lecture 4: advanced sql

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Lecture 4: Advanced SQL

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INTERSECT and EXCEPT:(missing from MySQL)

(SELECT R.A, R.BFROM R) INTERSECT(SELECT S.A, S.BFROM S)

SELECT R.A, R.BFROM RWHERE EXISTS(SELECT * FROM S WHERE R.A=S.A and R.B=S.B)

(SELECT R.A, R.BFROM R) EXCEPT(SELECT S.A, S.BFROM S)

SELECT R.A, R.BFROM RWHERE NOT EXISTS(SELECT * FROM S WHERE R.A=S.A and R.B=S.B)

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Null Values

• NULL in SQL is used as a placeholder for a missing or unknown values

• By default, a table column can hold a NULL value

First Last Birth_Date Immigration_DateOla Hansen 1989/09/01 1992/08/31Han Shin-Cho 1987/07/17 NULLJoe NULL 1990/04/21 1999/11/23

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Use of Null Values

• If x=Null then 4*(3-x)/7 is still NULL

• If x=Null then x=“Joe” is UNKNOWN• In SQL there are three boolean values:

FALSE = 0UNKNOWN = 0.5TRUE = 1

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Three-Valued Logic• C1 AND C2 = min(C1, C2)• C1 OR C2 = max(C1, C2)• NOT C1 = 1 – C1

Rule in SQL: include only tuples that yield TRUE

SELECT *FROM PersonWHERE (age < 25) AND (height > 6 OR weight > 190)

E.g.age=20height=NULLweight=200

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Three-Valued LogicUnexpected behavior:

Some Persons are not included !

SELECT *FROM PersonWHERE age < 25 OR age >= 25

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Working with Null ValuesCan test for NULL explicitly:

– x IS NULL– x IS NOT NULL

Now it includes all Persons

SELECT *FROM PersonWHERE age < 25 OR age >= 25 OR age IS NULL

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Explicit JoinsExplicit joins in SQL:

Product(name, category) Purchase(prodName, store)

Same as:

But Products that never sold will be lost !

SELECT Product.name, Purchase.storeFROM Product JOIN Purchase ON Product.name = Purchase.prodName

SELECT Product.name, Purchase.storeFROM Product, PurchaseWHERE Product.name = Purchase.prodName

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Left Outer JoinsLeft outer joins in SQL:

Product(name, category) Purchase(prodName, store)

SELECT Product.name, Purchase.store FROM Product LEFT OUTER JOIN Purchase ON Product.name = Purchase.prodName

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Name Category

Gizmo gadget

Camera Photo

OneClick Photo

ProdName Store

Gizmo Wiz

Camera Ritz

Camera Wiz

Name Store

Gizmo Wiz

Camera Ritz

Camera Wiz

OneClick NULL

Product Purchase

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Types of Joins - Summary• Inner join: same as Join, implicit Join• Left outer join:

– Same as Left join– Include the left tuple even if there’s no match

• Right outer join:– Same as right join– Include the right tuple even if there’s no match

• Full outer join:– Missing from MySQL (but can be emulated! how?)– Include the both left and right tuples even if there’s no match

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Modifying the Database

Three kinds of modifications• Insertions• Deletions• Updates

Sometimes they are all called “updates”

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InsertionsGeneral form:

Missing attributes NULL.May drop attribute names if give them in order.

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

INSERT INTO Purchase(buyer, seller, product, store) VALUES (‘Joe’, ‘Fred’, ‘wakeup-clock-espresso-machine’, ‘The Sharper Image’)

Example: Insert a new purchase to the database:

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Insertions

INSERT INTO PRODUCT(name)

SELECT DISTINCT Purchase.product FROM Purchase WHERE Purchase.date > “10/26/01”

The query replaces the VALUES keyword.Here we insert many tuples into PRODUCT

The number of columns must be identical

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Insertion: an Example

prodName is foreign key in Product.name

Suppose database got corrupted and we need to fix it:

name listPrice category

gizmo 100 gadgets

prodName buyerName price

camera John 200

gizmo Smith 80

camera Smith 225

Task: insert in Product all prodNames from Purchase

Product

Product(name, listPrice, category)Purchase(prodName, buyerName, price)

Purchase

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Insertion: an ExampleINSERT INTO Product(name)

SELECT DISTINCT prodName FROM Purchase WHERE prodName NOT IN (SELECT name FROM Product)

name listPrice category

gizmo 100 Gadgets

camera - -

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Insertion: an Example

INSERT INTO Product(name, listPrice)

SELECT DISTINCT prodName, price FROM Purchase WHERE prodName NOT IN (SELECT name FROM Product)

name listPrice category

gizmo 100 Gadgets

camera 200 -

camera ?? 225 ?? - ERROR!

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Deletions

DELETE FROM PURCHASE

WHERE seller = ‘Joe’ AND product = ‘Brooklyn Bridge’

Factoid about SQL: there is no way to delete only xoccurrences of a tuple that appears x+k times in a relation.

Example:

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Updates

UPDATE ProductSET listPrice= listPrice/2WHERE Product.name IN (SELECT product FROM Purchase WHERE Date =‘Oct, 25, 1999’);

Example:

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Updates Using Exists

UPDATE ProductSET listPrice = (SELECT AVG(price)

FROM PurchaseWHERE prodName=name)

WHERE EXISTS (SELECT *FROM PurchaseWHERE prodName=name);

Example:

Table Modifications - Summary

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INSERT INTO table_name (column1, column2,...)VALUES (value1, value2, ...)

INSERT INTO table_name (column1, column2,...)SELECT …

DELETE FROM table_name WHERE cond

UPDATE table_nameSET column1=expr1, column2=expr2,...WHERE cond