SQL SERVER Interview Questions-part5
- What are triggers?
Triggers are special kind of stored
procedures that get executed automatically when an INSERT, UPDATE or
DELETE operation takes place on a table.
- How
many triggers you can have on a table?
In SQL Server 6.5 you could define
only 3 triggers per table, one for INSERT, one for UPDATE and one for
DELETE. From SQL Server 7.0 onwards, this restriction is gone, and you
could create multiple triggers per each action.
- How
to invoke a trigger on demand?
In SQL Server 7.0 there's no way to
control the order in which the triggers fire. In SQL Server 2000 you could
specify which trigger fires first or fires last using sp_settriggerorder
Triggers can't be invoked on demand. They get triggered only when an
associated action (INSERT, UPDATE, DELETE) happens on the table on which
they are defined.
- What
are the uses of Triggers?
Triggers are generally used to
implement business rules, auditing. Triggers can also be used to extend
the referential integrity checks, but wherever possible, use constraints
for this purpose, instead of triggers, as constraints are much faster.
Till SQL Server 7.0, triggers fire only after the data modification
operation happens. So in a way, they are called post triggers. But in SQL
Server 2000 you could create pre triggers also. Search SQL Server 2000
books online for INSTEAD OF triggers.
- What
is a self join? Explain it with an example.
Self join is just like any other join,
except that two instances of the same table will be joined in the query.
Here is an example: Employees table which contains rows for normal
employees as well as managers. So, to find out the managers of all the
employees, you need a self join.
- What
is normalization?
The Process of organizing relational
data into tables is actually referred to as normalization.
- What
is a Stored Procedure?
Its nothing but a set of T-SQL
statements combined to perform a single task of several tasks. Its
basically like a Macro so when you invoke the Stored procedure, you
actually run a set of statements.
- Can
you give an example of Stored Procedure?
sp_helpdb , sp_who2, sp_renamedb are a
set of system defined stored procedures. We can also have user defined
stored procedures which can be called in similar way.
- What
is the basic difference between clustered and a non-clustered index?
The difference is that, Clustered
index is unique for any given table and we can have only one clustered
index on a table. The leaf level of a clustered index is the actual data
and the data is resorted in case of clustered index. Whereas in case of
non-clustered index the leaf level is actually a pointer to the data in
rows so we can have as many non-clustered indexes as we can on the db.
- When
do we use the UPDATE_STATISTICS command?
This command is basically used when we
do a large processing of data. If we do a large amount of deletions any
modification or Bulk Copy into the tables, we need to basically update the
indexes to take these changes into account. UPDATE_STATISTICS updates the
indexes on these tables accordingly.
