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SQL INTERVIEW PREPARATION PART-41

What is the difference between a LEFT JOIN and an INNER JOIN in SQL? Provide examples to illustrate the differences.

Answer:

INNER JOIN:
An INNER JOIN returns only the rows that have matching values in both tables. If there are rows in either table that do not have matches, they will not be included in the result set.

Example:
Suppose we have two tables, employees and departments:

employees table:
| employee_id | name | department_id |
|-------------|-------|---------------|
| 1 | John | 10 |
| 2 | Jane | 20 |
| 3 | Jim | 30 |

departments table:
| department_id | department_name |
|---------------|-----------------|
| 10 | HR |
| 20 | Finance |
| 40 | IT |

An INNER JOIN query to get the employees and their corresponding department names would be:

SELECT employees.name, departments.department_name
FROM employees
INNER JOIN departments ON employees.department_id = departments.department_id;

Result:
| name | department_name |
|-------|-----------------|
| John | HR |
| Jane | Finance |

LEFT JOIN:
A LEFT JOIN returns all the rows from the left table and the matched rows from the right table. If there is no match, the result is NULL on the side of the right table.

Example:
Using the same tables, a LEFT JOIN query to get all employees and their corresponding department names would be:

SELECT employees.name, departments.department_name
FROM employees
LEFT JOIN departments ON employees.department_id = departments.department_id;

Result:
| name | department_name |
|-------|-----------------|
| John | HR |
| Jane | Finance |
| Jim | NULL |

In this result, Jim is included even though there is no corresponding department in the departments table, showing a NULL for department_name.

Tip: Use an INNER JOIN when you want to retrieve only the records that have matching values in both tables. Use a LEFT JOIN when you want to retrieve all records from the left table and the matching records from the right table, filling in NULLs for non-matching rows. Understanding these joins is crucial for effectively querying relational databases and retrieving the desired data.

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SQL INTERVIEW PREPARATION PART-40

What are the differences between DELETE, TRUNCATE, and DROP commands in SQL? Provide examples.

Answer:

DELETE:
The DELETE command is used to remove rows from a table based on a specified condition. It can delete all rows or specific rows that match the condition. DELETE operations can be rolled back if they are part of a transaction.

Example:

DELETE FROM employees WHERE department_id = 10;

Key Points:
- Deletes specified rows.
- Can use a WHERE clause to filter rows.
- Triggers are fired.
- Slower compared to TRUNCATE for large data sets due to row-by-row deletion.
- Transactional and can be rolled back.

TRUNCATE:
The TRUNCATE command is used to remove all rows from a table. It is faster than DELETE because it does not log individual row deletions. TRUNCATE operations cannot be rolled back if they are not part of a transaction.

Example:

TRUNCATE TABLE employees;

Key Points:
- Deletes all rows from a table.
- Cannot use a WHERE clause.
- Resets table's identity column (if any).
- Does not fire triggers.
- Faster than DELETE due to minimal logging.
- Transactional and can be rolled back only if part of a transaction.

DROP:
The DROP command is used to remove a table or database entirely from the database. This operation deletes the table schema and all its data, and it cannot be rolled back.

Example:

DROP TABLE employees;

Key Points:
- Deletes the entire table schema and data.
- Irreversible and cannot be rolled back.
- Removes all associated objects like constraints, triggers, and indexes.
- Not transactional.

Tip: Use DELETE when you need to remove specific rows and want the operation to be logged and potentially rolled back. Use TRUNCATE for quickly removing all rows in a table while retaining the table structure. Use DROP when you need to permanently remove a table and its schema from the database.

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SQL INTERVIEW PREPARATION PART-41

What is SQL and what are its main components?

Answer:

SQL (Structured Query Language):
SQL is a standard programming language specifically designed for managing and manipulating relational databases. It allows users to create, read, update, and delete (CRUD) data stored in a relational database.

Main Components of SQL:

1. DDL (Data Definition Language):
- Purpose: Defines and modifies the structure of database objects.
- Commands:
- CREATE: Creates a new table, view, or other database objects.

       CREATE TABLE employees (
           employee_id INT PRIMARY KEY,
           first_name VARCHAR(50),
           last_name VARCHAR(50),
           hire_date DATE
       );

- ALTER: Modifies the structure of an existing table.

       ALTER TABLE employees ADD COLUMN salary DECIMAL(10, 2);

- DROP: Deletes a table, view, or other database objects.

       DROP TABLE employees;

2. DML (Data Manipulation Language):
- Purpose: Manipulates the data stored in the database.
- Commands:
- SELECT: Retrieves data from one or more tables.

       SELECT first_name, last_name FROM employees;

- INSERT: Adds new rows of data into a table.

       INSERT INTO employees (employee_id, first_name, last_name, hire_date)
       VALUES (1, 'John', 'Doe', '2024-07-28');

- UPDATE: Modifies existing data within a table.

       UPDATE employees SET salary = 60000 WHERE employee_id = 1;

- DELETE: Removes rows of data from a table.

       DELETE FROM employees WHERE employee_id = 1;

3. DCL (Data Control Language):
- Purpose: Controls access to the data within the database.
- Commands:
- GRANT: Provides specific privileges to users.

       GRANT SELECT ON employees TO user_name;

- REVOKE: Removes specific privileges from users.

       REVOKE SELECT ON employees FROM user_name;

4. TCL (Transaction Control Language):
- Purpose: Manages transactions within a database to ensure data integrity.
- Commands:
- COMMIT: Saves the changes made by the current transaction.

       COMMIT;

- ROLLBACK: Undoes the changes made by the current transaction.

       ROLLBACK;

- SAVEPOINT: Sets a savepoint within a transaction to which a rollback can occur.

       SAVEPOINT savepoint_name;

Tip: Freshers should focus on mastering the syntax and use cases of these commands to effectively interact with relational databases.

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POWER BI INTERVIEW PREPARATION PART-17

What is the use of slicers in Power BI?

Answer:
- Slicers are visual filters that allow users to segment and filter data dynamically on a report page.
- They provide a user-friendly way to interact with data by selecting specific values, which then updates all connected visualizations.

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SQL INTERVIEW PREPARATION PART-42

What is a JOIN in SQL and what are the different types of JOINs? Provide examples.

Answer:

JOIN:
A JOIN clause in SQL is used to combine rows from two or more tables based on a related column between them. JOINs allow querying data across multiple tables.

Types of JOINs:

1. INNER JOIN:
- Definition: Returns only the rows that have matching values in both tables.
- Example:

     SELECT employees.employee_id, employees.first_name, departments.department_name
     FROM employees
     INNER JOIN departments ON employees.department_id = departments.department_id;

2. LEFT JOIN (or LEFT OUTER JOIN):
- Definition: Returns all rows from the left table and the matched rows from the right table. If no match is found, NULL values are returned for columns from the right table.
- Example:

     SELECT employees.employee_id, employees.first_name, departments.department_name
     FROM employees
     LEFT JOIN departments ON employees.department_id = departments.department_id;

3. RIGHT JOIN (or RIGHT OUTER JOIN):
- Definition: Returns all rows from the right table and the matched rows from the left table. If no match is found, NULL values are returned for columns from the left table.
- Example:

     SELECT employees.employee_id, employees.first_name, departments.department_name
     FROM employees
     RIGHT JOIN departments ON employees.department_id = departments.department_id;

4. FULL JOIN (or FULL OUTER JOIN):
- Definition: Returns all rows when there is a match in either left or right table. If there is no match, the result is NULL from the side where there is no match.
- Example:

     SELECT employees.employee_id, employees.first_name, departments.department_name
     FROM employees
     FULL OUTER JOIN departments ON employees.department_id = departments.department_id;

5. CROSS JOIN:
- Definition: Returns the Cartesian product of both tables, i.e., it combines each row of the first table with all rows of the second table.
- Example:

     SELECT employees.first_name, departments.department_name
     FROM employees
     CROSS JOIN departments;

6. SELF JOIN:
- Definition: A join where a table is joined with itself.
- Example:

     SELECT e1.employee_id, e1.first_name, e2.first_name AS manager_name
     FROM employees e1
     INNER JOIN employees e2 ON e1.manager_id = e2.employee_id;

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POWER BI INTERVIEW PREPARATION PART-18

What are hierarchies in Power BI and how are they used?

Answer:
- Hierarchies in Power BI are used to represent data at different levels of granularity, such as Year > Quarter > Month > Day.
- They enable users to drill down into data to analyze it at various levels of detail.

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SQL INTERVIEW PREPARATION PART-43

What is a primary key and a foreign key in SQL? Explain with examples.

Answer:

Primary Key:
- Definition: A primary key is a column (or a set of columns) in a table that uniquely identifies each row in that table. Primary keys must contain unique values and cannot contain NULL values.
- Purpose: To ensure each record in a table is unique and to serve as a reference point for other tables.
- Example:

  CREATE TABLE employees (
      employee_id INT PRIMARY KEY,
      first_name VARCHAR(50),
      last_name VARCHAR(50),
      hire_date DATE
  );

In this example, employee_id is the primary key of the employees table. It uniquely identifies each employee.

Foreign Key:
- Definition: A foreign key is a column (or a set of columns) in one table that uniquely identifies a row in another table. The foreign key establishes a relationship between the two tables.
- Purpose: To maintain referential integrity between the related tables by ensuring that the value in the foreign key column(s) matches a value in the referenced primary key column(s).
- Example:

  CREATE TABLE departments (
      department_id INT PRIMARY KEY,
      department_name VARCHAR(50)
  );

  CREATE TABLE employees (
      employee_id INT PRIMARY KEY,
      first_name VARCHAR(50),
      last_name VARCHAR(50),
      hire_date DATE,
      department_id INT,
      FOREIGN KEY (department_id) REFERENCES departments(department_id)
  );

In this example, department_id in the employees table is a foreign key that references department_id in the departments table. This establishes a relationship between employees and their respective departments.

Key Differences:

| Feature | Primary Key | Foreign Key |
|---------------|---------------------------------------|----------------------------------------|
| Uniqueness | Must be unique | Can have duplicate values |
| NULL Values | Cannot be NULL | Can contain NULL values |
| Purpose | Uniquely identifies each row in a table| Establishes a relationship between tables |
| Table | Each table can have only one primary key | A table can have multiple foreign keys |
| Enforcement | Enforces entity integrity | Enforces referential integrity |

Example of Referential Integrity:

-- Insert into departments
INSERT INTO departments (department_id, department_name) VALUES (1, 'HR');
INSERT INTO departments (department_id, department_name) VALUES (2, 'Finance');

-- Insert into employees
INSERT INTO employees (employee_id, first_name, last_name, hire_date, department_id)
VALUES (101, 'Alice', 'Smith', '2024-01-15', 1); -- Valid: department_id 1 exists
INSERT INTO employees (employee_id, first_name, last_name, hire_date, department_id)
VALUES (102, 'Bob', 'Brown', '2024-02-20', 3); -- Invalid: department_id 3 does not exist, will cause an error

Tip: Understanding primary and foreign keys is essential for designing relational databases that maintain data integrity and support complex relationships between tables. Always ensure that primary keys are unique and non-null and that foreign keys correctly reference existing primary key values in related tables.

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Data Analytics

SQL INTERVIEW PREPARATION PART-44

What are aggregate functions in SQL? Provide examples.

Answer:

Aggregate Functions:
Aggregate functions in SQL perform calculations on a set of values and return a single value. They are often used with the GROUP BY clause to group the result set by one or more columns.

Common Aggregate Functions:

1. COUNT():
- Purpose: Returns the number of rows that match a specified condition.
- Example:

     SELECT COUNT(*) AS total_employees FROM employees;

2. SUM():
- Purpose: Returns the total sum of a numeric column.
- Example:

     SELECT SUM(salary) AS total_salary FROM employees;

3. AVG():
- Purpose: Returns the average value of a numeric column.
- Example:

     SELECT AVG(salary) AS average_salary FROM employees;

4. MIN():
- Purpose: Returns the minimum value in a set of values.
- Example:

     SELECT MIN(salary) AS minimum_salary FROM employees;

5. MAX():
- Purpose: Returns the maximum value in a set of values.
- Example:

     SELECT MAX(salary) AS maximum_salary FROM employees;

Using Aggregate Functions with GROUP BY:

Aggregate functions are commonly used with the GROUP BY clause to group rows that have the same values in specified columns into summary rows.

Example:

SELECT department_id, COUNT(*) AS total_employees, AVG(salary) AS average_salary
FROM employees
GROUP BY department_id;

In this example, the query groups the employees by their department_id and calculates the total number of employees and the average salary for each department.

Example Scenario:
Consider the following employees table:

+-------------+------------+--------+-------------+
| employee_id | first_name | salary | department_id|
+-------------+------------+--------+-------------+
| 1           | Alice      | 60000  | 1           |
| 2           | Bob        | 55000  | 1           |
| 3           | Carol      | 75000  | 2           |
| 4           | David      | 80000  | 2           |
| 5           | Eve        | 72000  | 3           |
+-------------+------------+--------+-------------+

Using aggregate functions:

-- COUNT example
SELECT COUNT(*) AS total_employees FROM employees;

-- SUM example
SELECT SUM(salary) AS total_salary FROM employees;

-- AVG example
SELECT AVG(salary) AS average_salary FROM employees;

-- MIN example
SELECT MIN(salary) AS minimum_salary FROM employees;

-- MAX example
SELECT MAX(salary) AS maximum_salary FROM employees;

-- Using GROUP BY
SELECT department_id, COUNT(*) AS total_employees, AVG(salary) AS average_salary
FROM employees
GROUP BY department_id;

Tip: Aggregate functions are powerful tools for summarizing and analyzing data in SQL. They are essential for generating reports and insights from large datasets. Practice using aggregate functions with and without the GROUP BY clause to understand their capabilities fully.

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Essential SQL Functions 👇👇

### DATE AND TIME FUNCTIONS:
- NOW(): Returns the current date and time.
- CURDATE(): Returns the current date.
- CURTIME(): Returns the current time.
- DATE(): Extracts the date part of a date or datetime expression.
- DATEDIFF(): Returns the number of days between two date values.
- YEAR(): Extracts the year.
- MONTH(): Extracts the month.
- DAY(): Extracts the day of the month.
- HOUR(): Extracts the hour.
- MINUTE(): Extracts the minute.
- SECOND(): Extracts the second.

### AGGREGATE FUNCTIONS:
- SUM(): Returns the sum of a set of values.
- AVG(): Returns the average value of a numeric column.
- MIN(): Returns the minimum value in a set of values.
- MAX(): Returns the maximum value in a set of values.
- COUNT(): Returns the number of rows that matches a specified condition.
- COUNT(*): Returns the number of rows in a table.
- COUNT(DISTINCT column_name): Returns the number of distinct values in a column.

### STRING FUNCTIONS:
- CONCAT(): Concatenates two or more strings.
- LENGTH(): Returns the length of a string.
- UPPER(): Converts a string to upper-case.
- LOWER(): Converts a string to lower-case.
- LEFT(): Extracts a number of characters from a string (starting from left).
- RIGHT(): Extracts a number of characters from a string (starting from right).
- SUBSTRING(): Extracts a substring from a string.

### NUMERIC FUNCTIONS:
- ROUND(): Rounds a number to a specified number of decimal places.
- FLOOR(): Returns the largest integer value less than or equal to a number.
- CEIL(): Returns the smallest integer value greater than or equal to a number.
- ABS(): Returns the absolute value of a number.

### INFORMATION FUNCTIONS:
- ISNULL(): Returns a specified value if the expression is NULL.
- COALESCE(): Returns the first non-null value in a list.
- NULLIF(): Returns NULL if the two expressions are equal.

### LOGICAL FUNCTIONS:
- IF(): Returns one value if a condition is TRUE, and another value if it is FALSE.
- CASE: Evaluates a list of conditions and returns one of multiple possible result expressions.
- AND: Combines two or more conditions and returns TRUE if all conditions are TRUE.
- OR: Combines two or more conditions and returns TRUE if any condition is TRUE.
- NOT: Reverses the value of a boolean expression.

### JSON FUNCTIONS:
- JSON_EXTRACT(): Extracts data from a JSON document.
- JSON_OBJECT(): Creates a JSON object from a list of key-value pairs.

### WINDOW FUNCTIONS:
- ROW_NUMBER(): Assigns a unique sequential integer to rows within a partition.
- RANK(): Assigns a rank to each row within a partition.
- DENSE_RANK(): Similar to RANK(), but without gaps in the ranking sequence.
- NTILE(): Divides rows into a specified number of approximately equal groups.

### OTHER FUNCTIONS:
- CAST(): Converts a value of one data type to another.
- CONVERT(): Converts a value of one data type to another.
- COALESCE(): Returns the first non-null expression among its arguments.

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Commonly used Python functions and methods:

### STRING FUNCTIONS:
- len(): Returns the length of a string.
- str.upper(): Converts a string to upper-case.
- str.lower(): Converts a string to lower-case.
- str.capitalize(): Capitalizes the first character of a string.
- str.split(): Splits a string into a list.
- str.join(): Joins elements of a list into a string.
- str.replace(): Replaces a specified phrase with another specified phrase.
- str.strip(): Removes whitespace from the beginning and end of a string.

### LIST FUNCTIONS:
- len(): Returns the length of a list.
- list.append(): Adds an item to the end of the list.
- list.extend(): Adds the elements of a list (or any iterable) to the end of the current list.
- list.insert(): Adds an item at a specified position.
- list.remove(): Removes the first item with the specified value.
- list.pop(): Removes the item at the specified position.
- list.index(): Returns the index of the first element with the specified value.
- list.sort(): Sorts the list.
- list.reverse(): Reverses the order of the list.

### DICTIONARY FUNCTIONS:
- dict.keys(): Returns a list of all the keys in the dictionary.
- dict.values(): Returns a list of all the values in the dictionary.
- dict.items(): Returns a list of tuples, each tuple containing a key and a value.
- dict.get(): Returns the value of the specified key.
- dict.update(): Updates the dictionary with the specified key-value pairs.
- dict.pop(): Removes the element with the specified key.

### TUPLE FUNCTIONS:
- len(): Returns the length of a tuple.
- tuple.count(): Returns the number of times a specified value appears in a tuple.
- tuple.index(): Searches the tuple for a specified value and returns the position of where it was found.

### SET FUNCTIONS:
- len(): Returns the length of a set.
- set.add(): Adds an element to the set.
- set.remove(): Removes the specified element.
- set.union(): Returns a set containing the union of sets.
- set.intersection(): Returns a set containing the intersection of sets.
- set.difference(): Returns a set containing the difference of sets.
- set.symmetric_difference(): Returns a set with elements in either the set or the specified set, but not both.

### NUMERIC FUNCTIONS:
- abs(): Returns the absolute value of a number.
- round(): Rounds a number to a specified number of digits.
- max(): Returns the largest item in an iterable.
- min(): Returns the smallest item in an iterable.
- sum(): Sums the items of an iterable.

### DATE AND TIME FUNCTIONS (datetime module):
- datetime.datetime.now(): Returns the current date and time.
- datetime.datetime.today(): Returns the current local date.
- datetime.datetime.strftime(): Formats a datetime object as a string.
- datetime.datetime.strptime(): Parses a string to a datetime object.

### FILE I/O FUNCTIONS:
- open(): Opens a file and returns a file object.
- file.read(): Reads the contents of a file.
- file.write(): Writes data to a file.
- file.readlines(): Reads all the lines of a file into a list.
- file.close(): Closes the file.

### GENERAL FUNCTIONS:
- print(): Prints to the console.
- input(): Reads a string from standard input.
- type(): Returns the type of an object.
- isinstance(): Checks if an object is an instance of a class or a tuple of classes.
- id(): Returns the identity of an object.

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SQL INTERVIEW PREPARATION PART-53

What is a subquery in SQL? Provide an example.

Answer:

Subquery:
A subquery, also known as an inner query or nested query, is a query within another SQL query and embedded within the WHERE, HAVING, FROM, or SELECT clauses. Subqueries can be used to perform operations that need to be completed in multiple steps.

Types of Subqueries:

1. Single-row Subquery:
- Returns a single row and single column.
- Example:

     SELECT first_name, last_name
     FROM employees
     WHERE salary > (SELECT AVG(salary) FROM employees);

2. Multi-row Subquery:
- Returns multiple rows and a single column.
- Example:

     SELECT first_name, last_name
     FROM employees
     WHERE department_id IN (SELECT department_id FROM departments WHERE location = 'New York');

3. Multi-column Subquery:
- Returns multiple columns and rows.
- Example:

     SELECT first_name, last_name
     FROM employees
     WHERE (department_id, salary) IN (SELECT department_id, MAX(salary) FROM employees GROUP BY department_id);

4. Correlated Subquery:
- Refers to columns in the outer query and executes once for each row selected by the outer query.
- Example:

     SELECT e1.first_name, e1.salary
     FROM employees e1
     WHERE e1.salary > (SELECT AVG(e2.salary) FROM employees e2 WHERE e2.department_id = e1.department_id);

Using Subqueries in Different Clauses:

1. In the SELECT Clause:
- Example:

     SELECT first_name, last_name, (SELECT department_name FROM departments WHERE departments.department_id = employees.department_id) AS department
     FROM employees;

2. In the FROM Clause:
- Example:

     SELECT AVG(salary)
     FROM (SELECT salary FROM employees WHERE department_id = 1) AS dept1_salaries;

3. In the WHERE Clause:
- Example:

     SELECT first_name, last_name
     FROM employees
     WHERE department_id = (SELECT department_id FROM departments WHERE department_name = 'HR');

4. In the HAVING Clause:
- Example:

     SELECT department_id, AVG(salary)
     FROM employees
     GROUP BY department_id
     HAVING AVG(salary) > (SELECT AVG(salary) FROM employees);

Example Scenario:
Consider the following employees and departments tables:

-- employees table
+-------------+------------+----------+-------------+
| employee_id | first_name | salary   | department_id|
+-------------+------------+----------+-------------+
| 1           | Alice      | 60000    | 1           |
| 2           | Bob        | 55000    | 1           |
| 3           | Carol      | 75000    | 2           |
| 4           | David      | 80000    | 2           |
| 5           | Eve        | 72000    | 3           |
+-------------+------------+----------+-------------+

-- departments table
+---------------+-----------------+---------+
| department_id | department_name | location|
+---------------+-----------------+---------+
| 1             | HR              | London  |
| 2             | Finance         | New York|
| 3             | IT              | San Francisco|
+---------------+-----------------+---------+

Using subqueries:

-- Single-row subquery example
SELECT first_name, last_name
FROM employees
WHERE salary > (SELECT AVG(salary) FROM employees);

-- Multi-row subquery example
SELECT first_name, last_name
FROM employees
WHERE department_id IN (SELECT department_id FROM departments WHERE location = 'New York');

-- Multi-column subquery example
SELECT first_name, last_name
FROM employees
WHERE (department_id, salary) IN (SELECT department_id, MAX(salary) FROM employees GROUP BY department_id);

-- Correlated subquery example
SELECT e1.first_name, e1.salary
FROM employees e1
WHERE e1.salary > (SELECT AVG(e2.salary) FROM employees e2 WHERE e2.department_id = e1.department_id);

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POWER BI INTERVIEW PREPARATION PART-19

What is a Power BI Gateway and why is it used?

Answer:
- A Power BI Gateway acts as a bridge, enabling secure data transfer between on-premises data sources and Power BI Service.
- It allows for scheduled data refreshes and real-time data access for reports and dashboards hosted in the Power BI Service.

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Data Analytics

SQL INTERVIEW PREPARATION PART-54

What are window functions in SQL? Explain with examples.

Answer:

Window Functions:
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. The rows retain their separate identities.

Common Window Functions:

1. ROW_NUMBER():
- Purpose: Assigns a unique sequential integer to rows within a partition of a result set.
- Example:

     SELECT first_name, last_name, salary,
            ROW_NUMBER() OVER (PARTITION BY department_id ORDER BY salary DESC) AS row_num
     FROM employees;

2. RANK():
- Purpose: Assigns a rank to each row within the partition of a result set. The rank of a row is one plus the number of ranks that come before it.
- Example:

     SELECT first_name, last_name, salary,
            RANK() OVER (PARTITION BY department_id ORDER BY salary DESC) AS rank
     FROM employees;

3. DENSE_RANK():
- Purpose: Similar to RANK(), but ranks are consecutive integers, meaning no gaps between rank values.
- Example:

     SELECT first_name, last_name, salary,
            DENSE_RANK() OVER (PARTITION BY department_id ORDER BY salary DESC) AS dense_rank
     FROM employees;

4. NTILE():
- Purpose: Divides the result set into a specified number of roughly equal parts, or buckets, and assigns a bucket number to each row.
- Example:

     SELECT first_name, last_name, salary,
            NTILE(4) OVER (ORDER BY salary DESC) AS quartile
     FROM employees;

5. LAG():
- Purpose: Provides access to a row at a given physical offset that comes before the current row.
- Example:

     SELECT first_name, last_name, salary,
            LAG(salary, 1, 0) OVER (ORDER BY salary DESC) AS previous_salary
     FROM employees;

6. LEAD():
- Purpose: Provides access to a row at a given physical offset that follows the current row.
- Example:

     SELECT first_name, last_name, salary,
            LEAD(salary, 1, 0) OVER (ORDER BY salary DESC) AS next_salary
     FROM employees;

7. FIRST_VALUE() and LAST_VALUE():
- Purpose: Returns the first and last value in an ordered set of values.
- Example:

     SELECT first_name, last_name, salary,
            FIRST_VALUE(salary) OVER (ORDER BY salary DESC) AS highest_salary,
            LAST_VALUE(salary) OVER (ORDER BY salary DESC) AS lowest_salary
     FROM employees;

8. SUM() OVER, AVG() OVER, MIN() OVER, MAX() OVER:
- Purpose: Performs aggregate calculations over a window of rows.
- Example:

     SELECT first_name, last_name, salary,
            SUM(salary) OVER (PARTITION BY department_id ORDER BY salary DESC) AS running_total
     FROM employees;

Tip: Window functions are essential for advanced SQL querying. They allow for complex calculations and data analysis without collapsing the result set into aggregated rows. Familiarize yourself with the syntax and use cases of each window function to effectively utilize them in your queries.

Here you can find SQL Interview Resources👇
https://whatsapp.com/channel/0029VaGgzAk72WTmQFERKh02

Share with credits: https://news.1rj.ru/str/sqlspecialist

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Data Analytics

Many people pay too much to learn Power BI, but my mission is to break down barriers. I have shared complete learning series to learn Power BI from scratch.

Here are the links to the Power BI series

Complete Power BI Topics for Data Analyst: https://news.1rj.ru/str/sqlspecialist/588

Part-1: https://news.1rj.ru/str/sqlspecialist/589

Part-2: https://news.1rj.ru/str/sqlspecialist/590

Part-3: https://news.1rj.ru/str/sqlspecialist/592

Part-4: https://news.1rj.ru/str/sqlspecialist/595

Part-5: https://news.1rj.ru/str/sqlspecialist/597

Part-6: https://news.1rj.ru/str/sqlspecialist/600

Part-7: https://news.1rj.ru/str/sqlspecialist/603

Part-8: https://news.1rj.ru/str/sqlspecialist/604

Part-9: https://news.1rj.ru/str/sqlspecialist/605

Part-10: https://news.1rj.ru/str/sqlspecialist/606

Part-11: https://news.1rj.ru/str/sqlspecialist/609

Part-12:
https://news.1rj.ru/str/sqlspecialist/610

Part-13: https://news.1rj.ru/str/sqlspecialist/613

Part-14: https://news.1rj.ru/str/sqlspecialist/614

More Power BI Resources: https://news.1rj.ru/str/PowerBI_analyst

I'll continue with learning series on Excel & Tableau. I am also planning to start with Interview Preparation Series as have already covered essential concepts of Python, SQL & Power BI in detail.

Thanks to all who support our channel and share the content with proper credits. You guys are really amazing.

Hope it helps :)

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26.6K views12:49

Data Analytics

POWER BI INTERVIEW PREPARATION PART-20

What are Quick Measures in Power BI?

Answer:
- Quick Measures in Power BI are pre-built calculations that allow users to create common measures without writing DAX code.
- They provide a simplified interface where users can input data fields and parameters to generate the desired calculation.

Hope it helps :)

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18.5K views14:25

Data Analytics

SQL INTERVIEW PREPARATION PART-55

Explain the difference between UNION and UNION ALL in SQL.

UNION:
- Combines the result sets of two or more SELECT statements into a single result set.
- Removes duplicate rows from the combined result set.
- The columns in the SELECT statements must have the same number of columns, in the same order, with compatible data types.

Example:

SELECT first_name, last_name FROM employees
UNION
SELECT first_name, last_name FROM managers;

This query returns a list of unique first and last names from both the employees and managers tables, removing duplicates.

UNION ALL:
- Combines the result sets of two or more SELECT statements into a single result set.
- Includes all rows from the result sets, including duplicates.
- The columns in the SELECT statements must also match in number, order, and data types.

Example:

SELECT first_name, last_name FROM employees
UNION ALL
SELECT first_name, last_name FROM managers;

This query returns all first and last names from both the employees and managers tables, including any duplicates.

### Key Differences:

1. Duplicates:
- UNION removes duplicates.
- UNION ALL keeps all duplicates.

2. Performance:
- UNION is slower due to the need to remove duplicates.
- UNION ALL is faster as it doesn't need to check for duplicates.

Tip: Use UNION ALL when you are sure that there are no duplicates or when you want to retain all entries. Use UNION when you need unique records in the result set.

Here you can find SQL Interview Resources👇
https://whatsapp.com/channel/0029VaGgzAk72WTmQFERKh02

Share with credits: https://news.1rj.ru/str/sqlspecialist

Hope it helps :)

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19.9K viewsedited 13:49

Data Analytics

POWER BI INTERVIEW PREPARATION PART-21

What is the Q&A feature in Power BI?

Answer:
- The Q&A feature in Power BI allows users to ask natural language questions about their data and get answers in the form of visualizations.
- It uses AI to interpret the question and return the most relevant chart or graph.

More Power BI Resources: https://news.1rj.ru/str/PowerBI_analyst

Hope it helps :)

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16.3K views17:14

Data Analytics

SQL INTERVIEW PREPARATION PART-56

What is the difference between DELETE, TRUNCATE, and DROP in SQL?

DELETE:
- DELETE is a DML (Data Manipulation Language) command used to remove specific rows from a table based on a WHERE clause.
- It logs each row deletion in the transaction log, making it slower and allowing the operation to be rolled back.
- Triggers are fired for the operation.

Example:

DELETE FROM employees WHERE department_id = 3;

This command deletes all rows from the employees table where the department_id is 3.

TRUNCATE:
- TRUNCATE is a DDL (Data Definition Language) command used to remove all rows from a table, effectively resetting it.
- It is faster than DELETE because it doesn't log individual row deletions; instead, it logs the deallocation of the data pages.
- Triggers are not fired for the operation, and it cannot be rolled back in some databases.

Example:

TRUNCATE TABLE employees;

This command removes all rows from the employees table but retains the table structure for future use.

DROP:
- DROP is a DDL command used to delete the entire table or database from the database system.
- It completely removes the table structure along with all the data and cannot be rolled back.

Example:

DROP TABLE employees;

This command completely removes the employees table from the database.

### Key Differences:

1. Purpose:
- DELETE removes specific rows.
- TRUNCATE removes all rows but retains the table structure.
- DROP removes the entire table or database.

2. Logging and Performance:
- DELETE logs each row deletion, making it slower.
- TRUNCATE deallocates data pages, making it faster.
- DROP simply removes the entire table structure.

3. Rollback:
- DELETE can be rolled back.
- TRUNCATE might not be rolled back depending on the database system.
- DROP cannot be rolled back.

Tip: Choose the appropriate command based on the task: use DELETE for selective row removal, TRUNCATE for clearing a table while keeping its structure, and DROP when you no longer need the table or database at all.

Share with credits: https://news.1rj.ru/str/sqlspecialist

Hope it helps :)

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19.3K views13:49

Data Analytics

POWER BI INTERVIEW PREPARATION PART-22

What is drillthrough in Power BI?

Answer:
- Drillthrough in Power BI allows users to navigate from a summary page to a detailed report page by right-clicking on a data point.
- It helps in providing deeper insights by focusing on specific data, such as drilling down from overall sales to details of a particular product category.

Hope it helps :)

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18.9K views16:15

Data Analytics

POWER BI INTERVIEW PREPARATION PART-23

What is a KPI visual in Power BI and how is it used?

Answer:
- A KPI (Key Performance Indicator) visual in Power BI is used to evaluate the performance of a metric against a target.
- It typically displays the current value, the target value, and an indicator (e.g., color or trend) showing whether the performance is on track, below, or above the target.

Hope it helps :)

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18.6K views16:15

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