WHERE R.bid=B.bid AND B.color=‘red’
EXCEPT
WHERE R.bid=B.bid AND B.color=‘green’

24. sailors
sid
sname
rating
Boats
bid
bname
age
color
Reserves
day
Example: Find sid’s of all sailors who have a rating of 10 or reserved boat 104
SELECT S.sid
FROM Sailor S
WHERE S.rating = 10
UNION
SELECT R.sid
FROM Reserves R
WHERE R.bid=104

25. Nested Queries
A nested query is a query that has another query embedded within it.
The embedded query is called a subquery.
The embedded query can be a nested query itself.
Queries may have very deeply nested structures.

26. Example: Find names of sailors who’ve reserved boat #103:
SELECT S.sname
FROM Sailors S
WHERE S.sid IN (SELECT R.sid
FROM Reserves R
WHERE R.bid=103)
A very powerful feature of SQL: a WHERE clause can itself contain an SQL query! (Actually, so can FROM and HAVING clauses.)
To find sailors who’ve not reserved #103, use NOT IN.
To understand semantics of nested queries, think of a nested loops evaluation: For each Sailors tuple, check the qualification by computing the subquery.

27. Correlated Nested Queries
In the previous example, the inner subquery has been completely independent of the outer query.
In general, the inner subquery could depend on the row currently being examined in the outer query.

28. Example: Find names of sailors who’ve reserved boat #103:
SELECT S.sname
FROM Sailors S
WHERE EXISTS (SELECT *
FROM Reserves R
WHERE R.bid=103 AND S.sid=R.sid)
EXISTS is another set comparison operator, which allows us to test whether a set is nonempty.
If UNIQUE is used, and * is replaced by R.bid, finds sailors with at most one reservation for boat #103. (UNIQUE checks for duplicate tuples; * denotes all attributes. Why do we have to replace * by R.bid?)
Illustrates why, in general, subquery must be re-computed for each Sailors tuple.

29. Set-comparison Operators
We’ve already seen IN, EXISTS and UNIQUE. We can also use NOT IN, NOT EXISTS and NOT UNIQUE.
Also available: op ANY, op ALL
Where op is one of the arithmetic comparison operators
SOME is also available, but it is just a synonym for ANY.
Example: Find sailors whose rating is greater than that of some sailor called Horatio:
SELECT *
FROM Sailors S
WHERE S.rating > ANY (SELECT S2.rating
FROM Sailors S2
WHERE S2.sname=‘Horatio’)

30. Example: Find the sailors with the highest rating.
sid
sname
rating
Boats
bid
bname
age
color
Reserves
day
Example: Find sailors whose rating is better than every sailor called Horatio.
SELECT *
FROM Sailors S
WHERE S.rating > ALL (SELECT S2.rating
FROM Sailors S2
WHERE S2.sname=‘Horatio’)
Example: Find the sailors with the highest rating.
SELECT *
FROM Sailors S
WHERE S.rating >= ALL (SELECT S2.rating
FROM Sailors S2)

31. Rewriting INTERSECT queries using IN
Example: Find sid’s of sailors who’ve reserved both a red and a green boat:
SELECT S.sid
FROM Sailors S, Boats B, Reserves R
WHERE S.sid=R.sid AND R.bid=B.bid AND B.color=‘red’
AND S.sid IN (SELECT S2.sid
FROM Sailors S2, Boats B2, Reserves R2
WHERE S2.sid=R2.sid AND R2.bid=B2.bid
AND B2.color=‘green’)
Similarly, EXCEPT queries can be re-written using NOT IN.
To find names (not sid’s) of Sailors who’ve reserved both red and green boats, just replace S.sid by S.sname in SELECT clause. (What about INTERSECT query? [ see p. 150 of the textbook])

32. Division in SQL Example: Find sailors who’ve reserved all boats.
sid
sname
rating
Boats
bid
bname
age
color
Reserves
day
SELECT S.sname
FROM Sailors S
WHERE NOT EXISTS
((SELECT B.bid
FROM Boats B)
EXCEPT
(SELECT R.bid
FROM Reserves R
WHERE R.sid=S.sid))
All boats
All boats reserved by S
Note that this query is correlated – for each sailor S, we check to
see if the set of boats reserved by S includes every boat.

33. An Alternative way to write the previous query without using EXCEPT
sailors
sid
sname
rating
Boats
bid
bname
age
color
Reserves
day
An Alternative way to write the previous query without using EXCEPT
Intuitively, for each sailor we check that there is no boat that has not been reserved by this sailor.
Boat b will be returned if it has been
reserved by sailor s; otherwise the result
will be empty.
SELECT S.sname
FROM Sailors S
WHERE NOT EXISTS (SELECT B.bid
FROM Boats B
WHERE NOT EXISTS (SELECT R.bid
FROM Reserves R
WHERE R.bid=B.bid
AND R.sid=S.sid))
Boats haven’t been
reserved by sailor s

34. Aggregate Operators SQL allows the use of arithmetic expressions.
SQL supports five aggregate operations, which can be applied on any column of a relation.
COUNT([DISTINCT] A)
The number of (unique) value in the A column.
SUM ( [DISTINCT] A)
The sum of all (unique) values in the A column.
AVG ([DISTINCT A)
The average of all (unique) values in the A column.
MAX (A)
The maximum value in the A column.
MIN (A)
The minimum value in the A column.

35. Example: Find the average age of all sailors
sid
sname
rating
Boats
bid
bname
age
color
Reserves
day
Example: Find the average age of all sailors
SELECT AVG (S.age)
FROM Sailors S
Example: Find the average age of sailors with rating of 10
SELECT AVG (S.age)
FROM Sailors S
WHERE S.rating = 10

36. Example: Find the name and age of the oldest sailor
sailors
sid
sname
rating
Boats
bid
bname
age
color
Reserves
day
Example: Find the name and age of the oldest sailor
SELECT S.sname, MAX (S.age)
FROM Sailors S
SELECT S.sname, S.age
FROM Sailors S
WHERE S.age =
(SELECT MAX (S2.age)
FROM Sailors S2)
SELECT S.sname, S.age
FROM Sailors S
WHERE (SELECT MAX (S2.age)
FROM Sailors S2)
= S.age
Equivalent to the second query, and is allowed in the SQL/92 standard, but is not supported in some systems.

37. Example: Count the number of Sailors
sid
sname
rating
Boats
bid
bname
age
color
Reserves
day
Example: Count the number of Sailors
SELECT COUNT (*)
FROM Sailors S
Example: Count the number of different sailor names
SELECT COUNT (DISTINCT S.name)
FROM Sailors S

38. sailors
sid
sname
rating
Boats
bid
bname
age
color
Reserves
day
Aggregate operations offer an alternative to the ANY and ALL constructs.
Example: Find the names of sailors who are older than the oldest sailor with a rating of 10.
SELECT S.name
FROM Sailors S
WHERE S.age > ( SELECT MAX (S2.age)
FROM Sailors S2
WHERE S2.rating = 10)

39. Group by and Having
So far, we’ve applied aggregate operators to all (qualifying) tuples. Sometimes, we want to apply them to each of several groups of tuples.
Consider: Find the age of the youngest sailor for each rating level.
In general, we don’t know how many rating levels exist, and what the rating values for these levels are!
Suppose we know that rating values go from 1 to 10; we can write 10 queries that look like this (!):
SELECT MIN (S.age)
FROM Sailors S
WHERE S.rating = i
For i = 1, 2, ... , 10:

40. The query can be expressed as follows
To write such queries, we need a major extension to the basic SQL query form, namely the Group BY clause.
The extension also includes an optional HAVING clause that can be used to specify qualifications over groups.
The query can be expressed as follows
SELECT S.rating, MIN (S.age)
FROM Sailors S
GROUP BY S.rating

41. The general format of GROUP BY and Having
SELECT [DISTINCT] target-list
FROM relation-list
WHERE qualification
GROUP BY grouping-list
HAVING group-qualification
The target-list contains (i) attribute names (ii) terms with aggregate operations (e.g., MIN (S.age)).
The attribute list (i) must be a subset of grouping-list. Intuitively, each answer tuple corresponds to a group, and these attributes must have a single value per group. (A group is a set of tuples that have the same value for all attributes in grouping-list.)

42. Conceptual Evaluation
SELECT [DISTINCT] target-list
FROM relation-list
WHERE qualification
GROUP BY grouping-list
HAVING group-qualification
Conceptual Evaluation
The cross-product of relation-list is computed, tuples that fail qualification are discarded, `unnecessary’ fields are deleted, and the remaining tuples are partitioned into groups by the value of attributes in grouping-list.
The group-qualification is then applied to eliminate some groups. Expressions in group-qualification must have a single value per group!
In effect, an attribute in group-qualification that is not an argument of an aggregate op also appears in grouping-list. (SQL does not exploit primary key semantics here!)
One answer tuple is generated per qualifying group.

43. Example: Find the age of the youngest sailor with age 18, for each rating with at least 2 such sailors
SELECT S.rating, MIN (S.age)
FROM Sailors S
WHERE S.age >= 18
GROUP BY S.rating
HAVING COUNT (*) > 1
Only S.rating and S.age are mentioned in the SELECT, GROUP BY or HAVING clauses; other attributes are `unnecessary’.
2nd column of result is unnamed. (Use AS to name it. See Q32 on
p. 155 of your textbook.)
Answer relation

44. For each red boat, find the number of reservations for this boat
sailors
sid
sname
rating
Boats
bid
bname
age
color
Reserves
day
For each red boat, find the number of reservations for this boat
Only columns that appear in the
GROUP BY clause can appear in
the HAVING clause, unless
they appear as arguments to an
aggregate operator in the HAVING
clause.
SELECT B.bid, COUNT (*) AS reservationcount
FROM Boats B, Reserves R
WHERE R.bid=B.bid
GROUP BY B.bid
HAVING B.color = ‘red’
SELECT B.bid, COUNT (*) AS reservationcount
FROM Boats B, Reserves R
WHERE R.bid=B.bid AND B.color=‘red’
GROUP BY B.bid

45. Example: Find the average age of sailors for each rating level that has at least two sailors.
sid
sname
rating
age 22
Dustin 7
45.0 29
Brutus 1
33.0 31
Lubber 8
55.5 32
Andy
25.5 58
Rusty 10
35.0 64
Horatio 71
Zorba
16.0 74 9 85
Art 3 95
Bob
63.5 96
Frodo
SELECT S.rating, AVG(S.age) AS avgage
FROM Sailor S
GROUP BY S.rating
HAVING COUNT (*) > 1 OR
SELECT S.rating, AVG(S.age) AS avgage
FROM Sailor S
GROUP BY S.rating
HAVING 1 < (SELECT COUNT (*)
FROM Sailors S2
WHERE S.rating = S2.rating)
Rating
avgage 3
38.2 7
40.0 8
40.5 10
25.5
We can use S.rating
inside the nested
subquery in the HAVING
because it has a single
value for the current group
of sailors
Instance S3 of Sailor
Answer

46. Example: Find the average age of sailors who are at least 18 years old for each rating level that has at least two sailors.
sid
sname
rating
age 22
Dustin 7
45.0 29
Brutus 1
33.0 31
Lubber 8
55.5 32
Andy
25.5 58
Rusty 10
35.0 64
Horatio 71
Zorba
16.0 74 9 85
Art 3 95
Bob
63.5 96
Frodo
SELECT S.rating, AVG(S.age) AS avgage
FROM Sailor S
WHERE S.age >=18
GROUP BY S.rating
HAVING 1 < (SELECT COUNT (*)
FROM Sailors S2
WHERE S.rating = S2.rating)
Rating
avgage 3
38.2 7
40.0 8
40.5 10
35.0
Note that the answer is very
similar to the previous one, with the
only difference being that for the
group 10, we now ignore the
sailor with age 16 while computing
the average.
Answer
Instance S3 of Sailor

47. Example: Find the average age of sailors who are at least 18 years old for each rating level that has at least two such sailors.
sid
sname
rating
age 22
Dustin 7
45.0 29
Brutus 1
33.0 31
Lubber 8
55.5 32
Andy
25.5 58
Rusty 10
35.0 64
Horatio 71
Zorba
16.0 74 9 85
Art 3 95
Bob
63.5 96
Frodo
SELECT S.rating, AVG(S.age) AS avgage
FROM Sailor S
WHERE S.age >=18
GROUP BY S.rating
HAVING 1 < (SELECT COUNT (*)
FROM Sailors S2
WHERE S.rating = S2.rating AND
S2.age >=18)
Rating
avgage 3
38.2 7
40.0 8
40.5
It differs from the answer of the
previous question in that there is no
tuple for rating 10, since there
is only one tuple with rating and
age >= 18.
Answer
Instance S3 of Sailor

48. Some other ways to write the previous query.
SELECT S.rating, AVG(S.age) AS avgage
FROM Sailor S
WHERE S.age >=18
GROUP BY S.rating
HAVING COUNT (*) > 1
SELECT Temp.rating, Temp.avgage
FROM (SELECT S.rating, AVG(S.age) AS avgage,
count (*) As ratingcount
FROM Sailor S
WHERE S.age >=18
GROUP BY S.rating) AS Temp
WHERE Temp.ratingcount > 1

49. sailors
sid
sname
rating
Boats
bid
bname
age
color
Reserves
day
Find those ratings for which the average age is the minimum over all ratings
SELECT S.rating
FROM Sailors S
WHERE S.age = (SELECT MIN (AVG (S2.age))
FROM Sailors S2
GROUP BY S2.rating)
Aggregate operations cannot be nested!
This query will not work even if the expression MIN(AVG(S2.age)), which is illegal,
is allowed. In the nested query, Sailors is partitioned into groups by rating, and the
average age is computed for each rating value. For each group, applying MIN to this
average age value for the group will return the same value.

50. Correct solution (in SQL/92):
SELECT Temp.rating, Temp.avgage
FROM (SELECT S.rating, AVG (S.age) AS avgage
FROM Sailors S
GROUP BY S.rating) AS Temp
WHERE Temp.avgage = (SELECT MIN (Temp.avgage)
FROM Temp)
It essentially computes a temporary table containing the average age for
each rating value and then finds the rating(s) for which this average age is
the minimum.

51. NULL Value field value unknown field attribute inapplicable
A new employee has not been assigned a supervisor yet.
field attribute inapplicable
An unmarried employee does not have a spouse
Employee
eno
ename
supervisor_eno
spouse_name

52. Complications caused by NULL values
Special operators provided to check if value is / is not NULL
The condition following ‘where’ clause eliminates FALSE or unknown
For a person who hasn’t been assigned a supervisor yet, is
supervisor_eno = true or false?
(We need a three value logic: true, false, unknown)
NOT unknown -> unknown
OR (TRUE, unknown) -> TRUE , OR (FALSE, unknown) -> unknown
AND (TRUE, unknown) -> unknown, AND (FALSE, unknown) -> FALSE

53. NULL is counted in COUNT(*) All other aggregate discard NULL values
Two rows are duplicates if matching columns are either equal or both NULL
implicitly NULL=NULL.
But for comparison in where clause, (NULL=NULL) = unknown.
NULL is counted in COUNT(*)
All other aggregate discard NULL values

54. General Constraints
Useful when more general ICs than keys are involved.
Can use queries to express constraint.
CREATE TABLE Sailors
( sid INTEGER,
sname CHAR(10),
rating INTEGER,
age REAL,
PRIMARY KEY (sid),
CHECK ( rating >= 1
AND rating <= 10 )

55. Constraints can be named.
When a boat is inserted into Reserves or an existing row is modified, the conditional expression in the CHECK constraint is evaluated. If it evaluates to false, the command is rejected.
CREATE TABLE Reserves
( sname CHAR(10),
bid INTEGER,
day DATE,
PRIMARY KEY (bid,day),
CONSTRAINT noInterlakeRes
CHECK (`Interlake’ <>
( SELECT B.bname
FROM Boats B
WHERE B.bid=bid)))

56. Domain Constraints
We can define a new domain using the CREATE DOMAIN statement, which uses CHECK constraints.
CREATE DOMAIN ratingval INTEGER DEFAULT 1
CHECK (VALUE >=1 AND VALUE <=10)

57. Distinct Type
This statement defines a new distinct type called ratingtype, with INTEGER as its source type.
Values of type ratingtype can be compared with each other, but they cannot be compared with values of other types.
Ratingtype values are treated as being distinct from values for the source type.
CREATE TYPE ratingtype AS INTEGER

58. Constraints Over Multiple Relations
CREATE TABLE Sailors
( sid INTEGER,
sname CHAR(10),
rating INTEGER,
age REAL,
PRIMARY KEY (sid),
CHECK
( (SELECT COUNT (S.sid) FROM Sailors S)
+ (SELECT COUNT (B.bid) FROM Boats B) < 100 )
Number of boats
plus number of
sailors is < 100
Awkward and wrong!
If Sailors is empty, the number of Boats tuples can be anything!

59. ASSERTION is the right solution; not associated with either table.
CREATE ASSERTION smallClub
CHECK
( (SELECT COUNT (S.sid) FROM Sailors S)
+ (SELECT COUNT (B.bid) FROM Boats B) < 100 )

60. Triggers
Trigger: procedure that starts automatically if specified changes occur to the DBMS
Three parts:
Event (activates the trigger)
Condition (tests whether the triggers should run)
Action (what happens if the trigger runs)

61. Triggers: Example (SQL:1999)
CREATE TRIGGER youngSailorUpdate
AFTER INSERT ON SAILORS
REFERENCING NEW TABLE NewSailors
FOR EACH STATEMENT
INSERT
INTO YoungSailors(sid, name, age, rating)
SELECT sid, name, age, rating
FROM NewSailors N
WHERE N.age <= 18

62. Appendix: Natural Join
Note that some systems may support natural join. This query
can be rewritten as
Some systems also support inner join, left join, right join and full join.
SELECT S.sname
FROM Sailors S, Reserves R
WHERE S.sid=R.sid AND R.bid=103
SELECT S.sname
FROM Sailors S natural join Reserves R
WHERE R.bid=103