oracle sql questions

8/8/2019 Oracle SQL Questions

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# ORACLE SQL QUESTIONS Answers

1 What is a DBA? A DBA is a Database Administrator responsible for tuning,

maintenance, setup, and design of a database.

2 What is dual? Dual is a table with one column called dummy whose data typeis VARCHAR2(1). Dual acts like a parrot; If you select user

from dual; it will give you the ORACLE user you are logged inas. If you select a number from dual you will get the same

number back. If you select a character array from dual, you

get the same character array back.

sqlplus bob/bob

SQL> select 40 from dual; [40]

SQL> select 'polly want a cracker' from dual; [polly want acracker]

SQL> select user from dual; [BOB]

SQL> select SYSDATE from dual; [15-APR-98]SQL> exit

3 What is an object in ORACLE? An object in ORACLE can be a table, view, package, package body, index, constraint, sequence, etc . . .

4 a)How do you see what tables

are owned by the user_id you

logged onto ORACLE with? b) How can I see other things?

c) Can I see other user's

objects?

a) select * from user_tables;

b) select * from cat;

select * from user_objects;

select * from user_sequences;

select * from all_tables where user = 'BOB'

select * from user_views;

c) In most cases you can see other peoples items. Sometimes if

information is not being shared you will not be able to tell whatis in another person's account.

select * from all_tables;

select * from all_objects;

5 What is a VIEW? A view is a query which when called looks like a table. It is not

a table. To see what views exists you can issue the following

SQL statement:

select * from user_views;. Or

select * from all_views where owner = 'BOB'; user_views,

all_tables, user_tables, all_sequences, user_sequences,user_tab_columns, all_tab_columns etc. . . are all VIEWS.

6 How do I tell what the columns

in a view or a table are?desc view_or_table_name;

7 What is a schema? The set of objects owned by a user account is called the user's

schema.

8 What is an index? An index is used on one or more columns on a table to decreasethe time it takes to find certain value(s).



9 Why might an index increase

the amount of time it takes to do

an insert, update, or delete.

An index may increase the time it takes to do an insert, update,

or delete if the index itself needs to be updated because of the

DML (data manipulation language) issued.

10 What is a constraint? When a constraint is applied to a table, every row in the tablemust satisfy the conditions specified in the constraint

definition. Examples of types of constraints are NOT NULL,UNIQUE, DEFAULT, and CHECK.

11 How do you see what tablefields are used in indexes or

constraints?

select index_owner, index_name, column_name,

column_position from all_ind_columns where table_name =

'MLOAN'; select owner, table_name, constraint_name,

column_name, position from all_cons_columns; The positionindicates what order the column_name appears in the index or

constraint. The position is important because the order of the

columns determines how SQL statements should be written. Itis important to note that the positions of columns in an index

can change (the DBA may decide to do this).

12 What is all_tab_columns or

user_tab_columns?

All_tab_columns is a view that has an entry for every column

of every table of every users schema. User_tab_columns is aview that has an entry for every column of every table in a

certain user's schema. When you use desc on a table to show

the columns of that table the order of the columns shown is

determined by the position that you can see when you look atthe column_id in all_tab_columns and/or user_tab_columns.

The DBA may change the order of these columns. It is very

important not to assume that the columns are in a certain order and will stay that way.

13 Why do indexes and constraintshave the same names?

select * from all_constraints c, all_indexes i where i.owner= c.owner and i.table_owner = c.owner and index_name =

constraint_name and i.table_name = c.table_name; AUNIQUE CONSTRAINT, or NOT NULL CONSTRAINT by

definition is an index; even though it is enforcing referential

integrity, it also provides a fast method of looking up certainvalues in certain columns in a table.

14 What does it mean if an

index_name or constraint_name

has a "_PK_" in it?

It means that this index or constraint is on one or more columns

of a table and it represents the primary key(s).

15 What is a primary key? The primary key of a table is the column or set of columns thatmakes every row in that table unique.

16 What is a foreign key? A foreign key from one table references a primary key in

another table.

17 a) What is a sequence?

b) How do you obtain the nextvalue from a sequence?

c) How do you create a

a)A sequence is like a ticket dispenser in a bakery. It usually

dispense tickets starting with 1 and progressing with 2 , 3 , 4and so on. This progression can either be ascending or

descending. In the above example the increment is 1. Like the



sequence?

d) Can you modify a sequence

once you have created it?e) How do I get rid of a

sequence.

dispenser, a sequence will not give the same number to one or

more people. (okay, Dispensers get new tapes and start over again with 1).

b) So, you want the next value from a sequence? The SQL

command is

SELECT sequence_name.nextval from dual; Once you havecalled for the next value from a sequence, you can use get the

current value;

SELECT sequence_name.currval from dual; CURRVAL can

not be used before NEXTVAL in any session.c)create sequence ticket_giver; Go ahead try it!!! If you want

to see where your sequence information is kept you can query

the VIEW user_sequences or all_sequences:

select * from user_sequences;

d) In most cases the answer to this question is No. You would

have to drop the sequence and recreate it with the new

attributes. ; In ORACLE versions greater than 7.3.3 you mayuse the alter sequence sql statement to change some attributes

of a sequence.

e) In most cases your Database Administrator would not wantyou dropping sequences that he or she has created. However, if

you create a sequence you may drop it. Drop sequence

ticket_giver; or Drop sequence sequence_name;.

18 What is a trigger? A trigger is like a bomb. It is specified to go off when certainevents happen. A trigger may be set to check values for an

insert into a row of data and alter them or cause an error. A

trigger may be specified to log changes to tables or user logons,

just about anything.

19 Is there an ORACLE command

available through the operating

system can tell you the meaningof an error number. What is it?

Yes, on UNIX you can use the command: oerr ora #. Although

ORACLE errors sometimes appear to be negative numbers,

they are positive numbers!

20 What can you tell me about

EXP and IMP commands?

You can call exp and imp respectively from the AIX command

line to export and import data from the database. For more

information about these commands type either exp help=y or imp help=y. These commands are extremely

useful for saving data sets for testing certain conditions (like

proving that an error in a program has been fixed.) Exportsinclude sequences, privledges, views, tables, packages,

indexes, constraints, grants, database links, and table_data. You

can choose to export a table definition and not the table data or to export all the tables or just one. Imports can be used to

transfer tables, etc. from one user to another.

21 What are some basic Data

Manipulation Commands

INSERT UPDATE TRUNCATE DELETE CREATE DROP,

etc . . .



(DML)?

22 What does DROP do? It removes an object from the database such as a table,

sequence, view, package, etc. . . Syntax: drop table

table_name; drop sequence sequence_name;

23 What does ALTER do? Alter allows you to add to or modify a database object.

24 Is ORACLE SQL case sensitive

or not?

Trick question. When you access views you have to give field

values in uppercase like

select * from all_tables where table_name =

'TICKET_TAKER';. When you are issuing a data modification

language command you can reference a table in any case that

you want. Data is case sensitive.

25 What is the difference betweena select and a bind variable?

select variables are retrieved and bind variables are certainvalues you specifiy in the where clause.

26 How do you tell what date and

time an ORACLE system thinks

it is?

select sysdate from dual;

27 If you have a large number of rows in a table and you just

need to know the unique values

of a column what SQLstatement would you use?

select distinct (some_column) from some_table;

28 What query would you execute

if you wanted to know

how many times each valueappears in the database?

select some_column, count(*) from some_table group by

some_column;

29 What if you just wanted toknow what value appeared

the most times in a specific fieldin the database?

select some_column, count(*) from some_table group bycount(*) desc where rownum < 2;

30 What if you wanted to know

what values appear

more than 15 times in a table?

select some_column, count(*) from some_table group by

count(*) having count(*) > 15;

31 What is something veryimportant to remember about

using GROUP BY?

Your group by items must be in the exact order of the selectitems.

32 What query would you use if you wanted the minimum datavalue in a select field in a table?

select min(some_column) from some_table;

33 What is wrong with the

following query:

select min(some_column),

some_other_column from

The value in the some_other_column field may not come from

the same row as the minimum value.



some_table where something =

:something;

34 What about

select min(some_column),

max(some_other_column)

from some table where something = something;

There is nothing wrong with this query.

35 If you wanted to convert thecharacter string '23-JAN-98'

into an ORACLE variable of

type DATE what commandwould you use?

to_date('23-JAN-98','DD-MON-YY')

36 If you wanted a get a date from

the database in a character array

format what command would

you use? Specifically the format

Year month day hour min secand am or pm.

To_char(date,'YYYYMMDD:HH.MI.SS A.M.')

37 What if you wanted to some all

of the positive integer values

from some column on sometable

select sum(some_field) from some_table where some_field >

0;

38 What is an outer join and how is

it used?

To select columns from two or more tables whether or not data

exists in one of the tables you are querying against.

39 What is dynamic SQL? Is itgood or bad? Why is it good or bad?

Dynamic SQL is good only if it does not fill up the SQLAREA.If bind variables are stored as :variable it is okay.

40 Why do we give a table another

name like

select * from mloan m?

This is done for many reasons:

One reason is that we may use a table more than once in a

query and the references may be distinguished by assigningdifferent letters to them:

select a.ltv_ratio, b.ltv_ratio from mloan a, mloan b where . . .

.

Another reason is that we may need to reference a table in the

SELECT portion in an EXISTS clause:

select a.sale_typ from mloan a where exists (select 'X' fromloan l where l.account_nbr = a.account_nbr.)

But the third reason is the most important, we do not want to

fill up the SQLAREA.

41 What is the SQLAREA and

why is it so important?

The SQLAREA is an important part of the database. Every

time a query is issued it is stored in the SQLAREA unless itmatches exactly to a query already in the SQLAREA letter by

letter case-sensitively. The only way to clear the SQLAREA is



to flush it (The DBA does this.)

Using a one character letter to reference a table takes up less

room in the SQLAREA.

Why do we want to use the least amount of letters possible?

Because if a query is already in the SQLAREA it can be re-used and it will take less time to execute because it has already

been run and because it has already been parsed and stored in

the SQLAREA. It is ideal to have as many re-usable queries in

the SQLAREA as possible. When the SQLAREA fills up,newer queries start writing over older queries.

ORACLE version 7.3 will CRASH if the SQLAREA isconstantly getting written over and over again and

consequently becomes fragmented; flushing the area on a

regular basis will not help. It is ORACLE's position that if the

SQLAREA is written over too often, let us say by issuing50,000 almost identical SQL queries that this is not a bug, but a

bad program design.

42 What keyword do you need todrop a table when other tables

rely on it.

Cascade

43 Do null values in certain

columns in a table effect query performance times?

YES - they increase the time it takes to perform a query!!

44 How do you concatenate two

values in SQL?

|| (double pipes) concat is not valid.

select user || sysdate from dual; [BOB15-APR-98] select user || ' foofoo ' || sysdate from dual; [BOB foofoo 15-APR-98]

45 What is rownum? ROWNUM can be used to select , delete, update some number

of rows minus one from a table. An order by clause may not be

used with ROWNUM. ROWNUM must always be specified as being greater than 1. You can not where rownum > 3; It will

not work. Update mloan set sale_typ = 'C' where rownum <

500; Updates 499 rows in no specific order. Sometimes theorder will be the order that the rows were entered into the

database.

46 What is the best manner todelete A LARGE NUMBER OFROWS from a table?

In small portions using rownum if necessary. Deleting a largenumber of rows at one time takes up ORACLE resources andcan increase the time it takes to delete the rows (as compared to

the time it would take to delete 500 rows at a time). Issuing a

commit statement after every delete will free up some of the

ORACLE resources and make further deletes faster.

47 What is commit? Why do we Commit saves the current database state. When you issue



use it? certain ORACLE commands other users can not see the

changes until you issue the commit command or exit the SQL

session you have opened.If you do not issue a commit and you make a mistake, (as long

as you do not truncate or drop a table) you can issue a rollback

command and the database will be restored to the point it wasat when you started the SQL session or issued the last commit.

To restore the database to this state, use the rollback command:

rollback;

You can also specify a commit point to rollback to by doingcommit savepoint1; then if you need to you can issue the

command rollback savepoint1;

48 a)What is the fastest way to

delete ALL OF THE ROWS ina table?

b)Is this recoverable? (rollback)

a)Truncate table some_table; This command is

unrecoverable. It removes all rows from a table. b) NO.

49 What is the fastest query you

can execute for just checking tosee if a value exists in a certain

column on a certain table?

select 'X' from dual where exists (select 'X' from some_table

where some_column = :some_value)

50 What is that 'X' thing? 'X' represents TRUE or FALSE. You could replace 'X' with

'apple' or 'foobar' or some other entity as long as it has singlequotes around it and it is not an ORACLE keyword.

51 Why do you select that 'X' thing

from dual?

You select 'X' from dual when you already have all of the table

values selected that you need.

52 When is it appropriate to use the

ORACLE keyword UNIQUE?

It is only appropriate to use the ORACLE keyword UNIQUE if

you are creating a unique index; one that will only allow theuse of different values in a column on a table. DO NOT USE

UNIQUE in place of DISTINCT!

53 Can you use GROUP BY andDISTINCT in the same

SIMPLE query?

NO. There is not reason to. I say SIMPLE query because youcan use the terms in different sub-clauses in the same query

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