SQL: Structured Query Languageweb.cs.wpi.edu/~cs3431/c15/lectures/Week 5/sqlQueries3.pdf · SQL:...

SQL: Structured Query Language

Instructor: Mohamed Eltabakh

meltabakh@cs.wpi.edu

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Part -3

More in SELECT Statement

l  Special handling for NULL values

l  Queries inside Insert/Update/Delete

l  Temp Tables

l  Nested subqueries

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Null Values l  Null means ‘unknown’ value

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

l  Null in predicates returns UNKNOWN l  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 sNumber FROM Student WHERE address = ‘320FL’;

sNumber 1

2

3 May or may not appear

Having Null in the data is problematic and needs special care…

Use of “IS NULL” or “IS NOT NULL”

l  Check if a value is null or not

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SELECT sNumber FROM Student WHERE address is not null AND address = ‘320FL’;

SELECT sNumber FROM Student WHERE address is null;

Select student numbers where the address is null

Remember: SELECT sNumber FROM Student WHERE address = null;

X

The returned value here is unknown

Use of “NVL” Function l  NVL( exp1, exp2)

l  If exp1 is null return exp2, otherwise return expr1

l  Can be used in projection list or in predicates

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SELECT sNumber FROM Student WHERE 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

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

MAX except COUNT()

l  Grouping l  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 cnt FROM Student GROUP BY address;

address sum cnt 320FL 1 1

null 1 2

500MA 2 1

More in SELECT Statement

l  Special handling for NULL values

l  Queries inside Insert/Update/Delete

l  Temp Tables

l  Nested subqueries

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Reminder About: Insert, Update, Delete l  This is performed using Data Manipulation

Language of SQL (DML)

l  Insertion l  Insert into Students values (‘1111’, …);!

l  Deletion l  Delete from Students;!l  Delete from Students Where sid = ‘1111’;!

l  Update l  Update Students Set GPA = GPA + 0.4;!l  Update Students Set GPA = GPA + 0.4 Where sid = ‘1111’;!

!10

Use of Select Inside Insert l  All records returned from the Select will be inserted

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INSERT INTO suppliers (supplier_id, supplier_name) SELECT account_no, name FROM externals Where code = 1;

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

Number of columns and data types should match

Use of Select Inside Delete

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Delete From Student Where sNumber in (Select sNumber from Registration Where grade = ‘F’);

sNumber sName address 1 Dave 320FL

2 Greg null

3 Matt null

4 Jan 500MA

sNumber courseID grade 4 DB1 A

2 DB1 F

3 DB2 A

4 DB2 F

Student Registration

l  Delete from Student all those who have grade ‘F’

Students number 2 & 4 will be deleted…

Use of Select Inside Delete

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Delete From Student Where sNumber not in (Select sNumber from Registration);

sNumber sName address 1 Dave 320FL

2 Greg null

3 Matt null

4 Jan 500MA

sNumber courseID grade 4 DB1 A

2 DB1 F

3 DB2 A

4 DB2 F

Student Registration

l  Delete from Student all those who do not have registration

Student number 1 will be deleted…

Use of Select Inside Update

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Update Registration Set grade = ‘B’ Where sNumber in (Select sNumber from Student Where sName =‘Matt’);

sNumber sName address 1 Dave 320FL

2 Greg null

3 Matt null

4 Jan 500MA

sNumber courseID grade 4 DB1 A

2 DB1 F

3 DB2 A

4 DB2 F

Student Registration

l  Update the grades of student ‘Matt’ to B

Remember…

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Delete From Student Where sNumber not in (Select sNumber from Registration);

Update Registration Set grade = ‘B’ Where sNumber in (Select sNumber from Student Where sName =‘Matt’);

In Delete and Update - The ‘From’ clause always has one table - The subquery can be only added in the ‘Where’ clause

More in SELECT Statement

l  Special handling for NULL values

l  Queries inside Insert/Update/Delete

l  Temp Tables

l  Nested subqueries

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Temp Tables

l  If a query is very complex, divide it into sub-queries

l  Store the intermediate results in “Temp Tables”

l  The got deleted automatically at the end of the session

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Example

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Create Global Temporary Table temp123 (x int, y varchar2(10));

Insert into temp123 Select …;

Temp table. Its data will be deleted at log out

Inserting some intermediate data

More selects that may reference temp123 …

Exit; intermediate data is automatically deleted

Reuse the intermediate data

More in SELECT Statement

l  Special handling for NULL values

l  Queries inside Insert/Update/Delete

l  Nested subqueries

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Nested Subquery l  SQL provides a mechanism for the nesting of

subqueries.

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

l  Subquery can appear in FROM or WHERE

clauses

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Nested Subquery in FROM Clause

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

l  Inner SELECT can be a full statement with all clauses l  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

Back to this Example

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Report the output relation O(CustomerName, Num_Loans, Num_Accounts) only for customers who have loans and accounts

SELECT customer_name, count(Distinct loan_number), count(Distinct account_number) FROM Depositor D, Borrower B WHERE D.customer_name = B.customer_name GROUP BY customer_name;

Find another way ??

Back to this Example

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Report the output relation O(CustomerName, Num_Loans, Num_Accounts) only for customers who have loans and accounts

SELECT D.customer_name, Num_Loan, Num_Acc FROM ( Select customer_name, count(*) As Num_Acc From Depositor Group By customer_name) As D,

( Select customer_name, count(*) As Num_Loan From Borrower Group By customer_name) As B WHERE D.customer_name = B.customer_name;

Nested Subquery in WHERE Clause

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

single scalar value l  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, sName FROM Student WHERE pNum =

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

SubQuery Returning a Relation (General Case)

l  Predicates may include any of (OP above) : l  s in R à True if tuple s appears in R l  s not in R à True if tuple s does not appear in R l  s = R à R must return a single value (otherwise invalid op)

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SELECT sNumber, sName FROM Student WHERE 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, sName FROM Student WHERE pNum IN

(SELECT pNumber FROM Professor WHERE address Like ‘%FL’);

CS3431

Example 3: Subqueries

sID courseID … …

1 CS101

2 CS300

1 CS202

4 CS500

4 CS203

1 CS303

Registration è R Q: Find the student ID taking the largest number of courses

sID CNT

1 3

2 1

4 2

SELECT sID, count(courseID) as CNT FROM R Group By sID;

Step 1

CS3431

Example 3: Subqueries

sID courseID … …

1 CS101

2 CS300

1 CS202

4 CS500

4 CS203

1 CS303

Registration è R Q: Find the student ID taking the largest number of courses

SELECT sID, count(courseID) as CNT FROM R Group By sID Having count(courseID) =

( SELECT Max(CNT) As Max From (SELECT count(courseID) As CNT

FROM R Group By sID) q

) CNT

3

1

2

Step 2

Max

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sID CNT

1 3

sID CNT

1 3

2 1

4 2

Comparison Using ALL and ANY

l  We took: Exists, IN, NOT IN

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

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SELECT sNumber, sName FROM Student WHERE 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, sName FROM Student WHERE pNum >= ALL

(SELECT pNumber FROM Professor WHERE address Like ‘%FL’);

This inner select returns 1 , 2