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.