1. 04 SQL & Relational Algebra
And Franchise Colleges
HSQ - DATABASES & SQL
04 SQL & Relational Algebra
By MANSHA NAWAZ
Section 04
Relational Algebra

2. SQL & Relational Algebra
Two data manipulation languages proposed by Codd:
Relational Algebra (How)
Relational Calculus (What)
SQL provides implementation of both
Both based on the concept of Relational Closure:
Relation
Operation
Section 04
Relational Algebra

3. Relational Algebra
Several Operators take in one or more tables, and produce a new table
(without destroying the original ones)
Resultant table is temporary
e.g., for outputting data for reporting purposes
or for use with another operator
Operators are:
Cartesian Product
Union
Difference
Intersection
Rows of/Restrict
Columns of/Project
Join
Divide
Section 04
Relational Algebra

4. <result table name> = Union of <table list>
Two tables with same columns
(i.e., same number and columns with matching domains)
Output table is with same attributes
with all rows in either or both of the input tables
<result table name> = Union of <table list>
Past_Emp:
Present_Emp
Q: R1 = Union of Past_Emp, Present_Emp
Section 04
Relational Algebra

5. <result table name> = Intersection of <table list>
Two tables with same columns
(i.e., same number and columns with matching domains)
Output table is with same attributes
with common rows from the input tables only
<result table name> = Intersection of <table list>
Past_Emp:
Present_Emp
Q: R1 = Intersection of Past_Emp, Present_Emp
Section 04
Relational Algebra

6. Rows of (Select)
One input table only
Output table is with same attributes as input table
but only with those rows that satisfy the given condition
<result table name> = Rows of <table> where <condition>
Received_Order:
Q: R1 = Rows of Received_Order where quantity >= 100
Section 04
Relational Algebra

7. Columns of (Project)
One input table only.
Output table contains only those columns that are specified
<result table name> = Columns of <table> under <attribute list>
Q: R1 = Columns of Received_Order under Part#
Received_Order:
Q: duplicates?
Section 04
Relational Algebra

8. Join
Two tables required
not necessarily with the same attributes
but at least one attribute in each table that has the same domain
Output table has all the attributes in the two tables
(possibly minus the common attribute, depending on the join type)
with the rows combined on the attributes specified in the “on” clause
<result table name> = Join of <table> on <attribute list>
Section 04
Relational Algebra

9. Q: R1 = Join of Received_Order, Part on Part#
Section 04
Relational Algebra

10. R1: This is a Natural join This is an Equi- join R1: Section 04
Relational Algebra

11. The previous ones were Inner joins
This is an Outer join
The previous ones were Inner joins
Section 04
Relational Algebra

12. Some tasks to try
Supplier:
Q: Find the part# of those parts that have been supplied by “S3”
Q: Find the names of those parts that were supplied in a quantity greater than 100
Q: Find the names of the suppliers who have supplied bolts in quantities less than 200 and have been supplying for longer than 10 years
NB Importance of order of the statements
Section 04
Relational Algebra

13. Summary Relational Algebra Union Difference Intersection
Rows of/Restrict
Columns of/Project
Join
Section 04
Relational Algebra

14. End of Lecture
Section 04
Relational Algebra