1 Lecture 04: SQL Wednesday, January 11, 2006. 2 Outline Two Examples Nulls (6.1.6) Outer joins...
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Transcript of 1 Lecture 04: SQL Wednesday, January 11, 2006. 2 Outline Two Examples Nulls (6.1.6) Outer joins...
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Lecture 04: SQL
Wednesday, January 11, 2006
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Outline
• Two Examples
• Nulls (6.1.6)
• Outer joins (6.3.8)
• Database Modifications (6.5)
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Two Examples
Store(sid, sname)Product(pid, pname, price, sid)
Find all stores that sell only products with price > 100
same as:
Find all stores s.t. all their products have price > 100)
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SELECT Store.nameFROM Store, ProductWHERE Store.sid = Product.sidGROUP BY Store.sid, Store.nameHAVING 100 < min(Product.price)
SELECT Store.nameFROM Store, ProductWHERE Store.sid = Product.sidGROUP BY Store.sid, Store.nameHAVING 100 < min(Product.price)
SELECT Store.nameFROM StoreWHERE Store.sid NOT IN (SELECT Product.sid FROM Product WHERE Product.price <= 100)
SELECT Store.nameFROM StoreWHERE Store.sid NOT IN (SELECT Product.sid FROM Product WHERE Product.price <= 100)
SELECT Store.nameFROM StoreWHERE 100 < ALL (SELECT Product.price FROM product WHERE Store.sid = Product.sid)
SELECT Store.nameFROM StoreWHERE 100 < ALL (SELECT Product.price FROM product WHERE Store.sid = Product.sid)
Almost equivalent…
Why both ?
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Two Examples
Store(sid, sname)Product(pid, pname, price, sid)
For each store, find its most expensive product
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Two Examples
SELECT Store.sname, max(Product.price)FROM Store, ProductWHERE Store.sid = Product.sidGROUP BY Store.sid, Store.sname
SELECT Store.sname, max(Product.price)FROM Store, ProductWHERE Store.sid = Product.sidGROUP BY Store.sid, Store.sname
SELECT Store.sname, x.pnameFROM Store, Product xWHERE Store.sid = x.sid and x.price >= ALL (SELECT y.price FROM Product y WHERE Store.sid = y.sid)
SELECT Store.sname, x.pnameFROM Store, Product xWHERE Store.sid = x.sid and x.price >= ALL (SELECT y.price FROM Product y WHERE Store.sid = y.sid)
This is easy but doesn’t do what we want:
Better:
But mayreturnmultiple product namesper store
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Two Examples
SELECT Store.sname, max(x.pname)FROM Store, Product xWHERE Store.sid = x.sid and x.price >= ALL (SELECT y.price FROM Product y WHERE Store.sid = y.sid)GROUP BY Store.sname
SELECT Store.sname, max(x.pname)FROM Store, Product xWHERE Store.sid = x.sid and x.price >= ALL (SELECT y.price FROM Product y WHERE Store.sid = y.sid)GROUP BY Store.sname
Finally, choose some pid arbitrarily, if there are manywith highest price:
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NULLS in SQL
• Whenever we don’t have a value, we can put a NULL
• Can mean many things:– Value does not exists
– Value exists but is unknown
– Value not applicable
– Etc.
• The schema specifies for each attribute if can be null (nullable attribute) or not
• How does SQL cope with tables that have NULLs ?
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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 = 0
UNKNOWN = 0.5
TRUE = 1
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Null Values
• 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)
SELECT *FROM PersonWHERE (age < 25) AND (height > 6 OR weight > 190)
E.g.age=20heigth=NULLweight=200
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Null Values
Unexpected behavior:
Some Persons are not included !
SELECT *FROM PersonWHERE age < 25 OR age >= 25
SELECT *FROM PersonWHERE age < 25 OR age >= 25
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Null Values
Can 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
SELECT *FROM PersonWHERE age < 25 OR age >= 25 OR age IS NULL
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OuterjoinsExplicit joins in SQL = “inner joins”:
Product(name, category) Purchase(prodName, store)
SELECT Product.name, Purchase.storeFROM Product JOIN Purchase ON Product.name = Purchase.prodName
SELECT Product.name, Purchase.storeFROM Product JOIN Purchase ON Product.name = Purchase.prodName
SELECT Product.name, Purchase.storeFROM Product, PurchaseWHERE Product.name = Purchase.prodName
SELECT Product.name, Purchase.storeFROM Product, PurchaseWHERE Product.name = Purchase.prodName
Same as:
But Products that never sold will be lost !
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Outerjoins
Left 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
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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Application
Compute, for each product, the total number of sales in ‘September’Product(name, category)
Purchase(prodName, month, store)
SELECT Product.name, count(*) FROM Product, Purchase WHERE Product.name = Purchase.prodName and Purchase.month = ‘September’ GROUP BY Product.name
SELECT Product.name, count(*) FROM Product, Purchase WHERE Product.name = Purchase.prodName and Purchase.month = ‘September’ GROUP BY Product.name
What’s wrong ?
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Application
Compute, for each product, the total number of sales in ‘September’Product(name, category)
Purchase(prodName, month, store)
SELECT Product.name, count(*) FROM Product LEFT OUTER JOIN Purchase ON Product.name = Purchase.prodName and Purchase.month = ‘September’ GROUP BY Product.name
SELECT Product.name, count(*) FROM Product LEFT OUTER JOIN Purchase ON Product.name = Purchase.prodName and Purchase.month = ‘September’ GROUP BY Product.name
Now we also get the products who sold in 0 quantity
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Outer Joins
• Left outer join:– Include the left tuple even if there’s no match
• Right outer join:– Include the right tuple even if there’s no match
• Full outer join:– 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 attribute NULL.May drop attribute names if give them in order.
INSERT INTO R(A1,…., An) VALUES (v1,…., vn) 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’)
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”
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
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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)
Product(name, listPrice, category)Purchase(prodName, buyerName, price)
Purchase
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Insertion: an Example
INSERT INTO Product(name)
SELECT DISTINCT prodName FROM Purchase WHERE prodName NOT IN (SELECT name FROM Product)
INSERT 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)
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 ?? - Depends on the implementation
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Deletions
DELETE FROM PURCHASE
WHERE seller = ‘Joe’ AND product = ‘Brooklyn Bridge’
DELETE FROM PURCHASE
WHERE seller = ‘Joe’ AND product = ‘Brooklyn Bridge’
Factoid about SQL: there is no way to delete only a single
occurrence of a tuple that appears twice
in a relation.
Example:
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Updates
UPDATE PRODUCTSET price = price/2WHERE Product.name IN (SELECT product FROM Purchase WHERE Date =‘Oct, 25, 1999’);
UPDATE PRODUCTSET price = price/2WHERE Product.name IN (SELECT product FROM Purchase WHERE Date =‘Oct, 25, 1999’);
Example: