SQL INTERVIEW PREPARATION PART-7

Explain the difference between GROUP BY and ORDER BY in SQL.

- GROUP BY: Groups rows that have the same values into summary rows.
- ORDER BY: Sorts the result set in ascending or descending order based on one or more columns.

Tips:
- Mention that GROUP BY is typically used with aggregate functions like COUNT, SUM, AVG, etc., while ORDER BY is used for sorting the result set.
- Provide an example to illustrate the distinction between the two clauses.

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SQL INTERVIEW PREPARATION PART-8

How do you find the nth highest salary from a table in SQL?

Answer:
You can use the LIMIT clause in combination with the ORDER BY clause to find the nth highest salary.

Example:

SELECT DISTINCT salary
FROM employees
ORDER BY salary DESC
LIMIT n-1, 1;

Replace 'n' with the desired rank of the salary.

Tip: Emphasize the importance of using DISTINCT to handle cases where there are duplicate salaries, and ensure the ORDER BY clause is sorting the salaries in descending order to find the nth highest salary.

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SQL INTERVIEW PREPARATION PART-8

How can you find the second highest salary in a table without using the LIMIT clause?

You can use a subquery to find the maximum salary that is less than the overall maximum salary.

Example:

SELECT MAX(salary)
FROM employees
WHERE salary < (SELECT MAX(salary) FROM employees);

Tip: Explain that this approach can be useful when the LIMIT clause is not supported or if you want to demonstrate proficiency in using subqueries.

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SQL INTERVIEW PREPARATION PART-9

What are window functions in SQL and can you provide an example?

Answer:
Window functions perform a calculation across a set of table rows that are somehow related to the current row. Unlike aggregate functions, window functions do not cause rows to become grouped into a single output row.

Example using ROW_NUMBER():

SELECT name, salary, department_id,
       ROW_NUMBER() OVER (PARTITION BY department_id ORDER BY salary DESC) as row_num
FROM employees;

In this example, ROW_NUMBER() assigns a unique rank to each row within each department, ordered by salary in descending order.

Tip: Highlight the usefulness of window functions for complex analytics and reporting tasks, where you need to perform calculations across rows while still returning individual rows. Explain other common window functions like RANK(), DENSE_RANK(), and NTILE().

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Glad to see the amazing response. I will start other series parallely with SQL Interview Series very soon :)

SQL INTERVIEW PREPARATION PART-10

Explain what a CTE (Common Table Expression) is and provide an example.

Answer:
A Common Table Expression (CTE) is a temporary result set that you can reference within a SELECT, INSERT, UPDATE, or DELETE statement. CTEs are defined using the WITH keyword and can improve the readability and organization of complex queries.

Example:

WITH EmployeeCTE AS (
    SELECT department_id, AVG(salary) as avg_salary
    FROM employees
    GROUP BY department_id
)
SELECT e.name, e.salary, e.department_id, c.avg_salary
FROM employees e
JOIN EmployeeCTE c ON e.department_id = c.department_id
WHERE e.salary > c.avg_salary;

In this example, the CTE EmployeeCTE calculates the average salary per department, which is then used in the main query to find employees earning above the average salary in their department.

Tip: Explain that CTEs can be particularly useful for breaking down complex queries into more manageable parts, improving both readability and maintainability. They also allow for recursive queries, which can be useful in hierarchical data structures.

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SQL INTERVIEW PREPARATION PART-11

What is a self join in SQL? Provide an example.

A self join is a regular join but the table is joined with itself. This is useful when the data is hierarchical or when you need to compare rows within the same table.

Example:

SELECT e1.name AS Employee, e2.name AS Manager
FROM employees e1
JOIN employees e2 ON e1.manager_id = e2.id;

In this example, the employees table is joined with itself to find the manager for each employee.

Tip: Explain that self joins are particularly useful for hierarchical data, such as organizational charts, and emphasize the importance of using table aliases (e.g., e1 and e2) to differentiate between the different instances of the same table.

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Power BI Interview Preparation Part-1

1. How can you optimize a Power BI report for performance?

Answer:
Optimizing a Power BI report for performance involves several strategies:
1. Data Modeling:
- Use star schema design to simplify relationships.
- Reduce the number of columns in tables by removing unnecessary data.
- Use numeric keys for relationships instead of text keys.

2. Query Optimization:
- Use Power Query to filter and aggregate data before loading it into Power BI.
- Disable auto-detect relationships to manually manage them.
- Reduce the number of calculated columns and use measures instead.

3. DAX Optimization:
- Use variables to store repeated calculations within DAX expressions.
- Avoid using complex or nested DAX functions that can slow down performance.
- Optimize DAX queries by reducing the number of operations and focusing on simpler calculations.

4. Data Refresh:
- Schedule incremental data refresh instead of full data refresh.
- Ensure efficient data source connectivity, such as using DirectQuery or Import mode appropriately.

5. Visualization Optimization:
- Limit the number of visuals on a single page to reduce the rendering time.
- Use simple visuals and avoid using too many custom visuals.
- Enable the "Optimize for Power BI" option in the settings.

Tips:
- Use the Performance Analyzer tool in Power BI Desktop to identify and diagnose performance bottlenecks.
- Regularly review and refine the data model and DAX formulas as the dataset grows and changes.
- Monitor the report's performance after deployment and make adjustments as necessary based on user feedback and performance metrics.

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SQL INTERVIEW PREPARATION PART-12

How can you detect and delete duplicate rows in a SQL table?

Answer:
To detect and delete duplicate rows, you can use a Common Table Expression (CTE) along with the ROW_NUMBER() window function to identify duplicates and then delete them.

Example to detect duplicates:

WITH DuplicateCTE AS (
    SELECT 
        *,
        ROW_NUMBER() OVER (PARTITION BY column1, column2, ... ORDER BY (SELECT NULL)) as row_num
    FROM 
        table_name
)
SELECT * FROM DuplicateCTE WHERE row_num > 1;

Example to delete duplicates:

WITH DuplicateCTE AS (
    SELECT 
        *,
        ROW_NUMBER() OVER (PARTITION BY column1, column2, ... ORDER BY (SELECT NULL)) as row_num
    FROM 
        table_name
)
DELETE FROM DuplicateCTE WHERE row_num > 1;

In these examples, replace column1, column2, ... with the columns that define a duplicate.

Tip: Emphasize the importance of carefully choosing the columns in the PARTITION BY clause to accurately identify duplicates. Also, mention that the ORDER BY (SELECT NULL) is used to avoid influencing the row numbering, but a specific column can be used if a specific order is needed.

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Power BI Interview Preparation Part-2

2. What are the differences between DirectQuery and Import mode in Power BI, and when should you use each?

Answer:

Import Mode:
- Data Storage: Data is imported and stored in the Power BI Desktop file.
- Performance: Generally faster performance because data is preloaded into memory.
- Refresh Frequency: Requires scheduled data refreshes to keep data up-to-date.
- Data Volume: Can handle larger datasets up to the Power BI dataset size limit.
- Latency: Low latency since data is already loaded into Power BI.
- When to Use: Ideal for static or infrequently updated data, when performance is critical, and when handling large datasets.

DirectQuery Mode:
- Data Storage: Data remains in the source system; only metadata is imported into Power BI.
- Performance: Dependent on the performance of the underlying data source.
- Refresh Frequency: Real-time or near-real-time data access, no need for scheduled refreshes.
- Data Volume: Limited by the performance of the underlying data source, but can handle very large datasets.
- Latency: Higher latency since each query is sent to the data source in real-time.
- When to Use: Ideal for real-time data analysis, when data changes frequently, and when dealing with data sources that cannot be fully imported due to size or security constraints.

Tips:
- Consider using a composite model to leverage the benefits of both Import and DirectQuery modes within the same report.
- Evaluate the performance of the underlying data source when using DirectQuery to ensure it can handle the expected query load.
- Optimize queries and data models in DirectQuery mode to minimize performance issues.

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SQL INTERVIEW PREPARATION PART-13

Explain the difference between DELETE and TRUNCATE commands in SQL.

Answer:
- DELETE: This command is used to delete rows from a table based on a condition. It is a DML (Data Manipulation Language) operation. Each row deletion is logged individually, and you can use a WHERE clause to specify which rows to delete. It can be rolled back if used within a transaction.

  DELETE FROM employees WHERE department_id = 10;
  

- TRUNCATE: This command is used to delete all rows from a table. It is a DDL (Data Definition Language) operation. It is faster than DELETE because it deallocates the data pages rather than logging individual row deletions. It cannot be rolled back in some databases because it does not log individual row deletions.

  TRUNCATE TABLE employees;
  

Tip: Mention that DELETE can be more flexible when you need to remove specific rows, while TRUNCATE is more efficient for removing all rows. Also, point out that TRUNCATE resets any auto-increment counters on the table and may require higher privileges compared to DELETE.

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Power BI Interview Preparation Part-3

3. What is a calculated column, and how does it differ from a measure in Power BI?

Answer:

Calculated Column:
- Definition: A calculated column is a new column that you add to your data model. It is computed using a DAX (Data Analysis Expressions) formula and is stored in the data model.
- Calculation Context: Calculated once when the data is loaded or refreshed and stored in the table.
- Use Case: Best for row-level calculations, such as creating a new column based on existing columns in the same row.
- Example: Adding a column for "Total Sales" by multiplying "Quantity" by "Unit Price" for each row.

  Total Sales = Sales[Quantity] * Sales[Unit Price]
  

Measure:
- Definition: A measure is a dynamic calculation that is computed at query time. It is also defined using DAX but is not stored in the data model; instead, it is recalculated as needed.
- Calculation Context: Computed dynamically based on the filter context of the report visuals (e.g., slicers, rows, columns).
- Use Case: Best for aggregate-level calculations, such as sums, averages, or more complex aggregations across the entire dataset.
- Example: Calculating the total sales for the filtered context in a report visual.

  Total Sales = SUM(Sales[Quantity] * Sales[Unit Price])
  

Key Differences:
- Storage: Calculated columns are stored in the data model, whereas measures are computed on the fly.
- Context: Calculated columns are static once created and independent of the visual context, while measures are dynamic and depend on the context of the report visual.
- Performance: Calculated columns can increase the data model size, potentially impacting performance. Measures do not affect the data model size but can impact performance if the calculations are complex and the data is large.

Tips:
- Use calculated columns sparingly to avoid unnecessary increases in model size.
- Prefer measures for calculations that need to be dynamic and context-aware.
- Regularly review your data model to ensure optimal use of calculated columns and measures.

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SQL INTERVIEW PREPARATION PART-14

What is the purpose of the COALESCE function in SQL? Provide an example.

Answer:
The COALESCE function returns the first non-null value in a list of arguments. It is useful for handling NULL values and providing default values.

Example:

SELECT name, COALESCE(phone, 'No Phone') as contact_number
FROM employees;

In this example, if the phone column is NULL, the COALESCE function will return 'No Phone' instead.

Tip: Highlight that COALESCE is particularly useful for dealing with potential NULL values in your data, and it can accept multiple arguments, returning the first non-null one. It is more versatile than the ISNULL function, which only handles two arguments.

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Power BI Interview Preparation Part-4 👇👇

4. Explain the concept of a "star schema" in data modeling and why it is preferred in Power BI.

Answer:

Star Schema:
- Definition: A star schema is a type of data modeling schema that organizes data into fact and dimension tables. The fact table is at the center, surrounded by dimension tables, creating a star-like structure.
- Fact Table: Contains the core transactional data, such as sales or orders, with keys to dimension tables and numeric metrics for analysis (e.g., sales amount, quantity).
- Dimension Tables: Contain denoscriptive attributes related to the facts, such as date, product, customer, and location details. These tables provide context and categories for slicing and dicing the data.

Advantages of a Star Schema in Power BI:
1. Simplified Queries: The star schema simplifies query logic, making it easier and faster for Power BI to process queries.
2. Improved Performance: By reducing the number of joins and leveraging optimized indexing, the star schema enhances performance.
3. Ease of Understanding: A star schema's clear and intuitive structure makes it easier for users to understand and navigate the data model.
4. Optimized Storage: It reduces redundancy and optimizes storage by separating transactional data (fact table) from denoscriptive attributes (dimension tables).
5. Flexibility: Allows for more flexible and efficient DAX calculations, improving the overall performance of reports.

Example:
Consider a sales data model with:
- Fact Table: Sales (columns: SalesID, ProductID, CustomerID, DateID, SalesAmount, Quantity)
- Dimension Tables:
- Product (columns: ProductID, ProductName, Category)
- Customer (columns: CustomerID, CustomerName, Region)
- Date (columns: DateID, Date, Month, Year)

Tips:
- Ensure that each dimension table has a primary key that uniquely identifies each record, and this key should be used as a foreign key in the fact table.
- Regularly review and optimize the schema to handle growing data volumes and evolving business requirements.
- Use Power BI's built-in relationship management tools to define and manage relationships between fact and dimension tables accurately.

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SQL INTERVIEW PREPARATION PART-15

What are indexes in SQL, and why are they used?

Indexes are database objects created on tables and views that improve the speed of data retrieval operations. They work by providing a fast way to look up rows based on the values of one or more columns.

Types of indexes:
- Clustered Index: Alters the physical order of the table and is limited to one per table. The table data is sorted according to the clustered index.
- Non-Clustered Index: Does not alter the physical order of the table. It creates a separate object within the table that points back to the original table rows.

Example:

CREATE INDEX idx_employee_name ON employees(name);

In this example, an index is created on the name column of the employees table.

Tip: Explain that while indexes can significantly speed up data retrieval, they can also slow down data modification operations (INSERT, UPDATE, DELETE) because the indexes need to be maintained. Emphasize the importance of choosing the right columns for indexing based on query patterns and database performance analysis.

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SQL INTERVIEW PREPARATION PART-16

Question 16: What is the difference between a primary key and a unique key in SQL?

Answer:
- Primary Key:
- Uniquely identifies each record in a table.
- Cannot contain NULL values.
- A table can have only one primary key.
- Often automatically creates a clustered index on the column(s).

- Unique Key:
- Ensures that all values in a column or a set of columns are unique across the table.
- Can contain NULL values, but each NULL is considered unique.
- A table can have multiple unique keys.
- Often creates a non-clustered index on the column(s).

Example:

CREATE TABLE employees (
    id INT PRIMARY KEY,
    email VARCHAR(255) UNIQUE
);

In this example, id is the primary key, ensuring no duplicate values and no NULLs, while email is a unique key, ensuring unique values but allowing NULLs.

Tip: Clarify that the primary key is a more stringent constraint than a unique key because it does not allow NULL values. Emphasize the importance of both constraints in maintaining data integrity and ensuring that records can be uniquely identified.

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SQL INTERVIEW PREPARATION PART-17

Explain what normalization is and the different normal forms in SQL.

Answer:
Normalization is the process of organizing the columns and tables of a relational database to reduce data redundancy and improve data integrity. The main goal is to divide large tables into smaller, related tables and define relationships between them.

The different normal forms are:

1. First Normal Form (1NF): Ensures that the table has no repeating groups or arrays. Each cell contains a single value, and each record is unique.
- Example: A table where each column contains only atomic (indivisible) values.

2. Second Normal Form (2NF): Meets all the requirements of 1NF and ensures that all non-key columns are fully dependent on the primary key.
- Example: Remove partial dependencies; every non-primary-key attribute should depend on the whole primary key.

3. Third Normal Form (3NF): Meets all the requirements of 2NF and ensures that all the columns are dependent only on the primary key.
- Example: Remove transitive dependencies; non-primary-key attributes should not depend on other non-primary-key attributes.

4. Boyce-Codd Normal Form (BCNF): A stricter version of 3NF where every determinant is a candidate key.
- Example: Ensure that for every functional dependency (A → B), A is a super key.

5. Fourth Normal Form (4NF): Meets all the requirements of BCNF and ensures that multi-valued dependencies are removed.
- Example: Remove multi-valued dependencies; a record should not have more than one independent multi-valued attribute.

6. Fifth Normal Form (5NF): Meets all the requirements of 4NF and ensures that join dependencies are properly handled.
- Example: Decompose tables further to eliminate redundancy and handle complex join dependencies.

Tip: Emphasize that while normalization helps in reducing redundancy and improving data integrity, it can sometimes lead to complex queries and may affect performance. Denormalization is often used in data warehousing to improve query performance by reducing the number of joins needed.

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SQL INTERVIEW PREPARATION PART-18

What is a stored procedure, and what are its advantages?

Answer:
A stored procedure is a prepared SQL code that you can save and reuse. It can contain multiple SQL statements and control-of-flow language. Stored procedures are stored in the database and can be executed by calling them directly.

Example:

CREATE PROCEDURE GetEmployeeDetails
    @EmployeeID INT
AS
BEGIN
    SELECT name, department_id, salary
    FROM employees
    WHERE id = @EmployeeID;
END;

Advantages:
1. Reusability: Once created, stored procedures can be reused multiple times in different programs or noscripts.
2. Performance: Stored procedures are precompiled, which can result in faster execution compared to dynamically building and executing queries.
3. Security: Stored procedures provide an additional layer of security by controlling access to data and preventing SQL injection attacks.
4. Maintainability: By centralizing business logic in the database, it is easier to maintain and update the logic.
5. Reduced Network Traffic: Executing a stored procedure can reduce network traffic because multiple SQL statements can be sent as a single call.

Tip: Highlight the importance of stored procedures in managing complex business logic and enhancing performance. Also, emphasize the security benefits, especially in terms of protecting against SQL injection attacks.

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SQL INTERVIEW PREPARATION PART-19

What is a CTE (Common Table Expression) and how is it used?

Answer:
- A CTE is a temporary result set that you can reference within a SELECT, INSERT, UPDATE, or DELETE statement.
- It is defined using the WITH keyword followed by a subquery.

Example:

   WITH Sales_CTE AS (
       SELECT SalesPersonID, SUM(SalesAmount) AS TotalSales
       FROM Sales
       GROUP BY SalesPersonID
   )
   SELECT SalesPersonID, TotalSales
   FROM Sales_CTE
   WHERE TotalSales > 10000;
   

Tips:
- CTEs can improve the readability and maintainability of complex queries.
- They are useful for breaking down complex queries into simpler parts.
- Mention that CTEs can be recursive, which is particularly useful for hierarchical data.

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Power BI Interview Preparation Part-5 👇👇

5. What is the difference between calculated columns and measures in Power BI?

Answer:

Calculated Columns:
- Definition: Calculated columns are new columns added to a table in Power BI, computed using a DAX formula.
- Calculation Context: Calculated columns are computed during data refresh and stored in the data model.
- Usage: Used for row-level calculations that involve values from other columns within the same row.
- Example: Calculating total sales by multiplying quantity and unit price for each row in a sales table.

Measures:
- Definition: Measures are dynamic calculations performed on-the-fly during query execution in Power BI.
- Calculation Context: Measures adapt to the current context defined by report filters, slicers, and other visual elements.
- Usage: Used for aggregated calculations such as sums, averages, or more complex aggregations across multiple rows or tables.
- Example: Calculating total sales across all products and regions, respecting the current filter context applied to a report.

Key Differences:
- Storage: Calculated columns are physically stored in the data model, increasing its size, whereas measures are not stored but computed dynamically.
- Context Sensitivity: Calculated columns are static and not context-sensitive, while measures dynamically adjust based on the report's current context.
- Performance: Measures generally have better performance for aggregations since they are calculated on-demand and can leverage query folding.

Tips:
- Use calculated columns for static, row-level computations that are needed for every row in a table.
- Use measures for dynamic, context-sensitive aggregations or calculations that depend on the user's interaction with the report.
- Consider the impact on model size and performance when deciding whether to use calculated columns or measures.

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