C20.0046: Database Management Systems Lecture #14

M.P. Johnson, DBMS, Stern/NYU, Spring 2005

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C20.0046: Database Management SystemsLecture #14

M.P. Johnson

Stern School of Business, NYU

Spring, 2005


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Summary: SQL queries Only SELECT, FROM required Can’t have HAVING without GROUP BY Can have GROUP BY without HAVING Any clauses used must appear in this order:

SELECT LFROM RsWHERE sGROUP BY L2HAVING s2ORDER BY L3

SELECT LFROM RsWHERE sGROUP BY L2HAVING s2ORDER BY L3


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New topic: Modifications Three kinds of modifications

1. Insertions

2. Deletions

3. Updates

Sometimes “update” used as a synonym for “modification”


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Insertions

General form:

Missing attribute NULL (or other default value)

Example: Insert a new purchase to the database:

INSERT INTO R(A1,…., An) VALUES(v1,….,vn)INSERT INTO R(A1,…., An) VALUES(v1,….,vn)

INSERT INTO Knights(name, britnatl, title) VALUES('Bill Gates', 'n', 'KBE')

INSERT INTO Knights(name, britnatl, title) VALUES('Bill Gates', 'n', 'KBE')


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Insertions If we’re sure we have all values in the right order,

can just say:

Only do this if you’re sure of order in which the table fields were defined

INSERT INTO R VALUES(v1,….,vn)INSERT INTO R VALUES(v1,….,vn)

INSERT INTO KnightsVALUES('R. Giuliani', 'n', 'KBE');INSERT INTO KnightsVALUES('Bernard Kerik', 'n', 'CBE');



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Insertions Can insert the result of a query; Scenario:

Product(name, etc.) Purchase(buyerssn, prodName, etc.) Maybe some purchases name missing products add those to the Product table Subquery replaces VALUES

INSERT INTO R(As) (query)INSERT INTO R(As) (query)


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Insertion example Product(name, listPrice, category) Purchase(prodName, buyerName, price) Premise: data corruption lose some Product data

every product referred to in Purchase should exist in Product, but some are missing

name listPrice category

Canon D10 1000 Camera

Canon D20 2000 Camera

prodName buyerName

Canon D10 Bill

Canon D10 Hilary

Canon D20 George

Product Purchase


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Insertion example


Canon D10 NULL NULL

Canon D20 NULL NULL


Product Product’

prodName buyerName

Canon D10 Bill

Canon D20 Hilary

Canon D20 George

Purchase

Canon D20 NULL NULLQ: Or do we get:

A: Depends on implementation!

INSERT INTO Product(name) SELECT prodName FROM Purchase WHERE prodName NOT IN (SELECT name

FROM Product)

INSERT INTO Product(name) SELECT prodName FROM Purchase WHERE prodName NOT IN (SELECT name

FROM Product)


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Deletions General form:

E.g.:

As usual, WHERE can contain subqueries Depending on the DBMS

Q: How do you delete just one row with SQL simpliciter? Oracle has the ROWID/ROWNUM pseudo-field…

DELETE FROM TableWHERE condition

DELETE FROM TableWHERE condition


DELETE FROM KnightsWHERE name = 'Bernard Kerik';


DELETE FROM KnightsWHERE name = 'Bernard Kerik';


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Updates General form:

Example:

As usual, WHERE can contain subqueries

UPDATE ProductSET field1 = value1,

field2 = value2WHERE condition

UPDATE ProductSET field1 = value1,

field2 = value2WHERE condition

UPDATE ProductSET price = price/2WHERE Product.name IN (SELECT product FROM Purchase WHERE Date = DATE'Oct, 25, 1999')

UPDATE ProductSET price = price/2WHERE Product.name IN (SELECT product FROM Purchase WHERE Date = DATE'Oct, 25, 1999')


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New topic: Defining schemata So far, have done queries and data manipulation Now doing data definition Recall data types:

INT or INTEGER (variant: SHORTINT) FLOAT or REAL: floating-point numbers

DOUBLE PRECISION: DECIMAL(n,d):

E.g. decimal(5,2): five decimal digits, with the decimal point two positions from the right: e.g. 123.45

DATE and TIME Character strings

Fixed length: CHAR(n) Variable length: VARCHAR(n)


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Creating tables Form:

E.g.:

CREATE TABLE Table-name ( field1 field-type, field2 field-type, … fieldn field-type)

CREATE TABLE Table-name ( field1 field-type, field2 field-type, … fieldn field-type)

No comma!

CREATE TABLE People ( name VARCHAR(30), ssn CHAR(9), age INT, city VARCHAR(30), gender CHAR(1), dob DATE)

CREATE TABLE People ( name VARCHAR(30), ssn CHAR(9), age INT, city VARCHAR(30), gender CHAR(1), dob DATE)


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Default Values Specify defaults when creating table:

The default default: NULL

CREATE TABLE People ( name VARCHAR(30), ssn CHAR(9), age SHORTINT DEFAULT 100, city VARCHAR(30) DEFAULT 'New York', gender BIT(1), dob DATE DEFAULT DATE '1900-01-01')

CREATE TABLE People ( name VARCHAR(30), ssn CHAR(9), age SHORTINT DEFAULT 100, city VARCHAR(30) DEFAULT 'New York', gender BIT(1), dob DATE DEFAULT DATE '1900-01-01')


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Deleting and modifying schemata Delete data, indices, schema:

Delete data and indices:

Either way, exercise extreme caution! Add or delete attributes:

Q: What’s put in the new fields?

DROP TABLE PersonDROP TABLE Person

TRUNCATE TABLE PersonTRUNCATE TABLE Person

ALTER TABLE Person ADD phone CHAR(12)

ALTER TABLE Person ADD phone CHAR(12)

ALTER TABLE Person DROP age

ALTER TABLE Person DROP age


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New topic: Indices Very important speeding up query processing Index on field(s) = data structure that makes

searches/comparisons on those fields fast

Suppose we have a relation Person (name, age, city)

Sequential scan of the whole Person file may take a very long time

SELECT *FROM PersonWHERE name = 'Waksal, Sam'

SELECT *FROM PersonWHERE name = 'Waksal, Sam'


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Creating Indices Syntax:

Here:

No searching by name is much faster How much faster? Log-time, say Base-what? Doesn’t matter, but say 2

If all New Yorkers, #comparisons: 8000000 log2(8000000) ~= 23 (i.e., 223 ~= 8000000)

CREATE INDEX index-name ON R(fields)CREATE INDEX index-name ON R(fields)

CREATE INDEX nameIndex ON Person(name)CREATE INDEX nameIndex ON Person(name)


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How do indices work? What the data structure?

Different possibilities 1st intuition: index on field f is an ordered list of all

values in the table’s f field each item has address (“rowid”) of its row

Where do we get the ordered list? 2nd intuition: put all f values in a BST

searching BST take log time (why?) DBMSs actually use a variant: B+Tree

See Ullman’s book or data structures texts…


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Creating Indices Indexes can be useful in range queries too:

CREATE INDEX ageIndex ON Person (age)CREATE INDEX ageIndex ON Person (age)

SELECT * FROM Person WHERE age > 25

SELECT * FROM Person WHERE age > 25


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Using indices Indices can be created on multiple attributes:

Helps in:

And in:

But not in:

Idea: our sorted list is sorted on age;city, not city;age

Q: In Movie, should index be on year;title or title;year?

CREATE INDEX doubleNdx ON Person (lname, fname)CREATE INDEX doubleNdx ON Person (lname, fname)

SELECT * FROM Person WHERE fname='Sam' AND lname = 'Waksal'

SELECT * FROM Person WHERE fname='Sam' AND lname = 'Waksal'

SELECT * FROM Person WHERE lname='Waksal'

SELECT * FROM Person WHERE lname='Waksal'

SELECT * FROM Person WHERE fname='Sam'

SELECT * FROM Person WHERE fname='Sam'


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The Index Selection Problem Big Q: Why not just index all (sequences of) fields?

how does the list/B+Tree stay up to date?

We are given a workload: a set of SQL queries and their frequencies

Q is: What indices should we build to speed up the workload?

Answer: Attributes in WHERE clauses (queries) favor an index Attributes in INSERT/UPDATE/DELETEs discourage an

index In many DBMSs: your primary key fields get indexed

automatically (why?)


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New topic: Views Stored relations physically exist and persist Views are relations that don’t

in some texts, “table” = stored relation = “base table” Basically names/references given to queries

maybe a relevant subset of a table Employee(ssn, name, department, project, salary)

Payroll has access to Employee, others only to Developers

CREATE VIEW Developers AS SELECT name, project FROM Employee WHERE department = 'Dev'

CREATE VIEW Developers AS SELECT name, project FROM Employee WHERE department = 'Dev'


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A Different View Person(name, city) Purchase(buyer, seller, product, store) Product(name, maker, category)

We have a new virtual table:NYCview(buyer, seller, product, store)

CREATE VIEW NYCview AS SELECT buyer, seller, product, store FROM Person, Purchase WHERE Person.city = 'New York' AND Person.name = Purchase.buyer



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A Different View

Now we can query the view:



SELECT name, NYCview.storeFROM NYCview, ProductWHERE NYCview.product = Product.name AND Product.category = 'Camera'



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What happens when we query a view?



SELECT name, Purchase.storeFROM Person, Purchase, ProductWHERE Person.city = 'New York' AND Person.name = Purchase.buyer AND Purchase.poduct = Product.name AND Product.category = 'Camera'

SELECT name, Purchase.storeFROM Person, Purchase, ProductWHERE Person.city = 'New York' AND Person.name = Purchase.buyer AND Purchase.poduct = Product.name AND Product.category = 'Camera'


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Can rename view fields

CREATE VIEW NYCview(NYCbuyer, NYCseller, prod, store) AS

SELECT buyer, seller, product, store FROM Person, Purchase WHERE Person.city = 'New York' AND Person.name = Purchase.buyer

CREATE VIEW NYCview(NYCbuyer, NYCseller, prod, store) AS

SELECT buyer, seller, product, store FROM Person, Purchase WHERE Person.city = 'New York' AND Person.name = Purchase.buyer


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Types of Views Views discussed here:

Used in databases Computed only on-demand – slow at runtime Always up to date

Sometimes talk about “materialized” views Used in data warehouses Pre-computed offline – fast at runtime May have stale data Maybe more later…


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Updating ViewsHow to insert a tuple into a table that doesn’t exist?Employee(ssn, name, department, project, salary)

If we make thefollowing insertion:

It becomes:

CREATE VIEW Developers AS SELECT name, project FROM Employee WHERE department = 'Development'

CREATE VIEW Developers AS SELECT name, project FROM Employee WHERE department = 'Development'

INSERT INTO Developers VALUES('Bill', ‘Word')

INSERT INTO Developers VALUES('Bill', ‘Word')

INSERT INTO Employee(ssn, name, dept, project, sal)VALUES(NULL, 'Bill', NULL, 'Word', NULL)

INSERT INTO Employee(ssn, name, dept, project, sal)VALUES(NULL, 'Bill', NULL, 'Word', NULL)


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Non-Updatable Views Person(name, city) Purchase(buyer, seller, product, store)

How can we add the following tuple to the view?('NYC', 'The Wiz')

We don’t know the name of the person who made the purchase

cannot set to NULL (why?)

CREATE VIEW CityStore AS SELECT Person.city, Purchase.store FROM Person, Purchase WHERE Person.name = Purchase.buyer

CREATE VIEW CityStore AS SELECT Person.city, Purchase.store FROM Person, Purchase WHERE Person.name = Purchase.buyer


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Constraints in SQL A constraint = a property that we’d like our

database to hold

The system will enforce the constraint by taking some actions: forbid an update or perform compensating updates


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Constraints in SQLConstraints in SQL: Keys, foreign keys Attribute-level constraints Tuple-level constraints Global constraints: assertions

The more complex the constraint, the harder it is to check and to enforce

simplest

Mostcomplex


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Keys

Or:

CREATE TABLE Product ( name CHAR(30) PRIMARY KEY, category VARCHAR(20))

CREATE TABLE Product ( name CHAR(30) PRIMARY KEY, category VARCHAR(20))

CREATE TABLE Product ( name CHAR(30), category VARCHAR(20) PRIMARY KEY (name))

CREATE TABLE Product ( name CHAR(30), category VARCHAR(20) PRIMARY KEY (name))


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Keys with Multiple Attributes

Name Category Price

Canon D10 Camera 1200


iPod G4 MP3 Player 250


CREATE TABLE Product ( name CHAR(30), category VARCHAR(20), price INT, PRIMARY KEY (name, category))

CREATE TABLE Product ( name CHAR(30), category VARCHAR(20), price INT, PRIMARY KEY (name, category))


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Other Keys There is at most one PRIMARY KEY; there can be

many UNIQUE Primary key v. candidate keys

CREATE TABLE Product ( productID CHAR(10), name CHAR(30), category VARCHAR(20), price INT, PRIMARY KEY (productID), UNIQUE (name, category))

CREATE TABLE Product ( productID CHAR(10), name CHAR(30), category VARCHAR(20), price INT, PRIMARY KEY (productID), UNIQUE (name, category))


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prodName is a foreign key to Product(name) name should be a key in Product Purchase ~ Product is many-one

NB: referenced field specified with parentheses, not dot

Foreign Key Constraints

CREATE TABLE Purchase ( prodName CHAR(30) REFERENCES Product(name), date DATETIME)

CREATE TABLE Purchase ( prodName CHAR(30) REFERENCES Product(name), date DATETIME)

Referentialintegrityin SQL


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Name Category

Canon D10 Camera

Canon D20 Camera

iPod 4G MP3 Player

ProdName Store

Canon D10 Wiz

Canon D10 Wiz

Canon D20 Best Buy

Product Purchase


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Foreign Key Constraints Or:

(name, category) must be a key (primary/unique) in Product (why?)

CREATE TABLE Purchase ( prodName CHAR(30), category VARCHAR(20), date DATETIME, FOREIGN KEY (prodName, category) REFERENCES Product(name, category)

CREATE TABLE Purchase ( prodName CHAR(30), category VARCHAR(20), date DATETIME, FOREIGN KEY (prodName, category) REFERENCES Product(name, category)


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Name Category

Canon D10 Camera

Canon D20 Camera

iPod 4G MP3 Player

ProdName Store

Canon D10 Wiz

Canon D10 Wiz

Canon D20 Best Buy

Product Purchase

What happens during updates?Types of updates: In Purchase: insert/update In Product: delete/update


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What happens during updates? SQL has three policies for maintaining

referential integrity:

Reject: violating modifications (default) Cascade: after a delete/update do a

delete/update Set-null: set foreign-key field to NULL


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Constraints on Attributes and Tuples Constraints on attributes:

NOT NULL -- obvious meaning... CHECK condition -- any condition on row itself

Some DBMS support subqueries here, but many don’t

Constraints on tuples CHECK condition


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CREATE TABLE Purchase ( prodName CHAR(30) CHECK (prodName IN SELECT Product.name FROM Product), date DATETIME NOT NULL)

CREATE TABLE Purchase ( prodName CHAR(30) CHECK (prodName IN SELECT Product.name FROM Product), date DATETIME NOT NULL)

How is thisdifferent from aForeign-Key?


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General Assertions Supported in SQL standard:

Implemented/approximated in MySQL and Oracle as stored procedures PL/SQL in Oracle

CREATE ASSERTION myAssert CHECK (NOT EXISTS( SELECT Product.name FROM Product, Purchase WHERE Product.name = Purchase.prodName GROUP BY Product.name HAVING count(*) > 200))

CREATE ASSERTION myAssert CHECK (NOT EXISTS( SELECT Product.name FROM Product, Purchase WHERE Product.name = Purchase.prodName GROUP BY Product.name HAVING count(*) > 200))


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Final Comments on Constraints Can give them names, and alter later

We need to understand exactly when they are checked

We need to understand exactly what actions are taken if they fail


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Example with nulls look at emp table

get names, salaries, commissions, total salaries

What if commission is null? nvl in Oracle, ifnull in MySQL


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Live examples Inner joins require an ON clause

Like a where clause Arbitrary boolean expression If always true (1=1), reduces to cross join

New compar op: BETWEEN a between 5 and 10 a >= 5 and a <= 10

Q: produce a list of employees with their salary grades emp, salgrade


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Live examples Q: produce a list of employees and their

bosses What if no boss? Or no subordinate?

Joins on emp, emp man: Comma-based Cross Natural Inner

Must use INNER JOIN in MySQL If want non-managers, do outer join…

No FULL OUTER JOIN in MySQL yet


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Live examples Q: produce list of bosses and underling-

counts, for bosses with >1 underling

Just add HAVING clause…


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Finally: R.A./SQL has limitations Can easily find Alice’s direct subordinates:

But: find all of King’s underlings Cannot compute “transitive closure” Cannot express in R.A./SQL! SQL is not “Turing-Complete”

Name Job Boss

King President NULL

Jones Manager King

Blake Manager King

Ford Analyst Jones

Scott Analyst Jones


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Live examples Examples from sqlzoo.net

L(C(R1 x … Rn)L(C(R1 x … Rn)

SELECT LFROM R1, …, Rn

WHERE C

SELECT LFROM R1, …, Rn

WHERE C

Documents

C20.0046: Database Management Systems Lecture #14