Oracle Interview question : what is dual table in oracle

Summary

This video series focuses on SQL interview questions for Oracle developers, specifically addressing the 'DUAL' table concept. The DUAL table, a dummy table with a single column 'DUMMY' and value 'X', is essential for completing SQL syntax when no data retrieval is needed. The video provides examples for beginners and intermediates, explaining the DUAL table's use in evaluating expressions, user-defined functions, and complex queries. It also discusses creating custom tables similar to DUAL and emphasizes the table's performance optimization and universal accessibility. The script concludes with examples of using DUAL for sequence columns, decode statements, and creative SQL queries, inviting viewers to engage with the content.

Takeaways

• 📘 The DUAL table in Oracle is a built-in dummy table created during Oracle installation and is available in the SYS schema.
• 🔑 DUAL has one column named 'DUMMY' with a data type of VARCHAR2(1), and it contains a single value 'X'.
• 💡 For beginners, the DUAL table is used to evaluate expressions or to compute information without selecting from a user-defined function.
• 🔍 Experienced users should explain the DUAL table with examples, emphasizing its role in completing SQL syntax.
• 🛠️ The DUAL table is particularly useful in Oracle when writing a SELECT statement that does not need to retrieve information from any other table.
• 📊 Use cases of DUAL include computing simple expressions like '1 + 2', finding the square root of a number, or using string functions like SUBSTRING.
• 👤 The SELECT USER FROM DUAL statement can be used to retrieve the current user's name without needing to access any other table.
• 🔄 Users can create their own table similar to DUAL, but Oracle's DUAL table is optimized for performance and does not require any permissions.
• 🔢 DUAL is used in PL/SQL for sequence-related pseudo columns like NEXTVAL and CURRVAL, and for executing DECODE statements.
• 📚 The transcript provides various examples of using DUAL, such as printing multiplication tables and creating pyramid structures for strings.
• 💌 The video encourages viewers to ask questions in the comments or via email, and mentions a blog with compiled questions for further learning.

Q & A

What is the purpose of the DUAL table in Oracle?

The DUAL table in Oracle is a dummy table created during installation, available in the SYS schema, and is used to complete the syntax of a SELECT statement when no actual data retrieval from a table is needed.

What is the structure of the DUAL table in Oracle?

The DUAL table has only one column named 'DUMMY' with a data type of VARCHAR2(1), and it contains a single value 'X'.

How can the DUAL table be used by beginners in SQL?

Beginners can use the DUAL table for evaluating expressions or computing results without the need to select from any other table, such as computing simple interest or finding the square root of a number.

What is the significance of the DUAL table for intermediate users?

Intermediate users can utilize the DUAL table with various examples and scenarios, including calling user-defined functions, working with sequence-related pseudo columns, and using it in PL/SQL code.

Can the DUAL table be replaced with any other table in Oracle?

While users can create a similar table with one column and one row to mimic the DUAL table, the DUAL table is optimized for performance and does not require any permissions for use.

What is the minimum requirement to write a SELECT statement in Oracle?

The minimum requirement to write a SELECT statement in Oracle is to have a SELECT clause followed by a FROM clause, and the DUAL table is used when no actual data retrieval is needed.

How can the DUAL table be used to demonstrate user-defined functions?

The DUAL table can be used to demonstrate user-defined functions by calling them in a SELECT statement with the DUAL table to return a single scalar value.

What is the use of the DUAL table in PL/SQL code?

In PL/SQL code, the DUAL table is used to execute statements like DECODE that cannot be used directly in expressions, thus avoiding syntax errors.

Can the DUAL table be used to print information in a pyramid structure?

Yes, the DUAL table can be used in conjunction with the ROWNUM pseudo column and the SUBSTR function to print information in a pyramid structure.

How can the DUAL table be used to print multiplication tables?

The DUAL table can be used with the CONNECT BY clause to print multiplication tables, demonstrating its utility in completing the SELECT statement syntax without needing to select from an actual data table.

What are some examples of using the DUAL table provided in the video script?

Examples include computing 1+2, finding the square root of 100, using the SUBSTRING function to print the first two characters, and using the DUAL table to get sequence-related pseudo column information like NEXTVAL and CURRVAL.

Outlines

00:00

📘 Introduction to Oracle's DUAL Table for Beginners

This paragraph introduces the concept of the DUAL table in Oracle databases, a topic commonly discussed in SQL interview questions. The DUAL table is a dummy table created during Oracle installation, available in the SYS schema, with a single column 'DUMMY' of data type VARCHAR2(1). It is used for evaluating expressions and retrieving information from user-defined functions without the need to select from a regular database table. The paragraph emphasizes the simplicity of the DUAL table for beginners and suggests that more experienced individuals should be prepared to explain its use with various examples.

05:01

🔍 Advanced Usage of Oracle's DUAL Table with Examples

This paragraph delves into the advanced uses of the DUAL table in Oracle, providing several examples to illustrate its functionality. It explains that the DUAL table is primarily used to complete the syntax of a SELECT statement when no actual data retrieval is needed, such as computing simple expressions or using built-in functions. Examples include computing simple interest, finding the square root of a number, and using string functions like SUBSTR. The paragraph also addresses the possibility of creating a custom table similar to DUAL and highlights the performance optimization and universal accessibility of the DUAL table for all users.

Keywords

💡DUAL Table

The DUAL table is a virtual table in Oracle databases that exists solely for the purpose of providing a reference point for queries that do not need to access any actual data tables. It is automatically created during Oracle installation and is available in the SYS schema. The DUAL table has only one column named 'DUMMY' with a data type of VARCHAR2(1), and it contains a single value 'X'. In the context of the video, the DUAL table is used to complete the syntax of a SELECT statement when no actual data retrieval is needed, such as computing expressions or using built-in functions.

💡Oracle

Oracle is a widely used relational database management system (RDBMS) developed by Oracle Corporation. It is known for its robustness, scalability, and performance. In the video, Oracle is the database system in which the DUAL table is a part of, and the script discusses its specific features and use cases within Oracle SQL.

💡SELECT Statement

A SELECT statement in SQL is used to retrieve data from a database. It is the most common type of query and is essential for data manipulation and retrieval. In the video, the SELECT statement is discussed in the context of using the DUAL table to complete its syntax when no data retrieval is necessary, such as in the example 'SELECT 1 + 2 FROM DUAL'.

💡Dummy Column

In the context of the DUAL table, the 'DUMMY' column is a placeholder column that contains a single value 'X'. It is used to satisfy the requirement of a SELECT statement to have a FROM clause, even when no actual data is being retrieved. The video script explains that this column is part of the DUAL table and is used in various examples.

💡Data Type

Data types in SQL define the type of data that can be stored in a database column. In the case of the DUAL table, the 'DUMMY' column has a data type of 'VARCHAR2(1)', which means it can store a string of up to one character. The video script mentions this data type as part of the explanation of the DUAL table's structure.

💡User-Defined Functions

User-defined functions (UDFs) in SQL are custom functions created by users to perform specific tasks. They can be used in SELECT statements to manipulate data. The video script provides an example where a user-defined function can be called using the DUAL table to complete the syntax of the SELECT statement.

💡Syntax

Syntax in the context of SQL refers to the set of rules that define how statements are written and structured. The video script discusses the importance of the DUAL table in fulfilling the syntax requirements of SELECT statements, especially when no actual data retrieval is needed.

💡Performance Optimization

Performance optimization in databases involves improving the speed and efficiency of queries. The video script mentions that the DUAL table is optimized for performance, making it preferable over creating a custom table with similar characteristics.

💡Permissions

In database systems, permissions refer to the access rights granted to users or roles to perform certain operations. The video script explains that the DUAL table does not require any special permissions and is available to all users, which is a significant advantage when using it in queries.

💡PL/SQL

PL/SQL is Oracle's procedural language extension to SQL, which allows for the writing of stored procedures, functions, and other procedural logic. The video script discusses the use of the DUAL table in PL/SQL code, particularly in the context of using the 'DECODE' function within a SELECT statement from DUAL.

💡Decode Function

The 'DECODE' function in Oracle SQL is similar to a 'CASE' statement in other SQL dialects. It is used for conditional logic within a SELECT statement. The video script provides an example of using the DECODE function with the DUAL table to avoid syntax errors in PL/SQL.

Highlights

Introduction to a video series on SQL second interview questions for Oracle developers.

Explaining the concept of a 'DUAL' table in Oracle, a common question for interviewees at various skill levels.

The DUAL table is a dummy table created during Oracle installation, available in the SYS schema with a single column 'DUMMY'.

The DUAL table is used for evaluating expressions and computing values without selecting from a user-defined function.

For beginners, the DUAL table is a simple table with one column and one row containing the value 'X'.

Intermediate level explanation involves using the DUAL table with examples to demonstrate its utility.

The DUAL table is used to complete the syntax of a SELECT statement when no actual data retrieval is needed.

Demonstration of using the DUAL table to compute simple mathematical expressions like 1 + 2.

Describing the DUAL table to show its data type and the value it contains.

Use of the DUAL table to compute simple interest with the formula P * n * R / 100.

Using the DUAL table to find the square root of a number using the SQRT function.

Examples of calling user-defined and built-in functions from a SELECT statement using the DUAL table.

The DUAL table does not require any permissions and is available to all users.

Exploring the possibility of creating a custom table similar to the DUAL table for specific use cases.

Performance optimization of the DUAL table and its benefits over custom tables for similar purposes.

Use of the DUAL table to access sequence-related pseudo columns like NEXTVAL and CURRVAL.

Addressing the error encountered when using DECODE directly in an expression and the solution involving the DUAL table.

Demonstrating the use of the DUAL table to print multiplication tables and other patterns without data retrieval.

Providing examples of using the DUAL table for complex SELECT statements involving functions like CONNECT BY.

Invitation for viewers to ask questions in the comment section or via email, with a link to a blog for further questions.

Encouragement to like, subscribe, and stay tuned for new videos on interview questions, concepts, tips, and tricks.

Transcripts

hello friends welcome to the video

series on SQL second interview questions

for Oracle developers so in this video

we'll see about a very simple question

called what is Buell table in Oracle

though the question is very simple this

is being asked most of the foremost of

the freshers are beginners sometimes

this question is being asked for

intermediate level of expertise also

however then you need to explain this

table with lot of examples okay

in this video we'll see what should be

your answer for beginner and

intermediate level of expertise to start

with dual is a very simple dummy table

which is created as part of Oracle

installation and this table will be

available in this schema cornices and

this table has only one column called

dummy and the data type of this column

is work at two off one this table will

have contact will contain only one value

called X so this this should be your

answer if you are a fresher and this

table is you can be used for evaluating

expressions or two just to compute some

expressions or to select information

from user-defined function so this

should be your answer if you are a

beginner or a fresher however if you are

in a bit experienced person then you

should explain with more with lot of

examples okay we'll see with few

examples to start with when you say

select star from D well as I mentioned

it HAP it will have a column called

dummy and you just print a value called

X you can in fact go and describe this

table which you you can just show the

data type of this than me call them so

we'll start seeing I will start saying

some examples now the main use case of

this dual table is in specially in

Oracle when we write a select statement

the minimum class to write the Select

statement is select and from you cannot

write a select statement with only

select class it has to end with the from

class so but in some cases like this

kind of example where you want compute

just one plus two in this case you don't

have to actually retrieve information

from any table so just to complete the

syntax of the Select statement you can

use this dual table so this is the very

simple use case and in fact this kind of

use case for this kind of use cases only

will be using the dual table so here is

another use case we are I'm just

computing the simple interest so P n R

by hundred so two

this type of information you don't have

to select from any other table you can

just select from dual table and just the

computer results will be printed so here

is another use case I am using I'm using

the dual table to find the square root

of 100 okay so you are using the sq

active function so here's another use

case you can say select user from d1 are

you say select say state from DL so all

the user-defined functions are inbuilt

functions can be called from the Select

statement with dual table as far as this

particular function is returning single

scalar value so here is another use case

I'm using the substring function just to

print the first two character from the

dual table so you know all these

examples if you see we are actually not

selecting any information from the table

this table is just used to complete the

syntax of our select statement there can

be another question from this point can

I use any other table other than a dual

table in fact a can can someone create

their own table very similar to the dual

table

yes user can create their own table

which can have only one column one row

in that that will behave very similar to

this dual table however a dual table is

optimized for performance so it's better

to use the dual table and one more thing

is that this table doesn't need any

permission for any other users it will

be available for all the users will see

with few more example so here is another

use case like where I am using the dual

table to get the sequence related pseudo

column like next one and caravel

information so this is typically used in

the PLC code also so one more very use

case and appeals equalness in PLC code

we cannot use a decode directly in the

expression when you try to use a decode

directly in the expression you will get

the error saying that to avoid this

we'll be using the decode as part of the

deal table so like this is this is the

real use case you will be using the dual

table to Eva execute your decode

statement in PL sequel something like

this you'll see one more use case so as

you can see here I am using the dual

table to print to the first

multiplication table it's not just only

from dual table in fact I am using the

role and photo column I'm using the

connect by class all these things but

the key learning here is that

you don't have to select this

information from any table because we

are in fact not expecting any data from

any table just to complete our select

statement syntax I'm just using the dual

table so let's see one more example so

here is another use case typically these

kind of questions will be again our grid

for a fashionable

how can you print a string in a pyramid

structure something like this so you can

use the dual table along with the random

photo column along with substring

functionality using the connect by

classes if you want any questions to be

answered you can share your questions in

the comment section or you can drop your

questions to this mail ad I have

compiled all these questions in this

blog link which is given in the

description if you would have learnt

something new in this video please like

it subscribe and stay tuned for new

future videos interview questions

concept videos tips and tricks thanks so

much for watching