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C20.0046: Database Management Systems Lecture #9. M.P. Johnson Stern School of Business, NYU Spring, 2008. Agenda. Subqueries, etc. Sets, etc. From last time: escaping single quotes. http://reddit.com/info/69jzv/comments/. Subqueries. Subquery: copy in Conrad’s selection for his ssn: - PowerPoint PPT Presentation
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M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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C20.0046: Database Management SystemsLecture #9
M.P. JohnsonStern School of Business, NYUSpring, 2008
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
2
Agenda Subqueries, etc.
Sets, etc.
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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From last time: escaping single quotes http://reddit.com/info/69jzv/comments/
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Subqueries Subquery: copy in Conrad’s selection for his ssn:
The subquery returns one value, so the = is valid If it returns more (or fewer), we get a run-time error
SELECT Purchase.prodnameFROM PurchaseWHERE buyerssn = (SELECT ssn FROM Person WHERE name = 'Christo')
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Operators on subqueries Several new operators applied to (unary)
selections:1. IN R2. EXISTS R3. UNIQUE R4. s > ALL R5. s > ANY R6. x IN R
> is just an example op Each expression can be negated with NOT
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Next: ALL opEmployees(name, job, divid, salary)Find which employees are paid more than all the programmers
SELECT nameFROM EmployeesWHERE salary > ALL (SELECT salary FROM Employees WHERE job='programmer')
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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ANY/SOME opEmployees(name, job, divid, salary)Find which employees are paid more than at least one vice president
SELECT nameFROM EmployeesWHERE salary > ANY (SELECT salary FROM Employees WHERE job='VP')
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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ANY/SOME opEmployees(name, job, divid, salary)Find which employees are paid more than at least one vice president
SELECT nameFROM EmployeesWHERE salary > SOME (SELECT salary FROM Employees WHERE job='VP')
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Existential/Universal ConditionsEmployees(name, job, divid, salary)
Division(name, id, head)
Find all divisions with an employee whose salary is > 100000
Existential: easy!
SELECT DISTINCT Division.nameFROM Employees, DivisionWHERE salary > 100000 AND divid=id
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Existential/Universal ConditionsEmployees(name, job, divid, salary)
Division(name, id, head)
Find all divisions in which everyone makes > 100000
Existential: easy!
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Existential/universal with IN
2. Select the divisions we didn’t find:
1. Find the other divisions: in which someone makes <= 100000:
SELECT nameFROM DivisionWHERE id IN (SELECT divid FROM Employees WHERE salary <= 100000
SELECT nameFROM DivisionWHERE id NOT IN (SELECT divid FROM Employees WHERE salary <= 100000
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Next: correlated subqueries Acc(name,bal,type…) Q: Who has the largest balance?
Can we do this with subqueries?
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Acc(name,bal,type,…) Q: Find holder of largest account
SELECT nameFROM AccWHERE bal >= ALL (SELECT bal FROM Acc)
Correlated Queries
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Correlated Queries So far, subquery executed once;
result used for higher query More complicated: correlated queries
“[T]he subquery… [is] evaluated many times, once for each assignment of a value to some term in the subquery that comes from a tuple variable outside the subquery” (Ullman, p286).
Q: What does this mean? A: That subqueries refer to vars from outer queries
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Acc(name,bal,type,…) Q2: Find holder of largest account of each type
SELECT name, typeFROM AccWHERE bal >= ALL (SELECT bal FROM Acc WHERE type=type)
Correlated Queries
correlation
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Acc(name,bal,type,…) Q2: Find holder of largest account of each type
Note:1. scope of variables2. this can still be expressed as single SFW
SELECT name, typeFROM Acc a1WHERE bal >= ALL (SELECT bal FROM Acc WHERE type=a1.type)
Correlated Queries
correlation
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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New topic: R.A./SQL Set Operators Relations are sets have set-theoretic ops
Venn diagrams
Union: R1 R2 Example:
ActiveEmployees RetiredEmployees
Difference: R1 – R2 Example:
AllEmployees – RetiredEmployees = ActiveEmployees
Intersection: R1 R2 Example:
RetiredEmployees UnionizedEmployees
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Set operations - exampleName Address Gender BirthdateFisher 123 Maple F 9/9/99Hamill 456 Oak M 8/8/88
Name Address Gender BirthdateFisher 123 Maple F 9/9/99Ford 345 Palm M 7/7/77
R:
S:
Name Address Gender BirthdateFisher 123 Maple F 9/9/99Hamill 456 Oak M 8/8/88Ford 345 Palm M 7/7/77
R S:
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Set operations - exampleName Address Gender BirthdateFisher 123 Maple F 9/9/99Hamill 456 Oak M 8/8/88
Name Address Gender BirthdateFisher 123 Maple F 9/9/99Ford 345 Palm M 7/7/77
R:
S:
R S: Name Address Gender BirthdateFisher 123 Maple F 9/9/99
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Set operations - exampleName Address Gender BirthdateFisher 123 Maple F 9/9/99Hamill 456 Oak M 8/8/88
Name Address Gender BirthdateFisher 123 Maple F 9/9/99Ford 345 Palm M 7/7/77
R:
S:
R - S: Name Address Gender BirthdateHamill 456 Oak M 8/8/88
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Set ops in SQL Orthodox SQL has set operators:
UNION, INTERSECT, EXCEPT Oracle SQL uses MINUS rather than EXCEPT See the Ullman page on more differences
These ops applied to queries:
(SELECT name FROM Person WHERE City = 'New York')
INTERSECT(SELECT custname FROM Purchase WHERE store='Kim''s')
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Boat examples Reserve(ssn,bmodel,color)
Q: Find ssns of sailors who reserved red boats or green boats
SELECT DISTINCT ssnFROM reserveWHERE color = 'red' OR color = 'green'
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Boat examples Reserve(ssn,bmodel,color)
Q: Find ssns of sailors who reserved red boats and green boats
SELECT DISTINCT ssnFROM reserveWHERE color = 'red' AND color = 'green'
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Boat examples Reserve(ssn,bmodel,color)
Q: Find ssns of sailors who reserved red boats and green boats
SELECT DISTINCT r1.ssnFROM reserve r1, reserve r2WHERE r1.ssn = r2.ssn AND r1.color = 'red' AND r2.color = 'green'
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Boat examples Reserve(ssn,bmodel,color)
Q: Find ssns of sailors who reserved red boats and green boats
(SELECT DISTINCT ssn FROM reserve WHERE color = 'red') INTERSECT(SELECT DISTINCT ssn
FROM reserve WHERE color = 'green')
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Boat examples Reserve(ssn,bmodel,color)
Q: Find ssns of sailors who reserved red boats or green boats
(SELECT DISTINCT ssn FROM reserve WHERE color = 'red') UNION (SELECT DISTINCT ssn
FROM reserve WHERE color = 'green')
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Boat examples Reserve(ssn,bmodel,color)
Q: Find ssns of sailors who reserved red boats but not green boats
(SELECT DISTINCT ssn FROM reserve WHERE color = 'red') EXCEPT (SELECT DISTINCT ssn
FROM reserve WHERE color = 'green')
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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(SELECT name, address FROM Cust1)
UNION(SELECT name FROM Cust2)
Union-Compatibility Situation: Cust1(name,address,…), Cust2(name,…) Want: report of all customer names and addresses
(if known) Can’t do:
Both tables must have same sequence of types Applies to all set ops
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Union-Compatibility Situation: Cust1(name,address,…), Cust2(name,…) Want: report of all customer names and addresses
(if known) But can do:
Resulting field names taken from first table
(SELECT name, address FROM Cust1)
UNION(SELECT name, '(N/A)' FROM Cust2)
Result(name, address)
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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First Unintuitive SQLism Looking for R (S T)
But what happens if T is empty?
See transcript of this in Oracle on sales
SELECT R.AFROM R, S, TWHERE R.A=S.A OR R.A=T.A
M.P. Johnson, DBMS, Stern/NYU, Spring 2008
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Review Examples from sqlzoo.net
SELECT LFROM R1, …, Rn
WHERE C
L(C(R1 x … Rn)