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The Oracle Database SystemQuerying the Data

Database CourseThe Hebrew University of Jerusalem

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Basic SQL Query

SELECT [Distinct] target-list

FROM from-list

WHERE condition;

•from-list: A list of relation names (possibly with a range-variable after each name)

•target-list: A list of fields of relations in relation-list

•condition: A boolean condition

•DISTINCT: Optional keyword to delete duplicates

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Basic SQL Query (continues)

SELECT [Distinct] target-list

FROM from-list

WHERE condition;

The result evaluation:

1. Compute the cross product of the tables in from-list.2. Delete all rows that do not satisfy condition.3. Delete all columns that do not appear in target-list.4. If Distinct is specified eliminate duplicate rows.

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Basic SQL Query (continues)

SELECT Distinct A1,…,An

FROM R1,…,Rm

WHERE C;

This translates to the expression in relational algebra:

A1,…,An (C(R1 x…x Rm))

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Example Tables Used

Reserves

sid bid Day

22

58

101

103

10/10/02

11/12/02

Sailors

sid sname rating age

22

31

58

Dustin

Lubber

Rusty

7

8

10

45.0

55.5

35.0

Boats

bid bname color

101

103

Nancy

Gloria

red

green

KeyKey

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Boat Names and Reservation Dates

SELECT bname,day

FROM Boats,Reserves

WHERE Boats.bid = Reserves.bid

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Boat Names and Reservation Dates

SELECT bname,day

FROM Boats,Reserves

WHERE Boats.bid = Reserves.bid

Reserves

sid bid day

22

22

101

101

10/10/02

10/10/02

58

58

103

103

11/12/02

11/12/02

Boats

bid bname color

101

103

Nancy

Gloria

red

green

101

103

Nancy

Gloria

red

green

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Boat Names and Reservation Dates

SELECT bname,day

FROM Boats,Reserves

WHERE Boats.bid = Reserves.bid

Reserves

sid bid day

22 101 10/10/02

58 103 11/12/02

Boats

bid bname color

101 Nancy red

103 Gloria green

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Boat Names and Reservation Dates

SELECT bname,day

FROM Boats,Reserves

WHERE Boats.bid = Reserves.bid

Reserves

day

10/10/02

11/12/02

Boats

bname

Nancy

Gloria

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Sailors Who Reserved Boat 103

SELECT sname

FROM Sailors, Reserves

WHERE Sailors.sid = Reserves.sid and

bid = 103;

sname

(Sailors.sid = Reserves.sid bid = 103 (Sailors x Reserves))

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Sailors Reserves

sid sname rating age sid bid day

22 Dustin 7 45.0 22 101 10/10/02

22 Dustin 7 45.0 58 103 11/12/02

31 Lubber 8 55.5 22 101 10/10/02

31 Lubber 8 55.5 58 103 11/12/02

58 Rusty 10 35.0 22 101 10/10/02

58 Rusty 10 35.0 58 103 11/12/02

Sailors x Reserves

Sailors Who Reserved Boat 103

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Sailors Reserves

sid sname rating age sid bid day

22 Dustin 7 45.0 22 101 10/10/02

22 Dustin 7 45.0 58 103 11/12/02

31 Lubber 8 55.5 22 101 10/10/02

31 Lubber 8 55.5 58 103 11/12/02

58 Rusty 10 35.0 22 101 10/10/02

58 Rusty 10 35.0 58 103 11/12/02

Sailors Who Reserved Boat 103

Sailors.sid = Reserves.sid bid = 103

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Sailors Reserves

sid sname rating age sid bid day

22 Dustin 7 45.0 22 101 10/10/02

22 Dustin 7 45.0 58 103 11/12/02

31 Lubber 8 55.5 22 101 10/10/02

31 Lubber 8 55.5 58 103 11/12/02

58 Rusty 10 35.0 22 101 10/10/02

58 Rusty 10 35.0 58 103 11/12/02

Sailors Who Reserved Boat 103

sname

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Range Variables

SELECT S.sname

FROM Sailors S, Reserves R

WHERE S.sid = R.sid and

R.bid = 103;

• Range variables are good style

• They are necessary if the same relation appears twice in the FROM clause

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A Few SELECT Options

Select all columns:SELECT *

FROM Sailors S;

Rename selected columns:SELECT S.sname AS Sailors_Name

FROM Sailors S;

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A Few SELECT Options Applying functions:

Mathematical manipulations:

SELECT (age-5)*2 FROM Sailors S;

Aggregate functions:

SELECT COUNT(*) FROM Sailors S;

SELECT MAX(age) FROM Sailors S;

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The WHERE Clause

Numerical and string comparison: !=,<>,=, <, >, >=, <=, between(between val1 AND val2)

String comparisson is according to the alphabetical order!

Logical components: AND, OR Null verification: IS NULL, IS NOT NULL

Example:SELECT sname

FROM Sailors

WHERE age>=40 AND rating IS NOT NULL ;

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The WHERE Clause (continues)

The LIKE operator: A pattern matching operator Basic format: colname LIKE pattern Example:

_ is a single character

% is 0 or more characters

SELECT sid FROM Sailors WHERE sname LIKE ‘B_%g’;

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Sailors Who’ve Reserved a Boat

SELECT sname

FROM Sailors S, Reserves R

WHERE S.sid = R.sid;

Would adding DISTINCT give a different result?

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Exercise Formulate a query that finds the names of

sailors who reserved a yellow boat.

Sailors

sid sname rating age

22

31

58

Dustin

Lubber

Rusty

7

8

10

45.0

55.5

35.0

Boats

bid bname color

101

103

Nancy

Gloria

red

green

Reserves

sid bid Day

22

58

101

103

10/10/02

11/12/02

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A Harder Exercise

1. Formulate a query that finds the bid of boats that are reserved in at least two different days

2. Fix the query you formulated to find the names of these boats

Hint: A relation may appear more than once in the FROM list…

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Union, Intersect and Except

Sqlplus supports the union, intersection and difference operators

The syntax: query1 UNION query2; query1 INTERSECT query2; query1 MINUS query2;

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Sailors who’ve reserved a red or green boat

SELECT S.sname

FROM Sailors S, Boats B, Reserves R

WHERE S.sid = R.sid and R.bid = B.bid and (B.color = ‘red’ or

B.color = ‘green’);

sname

(color = ‘red’ color = ‘green’

(Sailors ⋈ Reserves ⋈ Boats))

What would happen if we replaced or by and ?

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Sailors who’ve reserved red or green boat

SELECT S.sname

FROM Sailors S, Boats B, Reserves R

WHERE S.sid = R.sid and R.bid = B.bid and B.color = ‘red’

UNION

SELECT S.sname

FROM Sailors S, Boats B, Reserves R

WHERE S.sid = R.sid and R.bid = B.bid and B.color = ‘green’;

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The Second Version in Relational Algebra

sname

(color = ‘red’

(Sailors ⋈ Reserves ⋈ Boats))

sname

(color = ‘green’

(Sailors ⋈ Reserves ⋈ Boats))

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Nested Queries

A query may be nested within another through the following operators:

(NOT) IN (NOT) EXISTS ANY ALL

A query with nested queries is computed using nested loops

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The IN Operator

SELECT S.sname

FROM Sailors S

WHERE S.sid IN (SELECT R.sid

FROM Reserves R

WHERE R.bid = 103);

Names of sailors who’ve reserved boat 103:

What would happen if we wrote NOT IN?

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The EXISTS Operator

SELECT S.sname

FROM Sailors S

WHERE EXISTS (SELECT *

FROM Reserves R

WHERE R.bid = 103 and

S.sid = R.sid);

Names of sailors who’ve reserved boat 103:

What would happen if we wrote NOT EXISTS?

Notice the correlation between the examined row

and the inner query

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Set-Comparison Queries: ANY,ALL

SELECT *

FROM Sailors S1

WHERE S1.age > ANY (SELECT S2.age

FROM Sailors S2);

Sailors who are not the youngest:

We can also use op ALL (op is >, <, =, >=, <=, or <>)

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Another Exercise What does the following query compute?

Using IN and INTERSECT operators, formulate a query that finds the names of the sailors who reserved both green and red boats.

SELECT S.sname

FROM Sailors S, Boats B, Reserves R

WHERE S.sid = R.sid and R.bid = B.bid and B.color = ‘red’

INTERSECT

SELECT S.sname

FROM Sailors S, Boats B, Reserves R

WHERE S.sid = R.sid and R.bid = B.bid and B.color = ‘green’;

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Another Exercise (continues)

What does the following query compute?

SELECT S.snameFROM Sailors S WHERE NOT EXISTS (

(SELECT B.bid FROM Boats B)MINUS(SELECT R.bid FROM Reserves R

WHERE R.sid=S.sid));

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For The Brave Ones…

1. Formulate a query that computes the sid of all sailors who have not reserved a green boat.

2. Prove formally that in the Sailors-Reserves-Boats database, the query of question 1 cannot be as simple as a query of the form:

SELECT Vj.sid

FROM R1 V1, R2 V2…Rn Vn

WHERE C

Where C is a simple condition (contains only comparison clauses, AND and OR, and does not contain nested queries)Notice that the records in each table are arbitrary!