Download SQL: Queries, Constraints, Triggers

Database Management Systems
Chapter 5
SQL
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Date Calculation
•
TimeStampDiff(unit, date_expr1, date_expre2)
mysql> SELECT TIMESTAMPDIFF(MONTH,'2003-02-01','2003-05-01');
-> 3
mysql> SELECT TIMESTAMPDIFF(YEAR,'2002-05-01','2001-01-01');
> -1
mysql> SELECT TIMESTAMPDIFF(DAY,'2003-02-01','2003-05-01');
-> 89
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Data Calculations
CREATE TABLE SailorsNew
(sid INTEGER,
sname CHAR(30) NOT NULL,
rating INTEGER,
DOB DATE,
CONSTRAINT StudentsKey PRIMARY KEY (sid));
SELECT sname, DOB, CURDATE( ),
TIMESTAMPDIFF(YEAR, DOB,CURDATE( )) AS age
FROM SailorsNew;
SELECT sname, DOB, CURDATE( ),
TIMESTAMPDIFF(YEAR, DOB,CURDATE( )) AS age
FROM SailorsNew
Order By sname;
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Query Results
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Extract Information from Date
• Month(), Year(), DayOfMonth()
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Null Values

Field values in a tuple are sometimes
Unknown : a rating has not been assigned or
 Inapplicable : no spouse’s name for un-married
person.
 SQL provides a special value null for such
situations.


The presence of null complicates many
issues.
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Comparisons Using Null Values
We need a 3-valued logic : true, false and
unknown.
 Is rating>8 true or false when rating is equal
to null?



The answer is unknown.
Special operators to check if value is/is not
null.
IS NULL returns true is the value is null.
 IS NOT NULL returns false is the valule is null.

atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Logical Operator AND with One
Argument as Null Value
p
q
p AND q
True
True
True
True
False
False
True
Unknown (null)
Unknown (null)
False
False
False
False
Unknown (null)
False
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Logical Operator OR with One
Argument as Null Value
p
q
p OR q
True
True
True
True
False
True
True
Unknown (null)
True
False
False
False
False
Unknown (null)
Unknown
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Logical Operator NOT with a Null
Value
p
NOT p
True
False
False
True
Unknown (null)
Unknown (null)
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Null Value’s Impact on SQL
Constructs





WHERE clause eliminates rows that don’t evaluate to
true (false and null is eliminated).
When corresponding columns are either equal, or both
contain null, two rows are regarded as duplicates.
Arithmetic operations +, -, *, and / all return null if one
of their arguments is null.
COUNT (*) handles null values as other values.
Other aggregate operations ( SUM, AVG, MIN, MAX,
and variations using DISTINCT) simply discard null
values.

If it applies to only null values, the result is null.
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Disallowing Null Values
We can disallow null values by specifying NOT
NULL as part of field definition.
 The fields in a primary key are not allowed as
null.

CREATE TABLE Students
(sid CHAR(20),
name CHAR(30) NOT NULL,
login CHAR(20),
DOB Date,
gpa REAL)
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
More About Create Table
• Insert values with local data file
CREATE TABLE Sailors
(sid INTEGER,
sname CHAR(30) NOT NULL,
rating INTEGER,
age REAL,
CONSTRAINT StudentsKey PRIMARY KEY (sid));
Load Data Local Infile '~/Sailors.txt' Into Table Sailors;
1
2
3
Tom Hanks 11 25
Tom Cruise \N 26
Hello K 10 30
NULL
Tab Key \t
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Sailors.txt
Clone or Copy a Table
•
Clone a table
CREATE TABLE SailorsAnother LIKE Sailors;
•
Copy a table
CREATE TABLE SailorsThird
Select * FROM Sailors;
•
Copy part of a table
CREATE TABLE SailorsFourth
Select sid, sname FROM Sailors;
•
Rename a table
ALTER TABLE Sailors RENAME SailorsFifth;
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Integrity Constraints (Review)

An IC describes conditions that every legal
instance of a relation must satisfy.



Inserts/deletes/updates that violate IC’s are
disallowed.
Can be used to ensure application semantics (e.g.,
sid is a key), or prevent inconsistencies (e.g., sname
has to be a string, age must be < 100)
Types of IC’s: Domain constraints, primary key
constraints, foreign key constraints, general
constraints.
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Constraints over a Single Table

Table Constraints

CHECK conditional-expression
Mysql does
not support !
CREATE TABLE Sailors
(sid INTEGER,
sname CHAR(30) NOT NULL,
rating INTEGER,
age REAL,
CONSTRAINT StudentsKey PRIMARY KEY (sid),
CHECK (rating >=1 AND rating<=10) )
Rating must be an integer in the range of 1-10.
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Constraints over a Single Table
CREATE TABLE Reserves
(sid INTEGER,
bid INTEGER,
day DATE,
FOREIGN KEY (sid) REFERENCES Sailors,
FOREIGN KEY (bid) REFERENCES Boats,
CONSTRAINT noInterlakeRes
CHECK ( ‘Interlake’ <>
(SELECT B.bname
FROM Boats B
WHERE B.bid = Reserves.bid)
) )
Interlake boats cannot be reserved.
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
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)
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Advantages of using SQL triggers
• Check the integrity of data.
• Catch errors in business logic in the database
layer.
• Run the scheduled tasks because the triggers
are invoked automatically before or after a
change is made to the data in the tables.
• Audit the changes of data in tables.
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Disadvantages of using SQL triggers
It only can provide an extended validation, not all
validations.
• Some simple validations have to be done in the
application layer - JavaScript (client side) or JSP,
PHP, ASP.NET, Perl, etc. (server side)
• SQL triggers are invoked and executed invisibly
from client-applications therefore it is difficult to
figure out what happen in the database layer.
• SQL triggers may increase the overhead of the
database server.
•
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Trigger Syntax
CREATE TRIGGER trigger_name
trigger_time trigger_event ON tbl_name
FOR EACH ROW
trigger_body
trigger_time: { BEFORE | AFTER }
trigger_event: { INSERT | UPDATE | DELETE }
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Association
• Even data in tables are deleted or updated, if your
statement does not use INSERT, DELETE or
UPDATE statement to change data in a table, the
triggers associated with the table are not invoked.
• TRUNCATE TABLE statement removes all data of a table but
does not invoke the trigger associated with that table.
• Statements using the INSERT statement behind the
scenes such as REPLACE statement or LOAD
DATA statement will invoke the corresponding triggers
associated with the table.
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Trigger Example
Mysql> Delimiter /
Mysql> CREATE TABLE SailorThird LIKE Sailors/
Mysql> CREATE TRIGGER SailorUpdate
BEFORE INSERT ON SailorThird
FOR EACH ROW
BEGIN
IF NEW.rating IS NULL THEN SET NEW.rating = 0;
ELSEIF NEW.rating <1 THEN SET NEW.rating = 1;
ELSEIF NEW.rating > 10 THEN SET NEW.rating = 10;
END IF;
END;/
Mysql> Load Data Local Infile ‘~/SailorsNew.txt’ Into Table SailorsThird/
Mysql> Delimiter ;
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
To Stop an Operation with Trigger
CREATE TABLE Reserves
Create Trigger ReservesUpdate
(sid INTEGER,
Before Insert On Reserves
bid INTEGER,
For Each Row
day DATE,
FOREIGN KEY (sid) REFERENCES
Sailors,
Begin
If ‘Interlack’ = (Select B.bname
From Boats B
FOREIGN KEY (bid) REFERENCES
Boats,
Where B.bid = New.bid)
CONSTRAINT noInterlakeRes
Then
CHECK ( ‘Interlake’ <>
signal sqlstate ‘45000’;
(SELECT B.bname
FROM Boats B
WHERE B.bid = Reserves.bid)
) )
End If;
End;
;
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
Organize Triggers
• In database Information_Schema, Table
Triggers hold all information about triggers.
SELECT * FROM information_schema.triggers
WHERE trigger_schema = ‘DatabaseName’ AND
trigger_name = ‘TriggerName’;
SELECT * FROM information_schema.triggers
WHERE trigger_schema = 'database_name'
AND event_object_table = 'table_name';
SHOW TRIGGERS FROM classicmodels
WHERE `table` = 'employees';
atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke