1. January 17th – Subqueries
Cse 344
January 17th – Subqueries

2. Grouping and Aggregation
Purchase(product, price, quantity)
Find total quantities for all sales over $1, by product.
SELECT product, Sum(quantity) AS TotalSales
FROM Purchase
WHERE price > 1
GROUP BY product
How is this query processed?

3. Grouping and Aggregation
Purchase(product, price, quantity)
Find total quantities for all sales over $1, by product.
SELECT product, Sum(quantity) AS TotalSales
FROM Purchase
WHERE price > 1
GROUP BY product
Do these queries return the same number of rows? Why?
SELECT product, Sum(quantity) AS TotalSales
FROM Purchase
GROUP BY product

4. Grouping and Aggregation
Purchase(product, price, quantity)
Find total quantities for all sales over $1, by product.
SELECT product, Sum(quantity) AS TotalSales
FROM Purchase
WHERE price > 1
GROUP BY product
Do these queries return the same number of rows? Why?
SELECT product, Sum(quantity) AS TotalSales
FROM Purchase
GROUP BY product
Empty groups are removed, hence first query may return fewer groups

5. Grouping and Aggregation
1. Compute the FROM and WHERE clauses.
2. Group by the attributes in the GROUPBY
3. Compute the SELECT clause: grouped attributes and aggregates.
FWGS = From Where Group by Select (I made up that acronym) TM
FWGS
CSE au

6. 1,2: From, Where FWGS Product Price Quantity Bagel 3 20 1.50 Banana
0.5 50 2 10 4
WHERE price > 1
SELECT product, Sum(quantity) AS TotalSales
FROM Purchase
WHERE price > 1
GROUP BY product

7. 3,4. Grouping, Select FWGS Product Price Quantity Bagel 3 20 1.50
Banana
0.5 50 2 10 4
Product
TotalSales
Bagel 40
Banana 20
SELECT product, Sum(quantity) AS TotalSales
FROM Purchase
WHERE price > 1
GROUP BY product

8. Ordering Results FWGOS Purchase(pid, product, price, quantity, month)
SELECT product, sum(price*quantity) as rev
FROM Purchase
GROUP BY product
ORDER BY rev desc
Reminder students of the syntax of “as” if they have not seen already (just renaming)
Why need the alias for rev?
FWGOS TM
Note: some SQL engines want you to say ORDER BY sum(price*quantity) desc
CSE au

9. HAVING Clause Purchase(pid, product, price, quantity, month)
Same query as before, except that we consider only products
that had at least 30 sales.
SELECT product, sum(price*quantity)
FROM Purchase
WHERE price > 1
GROUP BY product
HAVING sum(quantity) > 30
HAVING clause contains conditions on aggregates.
CSE au

10. General form of Grouping and Aggregation
SELECT S
FROM R1,…,Rn
WHERE C1
GROUP BY a1,…,ak
HAVING C2
Why ?
S = may contain attributes a1,…,ak and/or any aggregates but NO OTHER ATTRIBUTES C1 = is any condition on the attributes in R1,…,Rn C2 = is any condition on aggregate expressions and on attributes a1,…,ak
CSE au

11. Semantics of SQL With Group-By
SELECT S
FROM R1,…,Rn
WHERE C1
GROUP BY a1,…,ak
HAVING C2
FWGHOS
Evaluation steps:
Evaluate FROM-WHERE using Nested Loop Semantics
Group by the attributes a1,…,ak
Apply condition C2 to each group (may have aggregates)
Compute aggregates in S and return the result
From Where Groupby Having Orderby Select
CSE au

12. Exercise Purchase(pid, product, price, quantity, month)
Compute the total income per month Show only months with less than 10 items sold Order by quantity sold and display as “TotalSold”
Here is a series of slides that discusses how to construct a query from scratch
CSE au

13. Exercise Purchase(pid, product, price, quantity, month)
Compute the total income per month Show only months with less than 10 items sold Order by quantity sold and display as “TotalSold”
FROM Purchase
CSE au

14. Exercise Purchase(pid, product, price, quantity, month)
Compute the total income per month Show only months with less than 10 items sold Order by quantity sold and display as “TotalSold”
FROM Purchase
GROUP BY month
CSE au

15. Exercise Purchase(pid, product, price, quantity, month)
Compute the total income per month Show only months with less than 10 items sold Order by quantity sold and display as “TotalSold”
FROM Purchase
GROUP BY month
HAVING sum(quantity) < 10
CSE au

16. Exercise Purchase(pid, product, price, quantity, month)
Compute the total income per month Show only months with less than 10 items sold Order by quantity sold and display as “TotalSold”
SELECT month, sum(price*quantity), sum(quantity) as TotalSold
FROM Purchase
GROUP BY month
HAVING sum(quantity) < 10
CSE au

17. Exercise Purchase(pid, product, price, quantity, month)
Compute the total income per month Show only months with less than 10 items sold Order by quantity sold and display as “TotalSold”
SELECT month, sum(price*quantity), sum(quantity) as TotalSold
FROM Purchase
GROUP BY month
HAVING sum(quantity) < 10
ORDER BY sum(quantity)
CSE au

18. WHERE vs HAVING WHERE condition is applied to individual rows
The rows may or may not contribute to the aggregate
No aggregates allowed here
Occasionally, some groups become empty and are removed
HAVING condition is applied to the entire group
Entire group is returned, or removed
May use aggregate functions on the group
CSE au

19. Mystery Query Purchase(pid, product, price, quantity, month)
What do they compute?
SELECT month, sum(quantity), max(price)
FROM Purchase
GROUP BY month
SELECT month, sum(quantity)
FROM Purchase
GROUP BY month
SELECT month
FROM Purchase
GROUP BY month

20. Mystery Query Purchase(pid, product, price, quantity, month)
What do they compute?
SELECT month, sum(quantity), max(price)
FROM Purchase
GROUP BY month
SELECT month, sum(quantity)
FROM Purchase
GROUP BY month
Lesson:
DISTINCT is a special case of GROUP BY
SELECT month
FROM Purchase
GROUP BY month

21. Aggregate + Join Product(pid,pname,manufacturer)
Purchase(id,product_id,price,month)
Aggregate + Join
For each manufacturer, compute how many products with price > $100 they sold
Group by x.manufacturer, y.month = create groups of the form (manufacturer, month)

22. Aggregate + Join Product(pid,pname,manufacturer)
Purchase(id,product_id,price,month)
Aggregate + Join
For each manufacturer, compute how many products with price > $100 they sold
Problem: manufacturer is in Purchase, price is in Product...
Group by x.manufacturer, y.month = create groups of the form (manufacturer, month)

23. Aggregate + Join Product(pid,pname,manufacturer)
Purchase(id,product_id,price,month)
Aggregate + Join
For each manufacturer, compute how many products with price > $100 they sold
Problem: manufacturer is in Purchase, price is in Product...
manu facturer
...
price
Hitachi
150
Canon
300
180
-- step 1: think about their join SELECT ...
FROM Product x, Purchase y
WHERE x.pid = y.product_id and y.price > 100
Group by x.manufacturer, y.month = create groups of the form (manufacturer, month)

24. Aggregate + Join Product(pid,pname,manufacturer)
Purchase(id,product_id,price,month)
Aggregate + Join
For each manufacturer, compute how many products with price > $100 they sold
Problem: manufacturer is in Purchase, price is in Product...
manu facturer
...
price
Hitachi
150
Canon
300
180
-- step 1: think about their join SELECT ...
FROM Product x, Purchase y
WHERE x.pid = y.product_id and y.price > 100
Group by x.manufacturer, y.month = create groups of the form (manufacturer, month)
-- step 2: do the group-by on the join
SELECT x.manufacturer, count(*)
FROM Product x, Purchase y
WHERE x.pid = y.product_id and y.price > 100
GROUP BY x.manufacturer
manu facturer
count(*)
Hitachi 2
Canon 1
...

25. Aggregate + Join Product(pid,pname,manufacturer)
Purchase(id,product_id,price,month)
Aggregate + Join
Variant: For each manufacturer, compute how many products with price > $100 they sold in each month
SELECT x.manufacturer, y.month, count(*)
FROM Product x, Purchase y
WHERE x.pid = y.product_id and y.price > 100
GROUP BY x.manufacturer, y.month
Group by x.manufacturer, y.month = create groups of the form (manufacturer, month)
manu facturer
month
count(*)
Hitachi
Jan 2
Feb 1
Canon 3
...

26. Including Empty Groups
FWGHOS
In the result of a group by query, there is one row per group in the result
Count(*) is never 0
SELECT x.manufacturer, count(*)
FROM Product x, Purchase y
WHERE x.pname = y.product
GROUP BY x.manufacturer
Mention that this actually triggered the famous count bug that happened in the 80s that took 5 years to discover and fix.
CSE au

27. Including Empty Groups
SELECT x.manufacturer, count(y.pid)
FROM Product x LEFT OUTER JOIN Purchase y
ON x.pname = y.product
GROUP BY x.manufacturer
Count(pid) is 0 when all pid’s in the group are NULL
Extra exercises:
1) List all manufacturers with more than 10 items sold. Return the manufacturer name and the number of items sold.
select manufacturer, sum(quantity) from Product, Purchase where pname=product group by manufacturer having sum(quantity) > 10;
2) List all manufacturers with more than 1 distinct product sold. Return the name of the manufacturer and the number of distinct products sold.
select manufacturer, count( distinct product) from Product, Purchase where pname=product group by manufacturer having count(distinct product) > 1;
3) List all products with more than 2 purchases. Return the name of the product, its product ID, and the max price at which it was sold
select R.pname, R.pid, max(R.price) from Product R, Purchase P where R.pname=P.product group by R.pname, R.pid having count(*) > 2;
4) Find manufacturers with at least 5 purchases in the same month. Return the manufacturer, month, and the total quantity of items sold
select R.manufacturer, P.month, sum(quantity) from Product R, Purchase P group by P.month, R.manufacturer having count(*) > 2;
CSE au

28. Subqueries A subquery is a SQL query nested inside a larger query
Such inner-outer queries are called nested queries
A subquery may occur in:
A SELECT clause
A FROM clause
A WHERE clause
Rule of thumb: avoid nested queries when possible
But sometimes it’s impossible, as we will see
Subqueries can return a single constant and this constant can be compared with another value in a WHERE clause
Subqueries can return relations that can be used in various ways in WHERE clauses
Subqueries can appear in FROM clauses, followed by a tuple variable that represents the tuples in the result of the subquery
You will understand why the last is true in a moment
CSE au

29. Subqueries…
Can return a single value to be included in a SELECT clause Can return a relation to be included in the FROM clause, aliased using a tuple variable Can return a single value to be compared with another value in a WHERE clause Can return a relation to be used in the WHERE or HAVING clause under an existential quantifier
Subqueries can return a single constant and this constant can be compared with another value in a WHERE clause
Subqueries can return relations that can be used in various ways in WHERE clauses
Subqueries can appear in FROM clauses, followed by a tuple variable that represents the tuples in the result of the subquery
CSE au

30. “correlated subquery”
1. Subqueries in SELECT
Product (pname, price, cid)
Company (cid, cname, city)
For each product return the city where it is manufactured
“correlated subquery”
SELECT X.pname, (SELECT Y.city FROM Company Y WHERE Y.cid=X.cid) as City
FROM Product X
Putting query here because it is easier to paste into SQLite:
SELECT X.pname, (SELECT Y.city FROM Company Y WHERE Y.cid=X.cid) As City FROM Product X;
What happens if the subquery returns more than one city?
We get a runtime error (and SQLite simply ignores the extra values…)
CSE au

31. We have “unnested” the query
Product (pname, price, cid)
Company (cid, cname, city)
1. Subqueries in SELECT
Whenever possible, don’t use a nested queries:
SELECT X.pname, (SELECT Y.city FROM Company Y WHERE Y.cid=X.cid) as City
FROM Product X =
SELECT X.pname, Y.city FROM Product X, Company Y WHERE X.cid=Y.cid
We have “unnested” the query
SELECT X.pname, Y.city
FROM Product X, Company Y
WHERE X.cid=Y.cid
CSE au

32. Product (pname, price, cid)
Company (cid, cname, city)
1. Subqueries in SELECT
Compute the number of products made by each company
SELECT DISTINCT C.cname, (SELECT count(*) FROM Product P WHERE P.cid=C.cid)
FROM Company C
SELECT DISTINCT C.cname, (SELECT count(*) FROM Product P WHERE P.cid=C.cid) FROM Company C;
SELECT C.cname, count(*) FROM Company C, Product P WHERE C.cid=P.cid GROUP BY C.cname;
SELECT C.cname, count(pname) FROM Company C left outer join Product P on C.cid=P.cid GROUP BY C.cname;
CSE au

33. Product (pname, price, cid)
Company (cid, cname, city)
1. Subqueries in SELECT
Compute the number of products made by each company
SELECT DISTINCT C.cname, (SELECT count(*) FROM Product P WHERE P.cid=C.cid)
FROM Company C
SELECT DISTINCT C.cname, (SELECT count(*) FROM Product P WHERE P.cid=C.cid) FROM Company C;
SELECT C.cname, count(*) FROM Company C, Product P WHERE C.cid=P.cid GROUP BY C.cname;
SELECT C.cname, count(pname) FROM Company C left outer join Product P on C.cid=P.cid GROUP BY C.cname;
SELECT C.cname, count(*)
FROM Company C, Product P
WHERE C.cid=P.cid
GROUP BY C.cname
Better: we can unnest using a GROUP BY
CSE au

34. Product (pname, price, cid)
Company (cid, cname, city)
1. Subqueries in SELECT
But are these really equivalent?
SELECT DISTINCT C.cname, (SELECT count(*) FROM Product P WHERE P.cid=C.cid)
FROM Company C
SELECT C.cname, count(*)
FROM Company C, Product P
WHERE C.cid=P.cid
GROUP BY C.cname
SELECT DISTINCT C.cname, (SELECT count(*) FROM Product P WHERE P.cid=C.cid) FROM Company C
SELECT C.cname, count(*) FROM Company C, Product P WHERE C.cid=P.cid GROUP BY C.cname
SELECT C.cname, count(pname) FROM Company C left outer join Product P on C.cid=P.cid GROUP BY C.cname
CSE au

35. Product (pname, price, cid)
Company (cid, cname, city)
1. Subqueries in SELECT
But are these really equivalent?
SELECT DISTINCT C.cname, (SELECT count(*) FROM Product P WHERE P.cid=C.cid)
FROM Company C
SELECT C.cname, count(*)
FROM Company C, Product P
WHERE C.cid=P.cid
GROUP BY C.cname
No! Different results if a company has no products
SELECT DISTINCT C.cname, (SELECT count(*) FROM Product P WHERE P.cid=C.cid) FROM Company C
SELECT C.cname, count(*) FROM Company C, Product P WHERE C.cid=P.cid GROUP BY C.cname
SELECT C.cname, count(pname) FROM Company C left outer join Product P on C.cid=P.cid GROUP BY C.cname
SELECT C.cname, count(pname)
FROM Company C LEFT OUTER JOIN Product P
ON C.cid=P.cid
GROUP BY C.cname
CSE au

36. Product (pname, price, cid)
Company (cid, cname, city)
2. Subqueries in FROM
Find all products whose prices is > 20 and < 500
SELECT X.pname FROM (SELECT * FROM Product AS Y WHERE price > 20) as X
WHERE X.price < 500
CSE au

37. Product (pname, price, cid)
Company (cid, cname, city)
2. Subqueries in FROM
Find all products whose prices is > 20 and < 500
SELECT X.pname FROM (SELECT * FROM Product AS Y WHERE price > 20) as X
WHERE X.price < 500
Try unnest this query !
CSE au

38. Product (pname, price, cid)
Company (cid, cname, city)
2. Subqueries in FROM
Find all products whose prices is > 20 and < 500
SELECT X.pname FROM (SELECT * FROM Product AS Y WHERE price > 20) as X
WHERE X.price < 500
Side note: This is not a correlated subquery. (why?)
Try unnest this query !
CSE au

39. 2. Subqueries in FROM
Sometimes we need to compute an intermediate table only to use it later in a SELECT-FROM-WHERE Option 1: use a subquery in the FROM clause Option 2: use the WITH clause
CSE au

40. Product (pname, price, cid)
Company (cid, cname, city)
2. Subqueries in FROM
SELECT X.pname FROM (SELECT * FROM Product AS Y WHERE price > 20) as X
WHERE X.price < 500 =
A subquery whose result we called myTable
WITH myTable AS (SELECT * FROM Product AS Y WHERE price > 20)
SELECT X.pname FROM myTable as X
WHERE X.price < 500
CSE au

41. 3. Subqueries in WHERE Product (pname, price, cid)
Company (cid, cname, city)
3. Subqueries in WHERE
Find all companies that make some products with price < 200
Conditions involving relations.
Remember to say: EXISTS R is true if and only if R is not empty
SELECT DISTINCT C.cname FROM Company C
WHERE EXISTS (SELECT * FROM Product P WHERE C.cid = P.cid and P.price < 200)
CSE au

42. Existential quantifiers
Product (pname, price, cid)
Company (cid, cname, city)
3. Subqueries in WHERE
Find all companies that make some products with price < 200
Existential quantifiers
Conditions involving relations.
Remember to say: EXISTS R is true if and only if R is not empty
SELECT DISTINCT C.cname FROM Company C
WHERE EXISTS (SELECT * FROM Product P WHERE C.cid = P.cid and P.price < 200)
CSE au

43. Existential quantifiers
Product (pname, price, cid)
Company (cid, cname, city)
3. Subqueries in WHERE
Find all companies that make some products with price < 200
Existential quantifiers
Using EXISTS:
Conditions involving relations.
Remember to say: EXISTS R is true if and only if R is not empty
SELECT DISTINCT C.cname FROM Company C
WHERE EXISTS (SELECT * FROM Product P WHERE C.cid = P.cid and P.price < 200)
SELECT DISTINCT C.cname
FROM Company C
WHERE EXISTS (SELECT * FROM Product P WHERE C.cid = P.cid and P.price < 200)
CSE au

44. Existential quantifiers
Product (pname, price, cid)
Company (cid, cname, city)
3. Subqueries in WHERE
Find all companies that make some products with price < 200
Existential quantifiers
Using IN
SELECT DISTINCT C.cname FROM Company C WHERE C.cid IN (SELECT P.cid FROM Product P WHERE P.price < 200)
SELECT DISTINCT C.cname
FROM Company C
WHERE C.cid IN (SELECT P.cid FROM Product P WHERE P.price < 200)
CSE au

45. Existential quantifiers
Product (pname, price, cid)
Company (cid, cname, city)
3. Subqueries in WHERE
Find all companies that make some products with price < 200
Existential quantifiers
Using ANY:
SQLITE DOES NOT SUPPORT ANY AND ALL!
S > ANY R is true iff s is greater than at least one value in R
SELECT DISTINCT C.cname FROM Company C WHERE 100 > ANY (SELECT price FROM Product P WHERE P.cid = C.cid)
SELECT DISTINCT C.cname
FROM Company C
WHERE 200 > ANY (SELECT price FROM Product P WHERE P.cid = C.cid)
CSE au

46. 3. Subqueries in WHERE SELECT DISTINCT C.cname FROM Company C
Product (pname, price, cid)
Company (cid, cname, city)
3. Subqueries in WHERE
Find all companies that make some products with price < 200
Existential quantifiers
Using ANY:
SQLITE DOES NOT SUPPORT ANY AND ALL!
S > ANY R is true iff s is greater than at least one value in R
SELECT DISTINCT C.cname FROM Company C WHERE 100 > ANY (SELECT price FROM Product P WHERE P.cid = C.cid)
SELECT DISTINCT C.cname
FROM Company C
WHERE 200 > ANY (SELECT price FROM Product P WHERE P.cid = C.cid)
Not supported in sqlite
CSE au

47. Existential quantifiers
Product (pname, price, cid)
Company (cid, cname, city)
3. Subqueries in WHERE
Find all companies that make some products with price < 200
Existential quantifiers
Now let’s unnest it:
SELECT DISTINCT C.cname
FROM Company C, Product P
WHERE C.cid = P.cid and P.price < 200
CSE au

48. Existential quantifiers
Product (pname, price, cid)
Company (cid, cname, city)
3. Subqueries in WHERE
Find all companies that make some products with price < 200
Existential quantifiers
Now let’s unnest it:
You can easily unnest existential quantifiers
SELECT DISTINCT C.cname
FROM Company C, Product P
WHERE C.cid = P.cid and P.price < 200
Existential quantifiers are easy! 
CSE au

49. 3. Subqueries in WHERE Product (pname, price, cid)
Company (cid, cname, city)
3. Subqueries in WHERE
Find all companies s.t. all their products have price < 200
same as:
Find all companies that make only products with price < 200
CSE au

50. Universal quantifiers
Product (pname, price, cid)
Company (cid, cname, city)
3. Subqueries in WHERE
Find all companies s.t. all their products have price < 200
same as:
Find all companies that make only products with price < 200
Universal quantifiers
CSE au

51. Universal quantifiers
Product (pname, price, cid)
Company (cid, cname, city)
3. Subqueries in WHERE
Find all companies s.t. all their products have price < 200
same as:
Find all companies that make only products with price < 200
Universal quantifiers
Universal quantifiers are hard! 
CSE au

52. 3. Subqueries in WHERE SELECT DISTINCT C.cname FROM Company C
Product (pname, price, cid)
Company (cid, cname, city)
3. Subqueries in WHERE
Find all companies s.t. all their products have price < 200
1. Find the other companies that make some product ≥ 200
SELECT DISTINCT C.cname
FROM Company C
WHERE C.cid IN (SELECT P.cid FROM Product P WHERE P.price >= 200)
SELECT DISTINCT C.cname FROM Company C WHERE C.cid IN (SELECT P.cid FROM Product P WHERE P.price >= 200);
SELECT DISTINCT C.cname FROM Company C WHERE C.cid NOT IN (SELECT P.cid FROM Product P WHERE P.price >= 200);
CSE au

53. 3. Subqueries in WHERE SELECT DISTINCT C.cname FROM Company C
Product (pname, price, cid)
Company (cid, cname, city)
3. Subqueries in WHERE
Find all companies s.t. all their products have price < 200
1. Find the other companies that make some product ≥ 200
SELECT DISTINCT C.cname
FROM Company C
WHERE C.cid IN (SELECT P.cid FROM Product P WHERE P.price >= 200)
2. Find all companies s.t. all their products have price < 200
SELECT DISTINCT C.cname FROM Company C WHERE C.cid IN (SELECT P.cid FROM Product P WHERE P.price >= 200);
SELECT DISTINCT C.cname FROM Company C WHERE C.cid NOT IN (SELECT P.cid FROM Product P WHERE P.price >= 200);
SELECT DISTINCT C.cname
FROM Company C
WHERE C.cid NOT IN (SELECT P.cid FROM Product P WHERE P.price >= 200)
CSE au

54. Universal quantifiers
Product (pname, price, cid)
Company (cid, cname, city)
3. Subqueries in WHERE
Find all companies s.t. all their products have price < 200
Universal quantifiers
Using EXISTS:
SELECT DISTINCT C.cname FROM Company C WHERE NOT EXISTS (SELECT * FROM Product P WHERE P.cid = C.cid and P.price >= 200);
SELECT DISTINCT C.cname
FROM Company C
WHERE NOT EXISTS (SELECT * FROM Product P WHERE P.cid = C.cid and P.price >= 200)
CSE au

55. Universal quantifiers
Product (pname, price, cid)
Company (cid, cname, city)
3. Subqueries in WHERE
Find all companies s.t. all their products have price < 200
Universal quantifiers
Using ALL:
Does not work in SQLite:
SELECT DISTINCT C.cname FROM Company C WHERE 200 > ALL (SELECT price FROM Product P WHERE P.cid = C.cid);
SELECT DISTINCT C.cname
FROM Company C
WHERE 200 >= ALL (SELECT price FROM Product P WHERE P.cid = C.cid)
CSE au

56. 3. Subqueries in WHERE SELECT DISTINCT C.cname FROM Company C
Product (pname, price, cid)
Company (cid, cname, city)
3. Subqueries in WHERE
Find all companies s.t. all their products have price < 200
Universal quantifiers
Using ALL:
Does not work in SQLite:
SELECT DISTINCT C.cname FROM Company C WHERE 200 > ALL (SELECT price FROM Product P WHERE P.cid = C.cid);
SELECT DISTINCT C.cname
FROM Company C
WHERE 200 >= ALL (SELECT price FROM Product P WHERE P.cid = C.cid)
Not supported in sqlite
CSE au

57. Question for Database Theory Fans and their Friends
Can we unnest the universal quantifier query? We need to first discuss the concept of monotonicity
CSE au

58. Monotone Queries Definition A query Q is monotone if:
Product (pname, price, cid)
Company (cid, cname, city)
Monotone Queries
Definition A query Q is monotone if:
Whenever we add tuples to one or more input tables, the answer to the query will not lose any of the tuples
CSE au

59. Monotone Queries Definition A query Q is monotone if:
Product (pname, price, cid)
Company (cid, cname, city)
Monotone Queries
Definition A query Q is monotone if:
Whenever we add tuples to one or more input tables, the answer to the query will not lose any of the tuples
Product
Company
pname
price
cid
Gizmo
19.99
c001
Gadget
999.99
c004
Camera
149.99
c003
cid
cname
city
c002
Sunworks
Bonn
c001
DB Inc.
Lyon
c003
Builder
Lodtz
pname
city
Gizmo
Lyon
Camera
Lodtz Q
CSE au

60. Monotone Queries Definition A query Q is monotone if:
Product (pname, price, cid)
Company (cid, cname, city)
Monotone Queries
Definition A query Q is monotone if:
Whenever we add tuples to one or more input tables, the answer to the query will not lose any of the tuples
Product
Company
pname
price
cid
Gizmo
19.99
c001
Gadget
999.99
c004
Camera
149.99
c003
cid
cname
city
c002
Sunworks
Bonn
c001
DB Inc.
Lyon
c003
Builder
Lodtz
pname
city
Gizmo
Lyon
Camera
Lodtz Q
Product
Company
pname
city
Gizmo
Lyon
Camera
Lodtz
iPad
pname
price
cid
Gizmo
19.99
c001
Gadget
999.99
c004
Camera
149.99
c003
iPad
499.99
cid
cname
city
c002
Sunworks
Bonn
c001
DB Inc.
Lyon
c003
Builder
Lodtz Q
So far it looks monotone...

61. Monotone Queries Definition A query Q is monotone if:
Product (pname, price, cid)
Company (cid, cname, city)
Monotone Queries
Definition A query Q is monotone if:
Whenever we add tuples to one or more input tables, the answer to the query will not lose any of the tuples
Product
Company
pname
price
cid
Gizmo
19.99
c001
Gadget
999.99
c004
Camera
149.99
c003
cid
cname
city
c002
Sunworks
Bonn
c001
DB Inc.
Lyon
c003
Builder
Lodtz
pname
city
Gizmo
Lyon
Camera
Lodtz Q
Q is not monotone!
Product
Company
pname
city
Gizmo
Lodtz
Camera
iPad
Lyon
pname
price
cid
Gizmo
19.99
c001
Gadget
999.99
c004
Camera
149.99
c003
iPad
499.99
cid
cname
city
c002
Sunworks
Bonn
c001
DB Inc.
Lyon
c003
Builder
Lodtz
c004
Crafter Q

62. Monotone Queries
Theorem: If Q is a SELECT-FROM-WHERE query that does not have subqueries, and no aggregates, then it is monotone.
CSE au

63. Monotone Queries
Theorem: If Q is a SELECT-FROM-WHERE query that does not have subqueries, and no aggregates, then it is monotone. Proof. We use the nested loop semantics: if we insert a tuple in a relation Ri, this will not remove any tuples from the answer
for x1 in R1 do
for x2 in R2 do …
for xn in Rn do
if Conditions
output (a1,…,ak)
SELECT a1, a2, …, ak
FROM R1 AS x1, R2 AS x2, …, Rn AS xn
WHERE Conditions
CSE au

64. Monotone Queries The query: is not monotone
Product (pname, price, cid)
Company (cid, cname, city)
Monotone Queries
The query: is not monotone
Find all companies s.t. all their products have price < 200

65. Monotone Queries The query: is not monotone
Product (pname, price, cid)
Company (cid, cname, city)
Monotone Queries
The query: is not monotone
Find all companies s.t. all their products have price < 200
pname
price
cid
Gizmo
19.99
c001
cid
cname
city
c001
Sunworks
Bonn
cname
Sunworks

66. Product (pname, price, cid)
Company (cid, cname, city)
Monotone Queries
The query: is not monotone Consequence: If a query is not monotonic, then we cannot write it as a SELECT-FROM-WHERE query without nested subqueries
Find all companies s.t. all their products have price < 200
pname
price
cid
Gizmo
19.99
c001
cid
cname
city
c001
Sunworks
Bonn
cname
Sunworks
pname
price
cid
Gizmo
19.99
c001
Gadget
999.99
cid
cname
city
c001
Sunworks
Bonn
cname

67. Queries that must be nested
Queries with universal quantifiers or with negation
CSE au

68. Queries that must be nested
Queries with universal quantifiers or with negation
Queries that use aggregates in certain ways
sum(..) and count(*) are NOT monotone, because they do not satisfy set containment
select count(*) from R is not monotone!
CSE au

69. Introduction to Data Management CSE 344
Lecture 7-8: SQL Wrap-up
Relational Algebra
CSE au

70. Announcements You received invitation email to @cs
You will be prompted to choose passwd
Problems with existing account?
In the worst case we will ask you to create a account just for this class
If OK, create the database server
Choose cheapest pricing tier!
Remember: WQ2 due on Friday

71. GROUP BY v.s. Nested Queries
Purchase(pid, product, quantity, price)
GROUP BY v.s. Nested Queries
SELECT product, Sum(quantity) AS TotalSales
FROM Purchase
WHERE price > 1
GROUP BY product
SELECT DISTINCT x.product, (SELECT Sum(y.quantity) FROM Purchase y WHERE x.product = y.product AND y.price > 1) AS TotalSales
FROM Purchase x
WHERE x.price > 1
At the beginning of lecture, we saw that we can express group by queries with a subquery in the select clause. Let’s take a look
at some more complicated examples along those lines. First… what if we also want to apply a predicate in the where clause.
We have to be careful, we need to use it twice.
Why twice ?
CSE au

72. More Unnesting Author(login,name) Wrote(login,url)
Find authors who wrote ≥ 10 documents:
CSE au

73. More Unnesting Author(login,name) Wrote(login,url)
Find authors who wrote ≥ 10 documents:
This is SQL by a novice
Attempt 1: with nested queries
SELECT DISTINCT Author.name
FROM Author
WHERE (SELECT count(Wrote.url) FROM Wrote WHERE Author.login=Wrote.login) >= 10
CSE au

74. More Unnesting Author(login,name) Wrote(login,url)
Find authors who wrote ≥ 10 documents:
Attempt 1: with nested queries
Attempt 2: using GROUP BY and HAVING
SELECT Author.name
FROM Author, Wrote
WHERE Author.login=Wrote.login
GROUP BY Author.name
HAVING count(wrote.url) >= 10
This is SQL by an expert
CSE au

75. Finding Witnesses Product (pname, price, cid)
Company (cid, cname, city)
Finding Witnesses
For each city, find the most expensive product made in that city
CSE au

76. Finding Witnesses Product (pname, price, cid)
Company (cid, cname, city)
Finding Witnesses
For each city, find the most expensive product made in that city
Finding the maximum price is easy…
SELECT x.city, max(y.price)
FROM Company x, Product y
WHERE x.cid = y.cid
GROUP BY x.city;
But we need the witnesses, i.e., the products with max price
CSE au

77. Finding Witnesses Product (pname, price, cid)
Company (cid, cname, city)
Finding Witnesses
To find the witnesses, compute the maximum price in a subquery (in FROM or in WITH)
WITH CityMax AS (SELECT x.city, max(y.price) as maxprice
FROM Company x, Product y
WHERE x.cid = y.cid
GROUP BY x.city)
SELECT DISTINCT u.city, v.pname, v.price
FROM Company u, Product v, CityMax w
WHERE u.cid = v.cid
and u.city = w.city
and v.price = w.maxprice;
CSE au

78. Finding Witnesses Product (pname, price, cid)
Company (cid, cname, city)
Finding Witnesses
To find the witnesses, compute the maximum price in a subquery (in FROM or in WITH)
SELECT DISTINCT u.city, v.pname, v.price
FROM Company u, Product v,
(SELECT x.city, max(y.price) as maxprice
FROM Company x, Product y
WHERE x.cid = y.cid
GROUP BY x.city) w
WHERE u.cid = v.cid
and u.city = w.city
and v.price = w.maxprice;
CSE au

79. Finding Witnesses Product (pname, price, cid)
Company (cid, cname, city)
Finding Witnesses
Or we can use a subquery in where clause
SELECT u.city, v.pname, v.price
FROM Company u, Product v
WHERE u.cid = v.cid
and v.price >= ALL (SELECT y.price FROM Company x, Product y WHERE u.city=x.city and x.cid=y.cid);
CSE au

80. Finding Witnesses Product (pname, price, cid)
Company (cid, cname, city)
Finding Witnesses
There is a more concise solution here:
SELECT u.city, v.pname, v.price
FROM Company u, Product v, Company x, Product y
WHERE u.cid = v.cid and u.city = x.city
and x.cid = y.cid
GROUP BY u.city, v.pname, v.price
HAVING v.price = max(y.price)
CSE au