SQL: Structured Query Language

Instructor: Mohamed Eltabakh meltabakh@cs.wpi.edu

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Part II

More on SQL SELECT

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Cartesian Product in SQL

SELECT *

FROM Student, Professor;

• In Relation Algebra: R x S

• In SQL, add R and S to FROM clause• No WHERE condition that links R and S

SELECT sName, pNumber

FROM Student, Professor;

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Cross Product - Example

sNumber sName address professor

1 Dave 320FL 1

2 Greg 320FL 1

3 Matt 320FL 2

Student

pNumber pName addr

1 MM 141FL

2 ER 201FL

Professor

sNumber sName address professor pNumber pName addr

1 Dave 320FL 1 1 MM 141FL

1 Dave 320FL 1 2 ER 201FL

2 Greg 320FL 1 1 MM 141FL

2 Greg 320FL 1 2 ER 201FL

3 Matt 320FL 2 1 MM 141FL

3 Matt 320FL 2 2 ER 201FL

SELECT *

FROM Student, Professor;

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Theta Join in SQL

SELECT *

FROM Student, Professor

WHERE Student.pNum = Professor.Number;

• In Relation Algebra: R ⋈C S

• In SQL, add R and S to FROM clause• WHERE condition that links R and S with the join condition C

Join condition5

Theta Join Example

sNumber sName pName

1 Dave MM

2 Greg MM

3 Matt ER

sNumber sName address profNum

1 Dave 320FL 1

2 Greg 320FL 1

3 Matt 320FL 2

Student

pNumber pName address

1 MM 141FL

2 ER 201FL

Professor

sNumber,sName,pName(Student ⋈(profNum=pNumber) Professor)

SELECT sNumber, sName, pName

FROM Student, Professor

WHERE profNum = pNumber;

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Theta Join Example

sNumber sName address profNum

1 Dave 320FL 1

2 Greg 320FL 1

3 Matt 320FL 2

Student

pNumber pName address

1 MM 141FL

2 ER 201FL

Professor

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If column names are the same use relationName.attrName

SELECT sName, pName, S.address

FROM Student S, Professor P

WHERE S.address = P.address;

sName,pName,S.address(ρS(Student) ⋈(S.address=P.address) ρP(Professor))

Natural Join

SELECT *

FROM Student , Professor

WHERE Student.pnumber = Professor.pnumber ;

Student Professor⋈

Reminder: Join columns must have same names in both relations (R S)⋈

SELECT *FROM Student NATURAL JOIN Professor;

Explicitly add the equality join condition

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Difference between the two Queries below

SELECT *

FROM Student , Professor

WHERE Student.pnumber = Professor.pnumber ;

Student Professor⋈

SELECT *FROM Student NATURAL JOIN Professor;

Explicitly add the equality join condition

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Common columns will appear once

Common columns will appear twice

Natural Join - Example

sNumber sName address pNumber

1 Dave 320FL 1

2 Greg 320FL 1

3 Matt 320FL 2

StudentpNumber pName addr

1 MM 141FL

2 ER 201FL

Professor

sNumber sName address pNumber pName addr

1 Dave 320FL 1 MM 141FL

2 Greg 320FL 1 MM 141FL

3 Matt 320FL 2 ER 201FL

Student Professor⋈10

Must be the same name

SELECT *

FROM Student natural join Professor;

Example Queries

SELECT *

FROM loan

WHERE amount > 1200 ;

SELECT L.loan_number

FROM loan L

WHERE L.amount > 1200 ;

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Example Queries

SELECT customer_name

FROM depositor

Union

SELECT customer_name

FROM borrower;

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Example Queries

SELECT customer_name

FROM borrower B, loan L

WHERE B.loan_number = L.loan_number

AND L.branch_name = “Perryridge”;

DBMS is smart enough !!!(Select first, then joins)

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Sorting: ORDER BY clause

New optional clause that you can add to the SELECT statement called “ORDER BY”

Allows sorting the returned records according to one or more fields

SELECT *

FROM Student

WHERE sNumber >= 1

ORDER BY pNumber, sName;

SELECT *

FROM Student

WHERE sNumber >= 1

ORDER BY pNumber ASC, sName DESC;

Default is ascending order

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-Order first based on the pNumber (ascending)

-If many records exist with the same pNumber - order them based on sName (descending)

Sorting: ORDER BY clause

sNumber sName address pNumber

1 Dave 320FL 1

2 Greg 320FL 1

3 Matt 320FL 2

Student

SELECT *

FROM Student

WHERE sNumber >= 1

ORDER BY pNumber,

sName DESC;

sNumber sName address pNumber

2 Greg 320FL 1

1 Dave 320FL 1

3 Matt 320FL 2

(pNumber, sName DESC) ( (sNumber >= 1) (Student))

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Duplicate Elimination in SQL

New optional keyword “DISTINCT”

Added in the SELECT clause

SELECT DISTINCT …

FROM …

…Eliminate any duplicates from the answer

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Duplicate Elimination: Example

SELECT DISTINCT sName, address

FROM Student;

(sName,address(Student)) ( (address) ( (sNumber > 1) (Student)))

sNumber sName address professor

1 Dave 320FL MM

2 Greg 320FL MM

3 Matt 320FL ER

Student

address

320FL

SELECT DISTINCT address

FROM Student

WHERE sNumber > 1;

sName address

Dave 320FL

Greg 320FL

Matt 320FL

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Always Remember…. Only SELECT and FROM clauses are mandatory

All the others are optional

You can mix and match the optional ones But if you add a clause, then keep it in its order

SELECT address

FROM Student

ORDER BY sNumber

WHERE sNumber > 1;X

SELECT address

FROM Student

ORDER BY sNumber;

SELECT DISTINCT address

FROM Student

WHERE sNumber > 1;

SELECT address

FROM Student

WHERE sNumber > 1

ORDER BY sNumber;18

Aggregation + GroupBy

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Possible Aggregations in SQL

SELECT COUNT (*) FROM Student;

SELECT COUNT (sNumber) FROM Student;

SELECT MIN (sNumber) FROM Student;

SELECT MAX (sNumber) FROM Student;

SELECT SUM (sNumber) FROM Student;

SELECT AVG (sNumber) FROM Student;

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Grouping & Aggregation in SQL

New optional clause called “GROUP BY”

If the SELECT statement has “WHERE” Then WHERE conditions are evaluated first, then records

are grouped

SELECT pNumber, COUNT (sName), Min(gpa) FROM Student GROUP BY pNumber;

First form groups for each pNumber

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Then count the records in each groupAnd get the minimum gpa for each group

GROUP BY: Example I

sNumber sName address pNumber

1 Dave 320FL 1

2 Greg 320FL 1

3 Matt 320FL 2

4 Jan 500MA 2

Student

SELECT count(*) AS CNT

FROM Student;

cnt count(*) (Student)

CNT

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SELECT pNumber, count(*) AS CNT

FROM Student

WHERE sNumber > 1

GROUP BY pNumber;

pNumber CNT

1 1

2 2

pNumber,cnt count(*) ( (sNumber > 1) (Student))

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GROUP BY: Example II

sNumber sName address pNumber

1 Dave 320FL 1

2 Greg 320FL 1

3 Matt 320FL 2

4 Jan 500MA 2

Student

SELECT pNumber,address, count(sName) AS CNT, sum(sNumber) AS SUM

FROM Student

WHERE sNumber >= 1

GROUP BY pNumber, address;

pNumber address CNT SUM

1 320FL 2 3

2 320FL 1 3

2 500MA 1 4

pNumber,address, CNT count(sName), SUM sum(sNumber) ( (sNumber > 1) (Student))

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Restrictions of GROUP BY If you group by A1, A2, …An, then any other column projected in

SELECT clause must be inside an aggregation function

SELECT pNumber, address, count(sName) AS CNT, sum(sNumber) AS SUM

FROM Student

WHERE sNumber > 1

GROUP BY pNumber, address;

SELECT pNumber, address, sName, sum(sNumber) AS SUM

FROM Student

WHERE sNumber > 1

GROUP BY pNumber, address;X

SELECT pNumber, count(sName) AS CNT, sum(sNumber) AS SUM

FROM Student

WHERE sNumber > 1

GROUP BY pNumber, address;

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HAVING Clause: Putting Condition on Groups

How to add conditions on each group? Select only the groups where the COUNT > 5

These conditions are after you build the groups (not before) Remember: WHERE conditions are executed before the

groups are formed

New optional clause called “HAVING”, added after the GROUP BY clause

SELECT pNumber, COUNT (sName) FROM Student GROUP BY pNumberHAVING SUM(sNumber) > 2;

Can reference aggregation inside HAVING

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HAVING Clause: Example

sNumber sName address pNumber

1 Dave 320FL 1

2 Greg 320FL 1

3 Matt 320FL 2

4 Jan 500MA 2

Student

SELECT pNumber,address, count(sName) AS CNT, sum(sNumber) AS SUM

FROM Student

WHERE sNumber > 1

GROUP BY pNumber, address

HAVING sum(sNumber) > 3;

pNumber address CNT SUM

2 500MA 1 4

(SUM> 3) (pNumber,address, CNT count(sName), SUM sum(sNumber) ( (sNumber > 1) (Student)))

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SELECT Statement Clauses

Optional clauses if added must be in the order above Order of execution

FROM Check which relations are used WHERE Filter records based on conditions GROUP BY Form groups HAVING Filter groups based on conditions ORDER BY Sort the data SELECT Form the projection list (output columns)

SELECT <projection list>FROM <relation names>WHERE <conditions>GROUP BY <grouping columns>HAVING <grouping conditions>ORDER BY <order columns>;

optional

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QuestionsSELECT <projection list>FROM <relation names>WHERE <conditions>GROUP BY <grouping columns>HAVING <grouping conditions>ORDER BY <order columns>;

optional

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More in SELECT Statement

Special handling for NULL values

Nested subqueries

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Null Values

Null means ‘unknown’ value

Any expression containing Null returns Null 5 + null null ‘ABC’ || null null

Null in predicates returns UNKNOWN Predicates usually return TRUE or FALSE

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Example

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sNumber sName address pNumber

1 Dave 320FL 1

2 Greg null 1

3 Matt null 2

4 Jan 500MA 2

Student

SELECT sNumberFROM StudentWHERE address = ‘320FL’;

sNumber

1

2

3 May or may not appear

Truth Table (Including UNKNOWN)

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Use of “IS NULL” or “IS NOT NULL”

Check if a value is null or not

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SELECT sNumberFROM StudentWHERE address is not nullAND address ‘320FL’;

SELECT sNumberFROM StudentWHERE address is null;

Select student numbers where the address is null

Remember:

SELECT sNumberFROM StudentWHERE address = null;

XThe returned value here is unknown

Use of “NVL” Function

NVL( exp1, exp2) If exp1 is null return exp2, otherwise return expr1

Can be used in projection list or in predicates

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SELECT sNumberFROM StudentWHERE nvl(address, ‘n/a’) <> ‘n/a’AND address ‘320FL’;

SELECT sNumber, nvl(address, ‘N/A’)FROM Student;

sNumber address

1 320FL

2 N/A

3 N/A

4 500MA

Null with Grouping & Aggregation

Aggregation Null is ignored with all aggregates, e.g., SUM, AVG, MIN,

MAX except COUNT(*)

Grouping Null is considered as a separate group

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Example

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sNumber sName address pNumber

1 Dave 320FL 1

2 Greg null 1

3 Matt null null

4 Jan 500MA 2

Student

SELECT address, sum(pNumber) as sum, count(*) as cntFROM StudentGROUP BY address;

address

sum cnt

320FL 1 1

null 1 2

500MA 2 1

More in SELECT Statement

Special handling for NULL values

Nested subqueries

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Nested Subquery

SQL provides a mechanism for the nesting of subqueries.

A subquery is a SELECT statement expression that is nested within another query

Subquery can appear in FROM or WHERE clauses

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Nested Subquery in WHERE Clause

Since the predicates has = : The inner statement must return one record with one column In this case, DBMS will automatically convert the relation to a

single scalar value Otherwise an error is generated

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SELECT *

FROM Student

WHERE pNumber =

(SELECT pNumber

FROM Professor

WHERE pName = ‘Mike’);

1- Execute this statement first to get the pNumber (inner SELECT)

2- Then, execute this statement once pNumber from the first step is known(outer SELECT)

CS3431

Example: Subqueries Retuning Scalar Value

sNumber sName address pNum

1 Dave 320FL 1

2 Greg 320FL 1

3 Matt 320FL 2

Student

pNumber pName address

1 MM 141FL

2 ER 201FL

Professor

sNumber sName

1 Dave

2 Greg

Select students of professor ‘MM’

SELECT sNumber, sNameFROM StudentWHERE pNum =

(SELECT pNumberFROM ProfessorWHERE pName=‘MM’);

SubQuery Returning a Relation (General Case)

Predicates may include any of (OP above) : Exists R True if R is not empty s in R True if tuple s appears in R s not in R True if tuple s does not appear in R

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SELECT sNumber, sNameFROM StudentWHERE pNum OP

(SELECT pNumber FROM Professor WHERE pName=‘MM’);

Inner Select (R)

Outer Select (S)

CS3431

Example 1: Subqueries Returning Relations

sNumber sName address pNum

1 Dave 320FL 1

2 Greg 320FL 1

3 Matt 320FL 2

4 Sam 30IN 3

Student

pNumber pName address

1 MM 141FL

2 ER 201FL

3 XY 30WA

Professor

sNumber sName

1 Dave

2 Greg

3 Matt

Select students of professors with address like ‘%FL’

SELECT sNumber, sNameFROM StudentWHERE pNum IN

(SELECT pNumberFROM ProfessorWHERE address Like ‘%FL’);

CS3431

Example 2: Subqueries Returning Relations

sNumber sName address pNum

1 Dave 320FL 1

2 Greg 320FL 1

3 Matt 320FL 2

4 Sam 30IN 3

Student

pNumber pName address

1 MM 141FL

2 ER 201FL

3 XY 30WA

Professor

sNumber sName

1 Dave

2 Greg

3 Matt

4 Sam

SELECT sNumber, sNameFROM StudentWHERE Exists

(SELECT pNumberFROM ProfessorWHERE address Like ‘%FL’);

Always true because it is not empty

Example 3: Subqueries Returning Relations

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Multi-column tuples

Comparison Using ALL and ANY

We took: Exists, IN, NOT IN

s > ALL R True if s > all values in R s > ANY R True if s > any value in R ‘>’ can be any of the other comparison operators, e.g., <, <=, >=, =, <> R must be relation with single column

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SELECT sNumber, sNameFROM StudentWHERE pNum OP

(SELECT pNumber FROM Professor WHERE pName=‘MM’);

Inner Select (R)

Outer Select (S)

Example

sNumber sName address pNum

1 Dave 320FL 1

2 Greg 320FL 1

3 Matt 320FL 2

4 Sam 30IN 3

Student

pNumber pName address

1 MM 141FL

2 ER 201FL

3 XY 30WA

Professor

sNumber sName

3 Matt

4 Sam

SELECT sNumber, sNameFROM StudentWHERE pNum >= ALL

(SELECT pNumberFROM ProfessorWHERE address Like ‘%FL’);

This inner select returns 1 , 2

Correlated Selects If the “inner” select references the “outer” select correlated

In this case, the inner select is executed with each record from the outer select

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Reference to the outer select

Meaning: For each supplier, execute the inner select, and then evaluate the WHERE clause

Returns: suppliers who do not have orders

Use of Inner Select with DML Commands

Inner select can be used with Insert, Update, Delete commands

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INSERT INTO suppliers (supplier_id, supplier_name)SELECT account_no, nameFROM externalsWhere code = 1;

Notice that there is no keyword “values” in this case

Nested Subquery in FROM Clause

Use the inner SELECT like any other table It is just built on the fly

Inner SELECT can be a full statement with all clauses ORDER BY clause does not make sense in the inner select

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SELECT *

FROM Student, (inner SELECT) AS q

WHERE … Table built on the fly

Example

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• Subquery 1 is computed on the fly

• It is treated as a normal table after that

Questions

Special handling for NULL values

Nested subqueries

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