Lecture 4: Advanced SQL
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Transcript of 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