Saturday, 14 April 2018

Join operation in relational algebra and SQL solved exercises

Join operation in relational algebra and SQL solved exercises

Question:

Consider two relations R and S with instances as follows;

Relation R
Relation S
a b c
------
1 2 3
4 5 6
7 8 9
c d e
------
3 4 5
7 8 9
9 0 1
5 5 5

How many rows would the following operations yield for the given data?
(a) cartesian product of R and S.
(b) natural join of R and S
(c) left outer join of R and S
(d) right outer join of R and S
(e) full outer join of R and S

Solution:

Operation
No. of rows
How?
Result
Cartesian product
12
No. of records in R multiply No. of records in S
3 * 4 = 12
a b c c d e
--------------
1 2 3 3 4 5
1 2 3 7 8 9
1 2 3 9 0 1
1 2 3 5 5 5
4 5 6 3 4 5
4 5 6 7 8 9
4 5 6 9 0 1
4 5 6 5 5 5
7 8 9 3 4 5
7 8 9 7 8 9
7 8 9 9 0 1
7 8 9 5 5 5
Natural join
2
Compares the value of the common attribute between relations R and S. In our example, attribute C is the common attribute.
If C’s value is common for pair of tuples from both relations, then that pair will form a new tuple in the result.
Between R and S we have only 2 tuples with common C values.
a b c d e
-------------
1 2 3 4 5
7 8 9 0 1

Left outer join
3
It is the natural join that includes all the records from left side relation (in our case R) but not all records from right side relation (in our case S).
If there is no tuple in right side relation that are related to some records of left side relation, then the right side values will be NULL.
Number of records = Number of records in right relation R
a b c d e
--------------
1 2 3 4 5
4 5 6 n n
7 8 9 0 1

Right outer join
4
Same as above with inclusion of all records from right side relation (S) with NULL values of missing left side relation (R)
Number of records = Number of records in left relation S
a b c d e
-------------
1 2 3 4 5
n n 7 8 9
7 8 9 0 1
n n 5 5 5

Full outer join
5
Same as above. Now both sides the missing records will be replaced with NULL.
2 with common C values in R and S + 1 record in R with S values as null + 2 records in S with R values as null = 5 records
a b c d e
-------------
1 2 3 4 5
4 5 6 n n
n n 7 8 9
7 8 9 0 1
n n 5 5 5


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Go to Natural join in DBMS page (SQL)
Go to Equi-join in DBMS page (SQL)







result of join operation in sql
result of natural join in relational algebra
outer joins in sql and relational algebra
left and right outer joins examples
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how join operation is performed in relational algebra
join operation exercises

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