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.