Questions and Answers :
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
(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 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 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
Can a primary key contain more than one columns ?
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 ?
data type is used for storing BLOB’s (binary large objects).
How will you delete duplicating rows from a base table ?
table_name A WHERE rowid>(SELECT min(rowid) from table_name B where
new_table AS SELECT DISTINCT * FROM old_table;
new_table TO old_table
DELETE FROM table_name A
WHERE rowid NOT IN (SELECT MAX(ROWID) FROM table_name
GROUP BY column_name)
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 (2
occurence of ‘-‘)
There is a string ‘120000 12 0 .125’ ,how you will find the position of the decimal place ?
INSTR(‘120000 12 0 .125′,1,’.’)
There is a ‘%’ sign in one field of a column. What will be the query to find it ?
‘\’ Should be used before ‘%’.
When you use WHERE clause and when you use HAVING clause ?
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.