Oracle Interview Questions

1. 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.)
2. 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.
3. What are various constraints used in SQL
NULL NOT NULL CHECK DEFAULT
4. What are different Oracle database objects
TABLES VIEWS INDEXES SYNONYMS SEQUENCES TABLESPACES etc
5. 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.
6. What is a view
A view is stored procedure based on one or more tables, its a virtual table.
7. What are various privileges that a user can grant to another user
SELECT CONNECT RESOURCE
8. 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.
9. Can a primary key contain more than one columns
Yes
10. How you will avoid duplicating records in a query
By using DISTINCT
11. 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.
12. Which datatype is used for storing graphics and images
LONG RAW data type is used for storing BLOB's (binary large objects).
13. 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)

14. What is difference between SUBSTR and INSTR
SUBSTR returns a specified portion of a string eg SUBSTR('BCDEF',4) output BCDE INSTR provides character position in which a pattern is found in a string.
eg INSTR('ABC-DC-F','-',2) output 7 (2nd occurence of '-')
15. There is a string '120000 12 0 .125' ,how you will find the position of the decimal place
INSTR('120000 12 0 .125',1,'.') output 13
16. There is a '%' sign in one field of a column. What will be the query to find it.
'\' Should be used before '%'.
17. When you use WHERE clause and when you use HAVING clause
HAVING clause is used when you want to specify a condition for a group function and it is written after GROUP BY clause The WHERE clause is used when you want to specify a condition for columns, single row functions except group functions and it is written before GROUP BY clause if it is used.
18. Which is more faster - IN or EXISTS
EXISTS is more faster than IN because EXISTS returns a Boolean value whereas IN returns a value.
Appropriate answer will be....
Result of the subquery is small Then "IN" is typicaly more appropriate. and Result of the subquery is big/large/long Then "EXIST" is more appropriate.
19. What is a OUTER JOIN
Outer Join--Its a join condition used where you can query all the rows of one of the tables in the join condition even though they dont satisfy the join condition.
20. How you will avoid your query from using indexes
SELECT * FROM emp Where emp_no+' '=12345;
i.e you have to concatenate the column name with space within codes in the where condition.
SELECT /*+ FULL(a) */ ename, emp_no from emp where emp_no=1234;
i.e using HINTS