1. What are two
methods of retrieving SQL?
2. What cursor type
do you use to retrieve multiple recordsets?
3.
What is the difference between a
"where" clause and a "having" clause?
"Where" is a kind of restiriction statement. You use where
clause to restrict all the data from DB.Where clause is using before result
retrieving. But Having clause is using after retrieving the data.Having clause
is a kind of filtering command.
4.
What is the basic form of a SQL statement to
read data out of a table?
The basic form to read data out of table is ‘SELECT * FROM table_name; ‘
An answer: ‘SELECT * FROM table_name WHERE xyz= ‘whatever’;’ cannot be called
basic form because of WHERE clause.
5.
What structure can you implement for the
database to speed up table reads?
Follow the rules of DB tuning we have to: 1] properly use indexes (
different types of indexes) 2] properly locate different DB objects across
different tablespaces, files and so on.3] create a special space (tablespace)
to locate some of the data with special datatype ( for example CLOB, LOB and …)
6.
What are the tradeoffs with having indexes?
1. Faster selects, slower updates. 2. Extra storage space to store
indexes. Updates are slower because in addition to updating the table you have
to update the index.
7.
What is a "join"?
‘join’ used to
connect two or more tables logically with or without common field.
8. What is "normalization"?
"Denormalization"? Why do you sometimes want to denormalize?
Normalizing data
means eliminating redundant information from a table and organizing the data so
that future changes to the table are easier. Denormalization means allowing
redundancy in a table. The main benefit of denormalization is improved
performance with simplified data retrieval and manipulation. This is done by
reduction in the number of joins needed for data processing.
9. What is a "constraint"?
A constraint allows you to apply simple
referential integrity checks to a table. There are four primary types of
constraints that are currently supported by SQL Server: PRIMARY/UNIQUE -
enforces uniqueness of a particular table column. DEFAULT - specifies a default
value for a column in case an insert operation does not provide one. FOREIGN
KEY - validates that every value in a column exists in a column of another
table. CHECK - checks that every value stored in a column is in some specified
list. Each type of constraint performs a specific type of action. Default is
not a constraint. NOT NULL is one more constraint which does not allow values
in the specific column to be null. And also it the only constraint which is not
a table level constraint.
10. What types of index data structures can you
have?
An index helps to
faster search values in tables. The three most commonly used index-types are: -
B-Tree: builds a tree of possible values with a list of row IDs that have the
leaf value. Needs a lot of space and is the default index type for most
databases. - Bitmap: string of bits for each possible value of the column. Each
bit string has one bit for each row. Needs only few space and is very
fast.(however, domain of value cannot be large, e.g. SEX(m,f);
degree(BS,MS,PHD) - Hash: A hashing algorithm is used to assign a set of
characters to represent a text string such as a composite of keys or partial
keys, and compresses the underlying data. Takes longer to build and is
supported by relatively few databases.
11.
What is a "primary key"?
A PRIMARY INDEX or PRIMARY KEY is something
which comes mainly from database theory. From its behavior is almost the same
as an UNIQUE INDEX, i.e. there may only be one of each value in this column. If
you call such an INDEX PRIMARY instead of UNIQUE, you say something about
your table design, which I am not able to explain in few words. Primary
Key is a type of a constraint enforcing uniqueness and data integrity for each
row of a table. All columns participating in a primary key constraint must
possess the NOT NULL property.
12.
What is a "functional dependency"?
How does it relate to database table design? - Functional dependency
relates to how one object depends upon the other in the database. for example,
procedure/function sp2 may be called by procedure sp1. Then we say that sp1 has
functional dependency on sp2.
13.
What is a "trigger"?
Triggers are stored
procedures created in order to enforce integrity rules in a database. A trigger
is executed every time a data-modification operation occurs (i.e., insert,
update or delete). Triggers are executed automatically on occurance of one of
the data-modification operations. A trigger is a database object directly
associated with a particular table. It fires whenever a specific statement/type
of statement is issued against that table. The types of statements are
insert,update,delete and query statements. Basically, trigger is a set of SQL
statements A trigger is a solution to the restrictions of a constraint. For
instance:
1.A database column cannot carry
PSEUDO columns as criteria where a trigger can.
2. A database constraint cannot refer old and new values for a row where
a trigger can.
14. Why can a "group by" or "order
by" clause be expensive to process?
Processing of "group by" or
"order by" clause often requires creation of Temporary tables to
process the results of the query. Which depending of the result set can be very
expensive.
15. What is
"index covering" of a query?
Index covering means that "Data can be found only using indexes,
without touching the tables"
16. What types of
join algorithms can you have?
17. What is a SQL view?
An output of a query can be stored as a view.
View acts like small table which meets our criterion. View is a precomplied SQL
query which is used to select data from one or more tables. A view is like a
table but it doesn’t physically take any space. View is a good way to present
data in a particular format if you use that query quite often. View can also be
used to restrict users from accessing the tables directly.
