Oracle Questions and Answers

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# What is Pseudo Colomn ?
Pseudo Colomn are : USER, UID, SYSDATE, ROWNUM, ROWID, NULL, AND LEVEL

# How to Display output in PL/SQL ?
we first set serveroutput on at SQL Prompt. and then in sql programming we specify as dbms_output.put_line(`hdjhd' `sakdha'); No Displays are allowed in PL/SQL programming with select statement

# Write about Commands and Keywords ?
Alter : ALTER TABLE EMP ADD .......... , MODIFY .......... ;
for a in 1..10 loop ........... end loop;
loop exit when a<1>>..............
exit, rollback, commit, savepoint. References: This keyword is used to refer two similar fields of different tables.


# What are different built in functions ?
CONCATENATE, INITCAP, LENGTH, LOWER, UPPER, LPAD, RPAD, LTRIM (removes the character specified left side of word), RTRIM, SUBSTR (displays values from specified location onwards), TRANSLATE (for single character), REPLACE (for more than one character ),CHR ( prints character of given ASCII value), ASCII (value of given character ). ABS, CEIL, FLOOR,SQRT, POWER, SIGN, TRUNC, ROUND, EXP, MOD, LOG, LN, GREATEST, LEAST, SYSDATE, ADD_MONTHS, MONTHS_BETWEEN.

# what are emp.sal%type and emp%rowtype ?
First one is to assign a variable of type sal only.
Second is to assign a complete row of type emp table.

# What are constraints ?
Constraint Clause
: This will constrain a single column or a group of columns in a table. and are used for maintaining integrity of the database. Different Constraints are : candidate keys, primary keys, foreign keys, check conditions. Can specify as a part of column definition, or at the end of the table (if more than one column).

Check Constraint : for validations to fields.

Naming Constraint (table constraints): Can name the constraint else system will name it by default.

# Different types of joins ?
Equi join : where emp.deptno = dept.deptno; Non Equi join : where e.sal >= g.losal; Outer join :***********e.deptno(+)=d.deptno;

# Different types of Keys ?
Foreign Key
: This is a referential integrity constraint and specifies the values of primary key in other table. and is mentioned as : DEPTNO number(2) REFERENCES DEPT(DEPTNO); The clause ON DELETE CASCADE added to REFERENCES clause tells Oracle to delete the dependent rows when we delete the row in the parent table.
Unique Key : If declared, then does not accept duplicate and also NULL values. And can have many Unique key fields.
Primary Key : Similar to Unique key but, also maintains an index on primary key and also used to connect two tables.

# Write about cursors ? Implicit (SQL is implicit by default ) and Explicit cursors.

EXIT WHEN C1%NOTFOUND, IF C1%FOUND THEN, IF C1%ISOPEN THEN, FOR LOOP in CURSOR will :
1) open the cursor.
2) fetches the records from cursor one by one.
3) keeps the track of number of records in the cursor.

Super Dynamism in Cursors
is obtained by passing the values as parameters( ie pass by value) ie., for x in c1(a,b) Multiple Cursors : These are nothing but nested cursors........... In Implicit Cursors there is no need of declaration of cursors, by default system will create a cursor whenever a query is written...and the conditions are like if SQL%NOTFOUND if SQL%ROWCOUNT > 2 will keep a count of the number of records updated. SQL is a keyword and cannot be used to name as a cursor name explicitly.

# Difference between group functions and single row functions ?
Group Function Single Row Function A group function operates A single row function on many rows returns one and result for one row. returns single result. Not allowed in Pl/sql procedural Allowed in Pl/Sql Procedural statements eg SUM(),AVG,MIN,MAX etc eg UPPER,LOWER,CHR...

# Difference between DECODE and TRANSLATE ?
DECODE is value by value TRANSLATE is character by character replacement. replacement. Ex SELECT DECODE('ABC','A',1,'B',2,'ABC',3) eg SELECT from dual; o/p 3 TRANSLATE('ABCGH', 'ABCDEFGHIJ', 1234567899) FROM DUAL; o/p 12378 (DECODE command is used to bring IF,THEN,ELSE logic to SQL.It tests for the IF values(s) and then aplies THEN value(s) when true, the ELSE value(s) if not.)

# Difference between TRUNCATE and DELETE ?
TRUNCATE deletes much faster than DELETE Truncate Delete It is a DDL statement It is a DML statement It is a one way trip,cannot One can Rollback ROLLBACK Doesn't have selective features (where clause) Has Doesn't fire database triggers Does It requires disabling of referential Does not require constraints.

# What are various constraints used in SQL ?
NULL, NOT NULL, CHECK, DEFAULT What are different Oracle database objects ? TABLES, VIEWS, INDEXES, SYNONYMS, SEQUENCES, TABLESPACES etc

# What is difference between Rename and Alias ?
Rename is a permanent name given to a table or column whereas Alias is a temporary name given to a table or column which do not exist once the SQL statement is executed.

# What is a CO-RELATED SUBQUERY ?
A CO-RELATED SUBQUERY is one that has a correlation name as table or view designator in the FROM clause of the outer query and the same correlation name as a qualifier of a search condition in the WHERE clause of the subquery. eg SELECT field1 from table1 X WHERE field2>(select avg(field2) from table1 Y where field1=X.field1); (The subquery in a correlated subquery is revaluated for every row of the table or view
named in the outer query.)

# What are various joins used while writing SUBQUERIES ?
Self join-Its a join foreign key of a table references the same table. Outer Join--Its a join condition used where One can query all the rows of one of the tables in the join condition even though they don't satisfy the join condition. Equi-join--Its a join condition that retrieves rows from one or more tables in which one or more columns in one table are equal to one or more columns in the second table.

# What is a view ?
A view is stored procedure based on one or more tables, it's a virtual table.

# What are various privileges that a user can grant to another user ?
SELECT, CONNECT, RESOURCES

# What is difference between UNIQUE and PRIMARY KEY constraints ?
A table can have only one PRIMARY KEY whereas there can be any number of UNIQUE keys. The columns that compose PK are automatically define NOT NULL, whereas
a column that compose a UNIQUE is not automatically defined to be mandatory must also specify the column is NOT NULL.

# Can a primary key contain more than one columns ?
Yes

# How you will avoid duplicating records in a query ?
By using DISTINCT

# What is difference between SQL and SQL*PLUS ?
SQL*PLUS is a command line tool where as SQL and PL/SQL language interface and reporting tool. Its a command line tool that allows user to type SQL commands to be executed directly against an Oracle database. SQL is a language used to query the relational database(DML,DCL,DDL). SQL*PLUS commands are used to format query result, Set options, Edit SQL commands and PL/SQL.

# Which datatype is used for storing graphics and images ?
LONG RAW data type is used for storing BLOB's (binary large objects).

# How will you delete duplicating rows from a base table ?
DELETE FROM table_name A WHERE rowid>(SELECT min(rowid) from table_name B where B.table_no=A.table_no); CREATE TABLE new_table AS SELECT DISTINCT * FROM old_table; DROP old_table RENAME new_table TO old_table DELETE FROM table_name A WHERE rowid NOT IN (SELECT MAX(ROWID) FROM table_name GROUP BY column_name)

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