5 different ways to find TOP N queries

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5 different ways to find TOP N queries

Let us write five different queries to list the top 4 highest paid employees from emp table.

1) Using ROWNUM
2) Using ROW_NUMBER()
3) Using RANK()
4) Using DENSE_RANK()
5) Using MINUS

rownum1

1) Using ROWNUM

select * from (select empno,salary,rownum as rk from emp order by salary desc) where rk < 5;

rownum_topN

2) Using ROW_NUMBER()

select * from (select empno,salary,row_number() over(order by salary desc) as rk from emp) where rk < 5;

rownumber_topN

3) Using RANK()

select * from (select empno,salary,rank() over(order by salary desc) as rk from emp) where rk < 5;

rank_topN

4) Using DENSE_RANK()

select * from (select empno,salary,dense_rank() over(order by salary desc) as rk from emp) where rk < 5;

denserank_topN

Note that dense_rank will return multiple records if there is a tie.

5) Using MINUS

select empno,salary from emp
minus
select empno,salary from (select empno,salary,rownum as rk from emp order by salary desc) where rk > 4;

minus_topN

You can also read – Nth highest salary 

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Rownum and order by in oracle

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ROWNUM

ROWNUM is a psuedocolumn to return the row number of the row in a table. The order will be based on how oracle selects a row from table.

rownum1

Examples with rownum

rownum2

rownum3

Note that rownum will be decided by Oracle itself ( Oracle will assign a number based on how it retrieves from storage/memory)

Order by

Order by is a sql clause which used to order a selected column

Example :-

orderby1

Order by and Rownum together

Why we should use rownum and Order by clause together ? Many experienced developers ( Mainly java developers ) think they can write rownum and order by in same where clause to get the top N query.

For example – To find lowest salary employee developers write like below

select * from emp where rownum = 1 order by salary asc ; —  wrong query

see the result

rownum_orderby_wrong.PNG

Result is wrong.  It should be salary 7500 ( Empno – 14 )

Why ? Oracle applies the ROWNUM first and then applies the order by clause.

See the correct query below

rownum_orderby_correct

So always apply the order by and in next level apply the rownum.

To find a lowest salary employee :-

select * from emp where rownum = 1 order by salary asc ; —  wrong query

select * from (select * from emp order by salary asc) where rownum = 1 — correct query 

Few more tips about rownum

  1. Do not write ROWNUM = 0 ( There is no record having ROWNUM = 0)
  2. Do not write ROWNUM <= 1 instead write ROWNUM = 1
  3. In general Oracle assigns ROWNUM while retrieving the records, it is not based on Primary key column
  4. Do not write ROWNUM > 1, which will no yield any results
  5. ROWNUM and ROW_NUMBER() is not same.

Please comment.

 

 

ROW_NUMBER oracle

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ROW_NUMBER is an example of analytical function. It returns a unique number for each record.

Consider below emp table data

EMPNO EMPNAME SALARY DEPTNO
10 Bill 12000 5
11 Solomon 10000 5
12 Susan 10000 5
13 Wendy 9000 1
14 Benjamin 7500 1
15 Tom 7600 1
16 Henry 8500 2
17 Robert 9500 2
18 Paul 7700 2

ROW_NUMBER()

Select empno, deptno, salary, row_number() over(order by salary) from emp;

ROW_NUMBER assigns a unique number by ordering by salary ascending by default

ROW_NUMBER simple example
ROW_NUMBER simple example

You can use row_number to find top N records

Below query returns first 3rd to 5th records

select * from
(
select empno, deptno, salary, row_number() over(order by salary) as rn from emp
) where rn between 3 and 5
;

ROW_NUMBER() with partition by clause

select empno, deptno, salary,
row_number() over(partition by deptno order by salary)
from emp;

Returns unique number within the window group, ie deptno. Each department it will find the employees based on their salary.

Row_number_partitionby.PNG
ROW_NUMBER with partition by clause

ROW_NUMBER() with NULLS LAST

This ignores the records having salary as null and give least priority for ranking

Select empno, deptno, salary,
row_number() over(order by salary nulls last)
from emp;

ROW_NUMBER with NULLS LAST
ROW_NUMBER with NULLS LAST

Difference between rank and dense_rank Oracle

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What is the difference between RANK and DENSE_RANK functions in Oracle ?

In simple words both does the same except RANK skips if there is a tie and DENSE_RANK does not. So remember to use DENSE_RANK to use in Nth largest queries.

Detailed explanation of RANK and DESNSE_RANK given in another posts

RANK Function

DENSE_RANK Function

Example :-

Consider below emp table data

EMPNO EMPNAME SALARY DEPTNO
10 Bill 12000 5
11 Solomon 10000 5
12 Susan 10000 5
13 Wendy 9000 1
14 Benjamin 7500 1
15 Tom 7600 1
16 Henry 8500 2
17 Robert 9500 2
18 Paul 7700 2

select empno,salary,
rank() over(order by salary asc) “rank”,
dense_rank() over(order by salary asc) “dense_rank”
from emp;

See the result

Difference between RANK and DENSE_RANK
Difference between RANK and DENSE_RANK

RANK function skips number 8 while DENSE_RANK does not.

sys_context Function Oracle and its usage

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SYS_CONTEXT function returns set of defined values within the database(based out of parameters). This can be called from both SQL and PLSQL.

Example:-

SELECT SYS_CONTEXT (‘USERENV’, ‘NLS_DATE_FORMAT’)  FROM DUAL;

return NLS_DATE_FORMAT as DD-MON-RR ( In my session ). USERENV is the parameter

Another example below

SYS_CONTEXT
SYS_CONTEXT

 

Below few list of all SYS_CONTEXT parameters (most important )

Parameter Description
CURRENT_SQL First 4000 bytes of the current SQL.
CURRENT_SQL_LENGTH Current SQL Length
DB_DOMAIN DB Domain
DB_NAME DB Name
HOST Host Name
INSTANCE Instance ID
INSTANCE_NAME Instance Name
IP_ADDRESS IP Address
ISDBA Yes if logged as DBA
LANG Language Country
LANGUAGE Language Detail
MODULE Tool Name
NETWORK_PROTOCOL Eg:- TCP
NLS_CALENDAR Eg:- Gregorian
NLS_CURRENCY Currency
NLS_DATE_FORMAT Date format
NLS_DATE_LANGUAGE Date Language
NLS_SORT Eg:- Binary
NLS_TERRITORY NLS country
OS_USER OS User Name
SERVER_HOST Server Host
SERVICE_NAME Service Name
SESSION_USER Session User Name
SESSION_USERID Session User ID
SESSIONID Session Id
SID Process ID
TERMINAL Client Machine Name

For complete list please read Oracle Documentation

Usage

If I want to create audit table with information like userid, machine name, ip address, SYS_CONTEXT will help. See below sample

Create table emp_audit
(
emp_id number,
create_date timestamp default systimestamp,
created_by varchar2(50) default sys_context (‘userenv’, ‘session_user’),
ipaddress varchar2(50) default sys_context (‘userenv’,’IP_ADDRESS’),
module varchar2(50) default sys_context (‘userenv’,’MODULE’)
);

insert into emp_audit(emp_id) values (1000);

select * from emp_audit;

SYS_CONTEXT Example
SYS_CONTEXT Example

You can see SYS_CONTEXT used in another post Simple Debugging Tool

TO_DATE Oracle

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TO_DATE function in Oracle is the most useful as well as ‘annoying’ function.

TO_DATE function converts string into date data types.

Eg :-

select to_date(‘2016/11/13′,’yyyy/mm/dd’) from dual;

returns  2016/11/13

Note that Oracle converts string ‘2016/11/13’into Date ‘2016/11/13’. Now this value can be inserted into a table having a column date data type.

Note :- Before doing any testing on date columns make sure the default date format settings

For example in my sqldeveloper tool, go to Tools–> Preferences –> Database –> NLS

image7
SQLDEVELOPER date format

Below sql should have returned 2016/11/13, but shows 13-NOV-16 which is in sqldeveloper nls date format. This is the most common mistake developers make.

image8

Alternatively you can use below sql also to change the date format.

alter session set nls_date_format = ‘dd-mon-yyyy’;

The above SQL can be used from SQLPLUS also.

If you do not want to change the nls settings, you can use TO_CHAR function also in required format, see below example.

select to_char(to_date(‘2016/11/13′,’yyyy/mm/dd’),’yyyy/mm/dd’) from dual;

(TO_CHAR and TO_DATE)

image9

Important :-  Oracle stores date data type with time.

date_withtime

Below are the most used formats with TO_DATE

Format Description Example Value
AD AD or BC to_char(to_date(‘2016/11/13 AD’,’yyyy/mm/dd AD’),’AD’) AD
BC AD or BC to_char(to_date(‘2016/11/13 BC’,’yyyy/mm/dd AD’),’AD’) BC
D Day of week (1 7) to_char(to_date(‘2016/11/13′,’yyyy/mm/dd’),’D’) 1
DAY Name of day to_char(to_date(‘2016/11/13′,’yyyy/mm/dd’),’DAY’) SUNDAY
DD Day of month (1 31) to_char(to_date(‘2016/11/13′,’yyyy/mm/dd’),’DD’) 13
DDD Day of year (1 366) to_char(to_date(‘2016/11/13′,’yyyy/mm/dd’),’DDD’) 318
DY Abbr name of day to_char(to_date(‘2016/11/13′,’yyyy/mm/dd’),’DY’) SUN
HH Hour of day (1 12) to_char(to_date(‘2016/11/13 01:12:35′,’yyyy/mm/dd hh:mi:ss’),’HH’) ‘01’
HH12 Hour of day (1 12) to_char(to_date(‘2016/11/13 04:12:35 PM’,’yyyy/mm/dd hh:mi:ss PM’),’HH12′) ‘04’
HH24 Hour of day (0 23) to_char(to_date(‘2016/11/13 04:12:35 PM’,’yyyy/mm/dd hh:mi:ss PM’),’HH24′) 16
IW Week of year to_char(to_date(‘2016/11/13′,’yyyy/mm/dd’),’IW’) 45
J Julian day to_char(to_date(‘2016/11/13′,’yyyy/mm/dd’),’J’) 2457706
MI Minute (0 59) to_char(to_date(‘2016/11/13 01:12:35′,’yyyy/mm/dd hh:mi:ss’),’MI’) 12
MM Month (01 etc) to_char(to_date(‘2016/11/13 01:12:35′,’yyyy/mm/dd hh:mi:ss’),’MM’) 11
MON Abbr month to_char(to_date(‘2016/11/13 01:12:35′,’yyyy/mm/dd hh:mi:ss’),’MON’) NOV
MONTH Name of month to_char(to_date(‘2016/11/13 01:12:35′,’yyyy/mm/dd hh:mi:ss’),’MONTH’) NOVEMBER
AM AM or PM to_char(to_date(‘2016/11/13 01:12:35 AM’,’yyyy/mm/dd hh:mi:ss AM’),’AM’) AM
PM AM or PM to_char(to_date(‘2016/11/13 01:12:35 PM’,’yyyy/mm/dd hh:mi:ss AM’),’AM’) PM
Q Quarter of year to_char(to_date(‘2016/11/13 01:12:35 AM’,’yyyy/mm/dd hh:mi:ss AM’),’Q’) 4
RRRR Round year to_char(to_date(‘2016/11/13 01:12:35 AM’,’yyyy/mm/dd hh:mi:ss AM’),’RRRR’) 2016
SS Second (0 59) to_char(to_date(‘2016/11/13 01:12:35 AM’,’yyyy/mm/dd hh:mi:ss AM’),’SS’) 35
WW Week of year (1 53) to_char(to_date(‘2016/11/13 01:12:35 AM’,’yyyy/mm/dd hh:mi:ss AM’),’WW’) 46
W Week of month (1 5) to_char(to_date(‘2016/11/13 01:12:35 AM’,’yyyy/mm/dd hh:mi:ss AM’),’W’) 2
YYYY Last 4 digit(s) of year to_char(to_date(‘2016/11/13 01:12:35 AM’,’yyyy/mm/dd hh:mi:ss AM’),’YYYY’) 2016
YY Last 2 digit(s) of year to_char(to_date(‘2016/11/13 01:12:35 AM’,’yyyy/mm/dd hh:mi:ss AM’),’YY’) ‘16’

Always check below before you start using TO_DATE function

  1. NLS_DATE_FORMAT ( SELECT sys_context (‘userenv’,’NLS_DATE_FORMAT’) FROM DUAL )

or

2. use TO_CHAR for formating

Also read TO_CHAR Function

10 must-know Oracle Interview questions

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Interviewing a candidate for a particular job is a tedious task, there are many factors which effect your selection. Managers has to decide within short time span with minimum questions.So if we know the most possible or probable questions before interview we can be prepared. Here 10 must-know Interview questions ( there is no ranking )

1. What is the difference between Procedure and Function ?

Answer:-

1) Function must return a value and procedure need not.

2) Function can be used in SQL with some restrictions. Procedure cannot be called directly from SQL.

2. What is the difference between DELETE and TRUNCATE ?

Answer:-

1) DELETE is a DML command and TRUNCATE is a DDL command.

2) TRUNCATE re-set the memory blocks after execution and much faster than DELETE in most of the circumstances.

3. What is the difference between PRIMARY KEY and UNIQUE KEY constraints ?

Answer:-

1) UNIQUE KEY columns can have null values but PRIMARY KEY column cannot accept null values.

2) A table can have only one PRIMARY KEY column (composite primary key is ok) but many UNIQUE KEY columns allowed.

4. What is Cartesian Product ?

Answer:-

1) If two or more tables are joining without join condition will result into Cartesian products.

2) If table A has 2 rows and table B has 4 rows then Cartesian product between A and B will return 8 rows ( 2 multiply by 4 )

5. What is a Transaction ?

Answer:-

1) Transaction is a logical unit of work which will end in a consistent status

2) In Oracle we can control transactions using Save points, Commit, rollback etc

6. What is a Package, why do we go for packages ?

Answer:-

1) Package is a collection of one more program units having Procedure, Function etc

2) Depends on business logic, all related program units can be tied into a single package for better maintenance, readability etc

7. What is triggers, where we use triggers ?

Answer:-

1) Procedural code which automatically executes based on an event like insert,update, delete etc

2) Auditing, Logging etc

8. How to do sql performance tuning ?

Answer:-

1) Basic sql performance starts with EXPLAIN PLAN

2) Explain cost, index, access methods, plan etc

9. What is the most challenging/difficult feature in Oracle ?

Answer:-

1) This is the most tricky question, answer based on your experience.

2) Do not answer with a trivial problem, try something advanced feature

10. What is a materialized view ?

Answer:-

1) A materialized view, also called as snapshot,which contains the results of a query.

2) Generally used to create summary tables based on aggregations of a table data.

Please note that I have given the short description of the answers. One question answer may trigger another question. So be prepare the Oracle fundamentals deeply before the interview.

Also read Oracle Interview Questions and Answers