The document compares differences between correlated and nested subqueries, weak and strong entity sets, char and varchar data types in SQL Server, and SQL Server 2005 and 2008. A correlated subquery runs once for each outer query row and references the outer row, while a nested subquery runs once and does not reference the outer row. A weak entity set lacks a primary key while a strong entity set has one. Char occupies a fixed length while varchar occupies variable length plus 2 bytes for the length. SQL Server 2005 introduced XML datatype while 2008 encrypted entire databases and separated date and time datatypes.

1. 1.Difference between Correlated subquery and Nested subquery
S.No Correlated subquery Nested subquery
1 Correlated subquery runs once for Nested subquery runs only once for the
each row selected by the outer entire nesting (outer) query. It does not
query. It contains a reference to a contain any reference to the outer
value from the row selected by the query row.
outer query.
2 Correlated subquery follows down Nested subquery follows top-down
to top approach i.e., main query approach i.e., child query is executed
is executed first(even though first and then parent .
parenthesis are present) and then
child query. We can also say:In a subquery
Outer query condition is used in the
We can also say: in a Correlated the inner query.
subquery,Inner query condition is
used in the outer query
4 Example: Example:
select e1.empname, e1.basicsal, select empname, basicsal, deptno from
e1.deptno from emp e1 emp
where e1.basicsal = (select where (deptno, basicsal) in (select
max(basicsal) from emp e2 where deptno, max(basicsal) from emp group
e2.deptno = e1.deptno) by deptno)
2.Difference between Weak Entity Set and Strong Entity Set
S.No Weak Entity Set Strong Entity Set
1 An entity set which does not An entity set which does have a
possess sufficient attributes to primary key is called a strong entity
form a primary key is known as a set.
weak entity set.
2 Member of a weak entity set is a Member of a strong entity set is a
subordinate entity. dominant entity.
3 Example: Example:
Specific Set of all
Person,Company,Event,Plant Persons,Companies,Trees,Holidays

2. 3.Difference between char and varchar data types in Sql Server
S.No Char Varchar
1 Fixed length memory storage Variable length memory
storage(Changeable)
2 CHAR takes up 1 byte per VARCHAR takes up 1 byte per
character character, + 2 bytes to hold length
information
3 Use Char when the data entries in Use Varchar when the data entries in a
a column are expected to be the column are expected to vary
same size like phone number considerably in size like address
4 Ex: Ex:
Declare test Char(100); Declare test VarChar(100);
test="Test" - test="Test" -
Then "test" occupies 100 bytes Then "test" occupies only 4+2=6
first four bytes with values and bytes. first four bytes for value and
rest with blank data other two bytes for variable length
information.
4.Difference between Sql Server 2005 and Sql Server 2008
S.No Sql Server 2005 Sql Server 2008
1 XML datatype is introduced. XML datatype is used.
2 Cannot encrypt the entire Can encrypt the entire database
database. introduced in 2008.
3 Datetime is used for both date and Date and time are seperately used for
time. date and time
4 No table datatype is included. Table datatype introduced.
5 SSIS is started using. SSIS avails in this version.
6 CMS is not available. Central Management Server(CMS) is
Introduced.
7 PBM is not available Policy based management(PBM)
server is Introduced.