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SQL / SQL Plus Interview Questions with Answers - Set 1
Posted Date: 06 Feb 2008 Resource Type: Articles/Knowledge Sharing Category: General
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Posted By: Bala Member Level: Diamond
Rating:  Points: 5
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SQL / SQL Plus Interview Questions with Answers
1. How can variables be passed to a SQL routine?
Expected answer: By use of the & symbol. For passing in variables the numbers
1-8 can be used (&1, &2,...,&8) to pass the values after the command into the
SQLPLUS session. To be prompted for a specific variable, place the ampersanded
variable in the code itself:
"select * from dba_tables where owner=&owner_name;" . Use of double
ampersands tells SQLPLUS to resubstitute the value for each subsequent
use of the variable, a single ampersand will cause a reprompt for the
value unless an ACCEPT statement is used to get the value from the user.
2. You want to include a carriage return/linefeed in your output from a SQL script, how can you do this?
Expected answer: The best method is to use the CHR() function (CHR(10) is a return/linefeed) and the concatenation function "||". Another method, although it is hard to document and isn?t always portable is to use the return/linefeed as a part of a quoted string.
3. How can you call a PL/SQL procedure from SQL?
Expected answer: By use of the EXECUTE (short form EXEC) command.
4. How do you execute a host operating system command from within SQL?
Expected answer: By use of the exclamation point "!" (in UNIX and some other OS) or the HOST (HO) command.
5. You want to use SQL to build SQL, what is this called and give an example
Expected answer: This is called dynamic SQL. An example would be:
set lines 90 pages 0 termout off feedback off verify off
spool drop_all.sql
select ?drop user ?||username||? cascade;? from dba_users
where username not in ("SYS?,?SYSTEM?);
spool off Essentially you are looking to see that they know to include a command (in this case DROP USER...CASCADE;) and that you need to concatenate using the ?||? the values selected from the database.
6. What SQLPlus command is used to format output from a select?
Expected answer: This is best done with the COLUMN command.
7. You want to group the following set of select returns, what can you group on?
Max(sum_of_cost), min(sum_of_cost), count(item_no), item_no
Expected answer: The only column that can be grouped on is the "item_no" column, the rest have aggregate functions associated with them.
8. What special Oracle feature allows you to specify how the cost based system treats a SQL statement?
Level: Intermediate to high Expected answer: The COST based system allows the use of HINTs to control the optimizer path selection. If they can give some example hints such as FIRST ROWS, ALL ROWS, USING INDEX, STAR, even better.
9. You want to determine the location of identical rows in a table before attempting to place a unique index on the table, how can this be done?
Level: High Expected answer: Oracle tables always have one guaranteed unique column, the rowid column. If you use a min/max function against your rowid and then select against the proposed primary key you can squeeze out the rowids of the duplicate rows pretty quick. For example: select rowid from emp e where e.rowid > (select min(x.rowid) from emp x where x.emp_no = e.emp_no); In the situation where multiple columns make up the proposed key, they must all be used in the where clause.
10. What is a Cartesian product?
Expected answer: A Cartesian product is the result of an unrestricted join of two or more tables. The result set of a three table Cartesian product will have x * y * z number of rows where x, y, z correspond to the number of rows in each table involved in the join.
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