SQL Database Queries
Today I
am going to share some basic pointers related SQL Database which comes as handy
concepts for a tester:
1. What is the difference between a
"where" clause and a "having" clause?
"Where"
is a kind of restriction 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.
2. 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.
3. 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 …)
4. 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.
5. What is a "join"?
‘join’ used to connect two or more tables
logically with or without common field.
6. 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.
7. 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- 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.
8. 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 thestudentdaily.com 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.
9. 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.
10. 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.
11. 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
12. 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.
13. 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
Happy
Testing!!!!
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