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Which LIKE pattern finds names that begin with the letter A?
Anonymous Quiz
31%
A) LIKE '%A'
7%
B) LIKE '_A%'
47%
C) LIKE 'A%'
15%
D) LIKE '%A%'
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๐Ÿš€ Data Science Roadmap 2026

๐Ÿ“ Phase 3: SQL for Data Science

๐Ÿ“– Topic 3 โ€” ORDER BY

ORDER BY is used to sort the rows returned by a SQL query. It helps you arrange data in a meaningful order, such as:

โ€ข Highest salary to lowest salary

โ€ข Lowest price to highest price

โ€ข Newest date to oldest date

โ€ข Aโ€“Z or Zโ€“A

โ€ข Highest sales to lowest sales

๐Ÿ”น Basic Syntax

SELECT column1, column2
FROM table_name
ORDER BY column_name;


By default, SQL sorts in ascending order.

SELECT *
FROM employees
ORDER BY salary;


This generally returns employees from the lowest salary to the highest salary.

๐Ÿ”น ASC โ€” Ascending Order

ASC means ascending.

SELECT *
FROM employees
ORDER BY salary ASC;


For numbers: 10000, 25000, 40000, 75000, 100000

For text: Amazon, Apple, Google, Microsoft

For dates: 2024-01-01, 2024-05-15, 2024-12-20, 2025-03-10

ASC is usually the default, so ORDER BY salary; is equivalent to ORDER BY salary ASC;

๐Ÿ”น DESC โ€” Descending Order

DESC means descending.

SELECT *
FROM employees
ORDER BY salary DESC;


Example: 100000, 75000, 40000, 25000, 10000

This is especially useful when you want to find:

โ€ข Highest-paid employees

โ€ข Top-selling products

โ€ข Most expensive products

โ€ข Most recent transactions

โ€ข Highest-performing regions

๐Ÿ”น Sorting Text

You can sort text columns alphabetically.

SELECT employee_name, department
FROM employees
ORDER BY employee_name ASC;


SELECT employee_name, department
FROM employees
ORDER BY employee_name DESC;


๐Ÿ”น Sorting Dates

To find the most recent transactions:

SELECT transaction_id, transaction_date, amount
FROM transactions
ORDER BY transaction_date DESC;


To see transactions from oldest to newest:

SELECT transaction_id, transaction_date, amount
FROM transactions
ORDER BY transaction_date ASC;


๐Ÿ”น Sorting by Multiple Columns

This is very important. Suppose you want to sort employees:

1. By department

2. Within each department, by salary from highest to lowest

SELECT employee_name, department, salary
FROM employees
ORDER BY department ASC, salary DESC;


SQL first sorts by department. When multiple rows have the same department, it then uses salary to determine their order.

Example:

Finance 90000

Finance 70000

Finance 50000

HR 85000

HR 60000

IT 120000

IT 95000

๐Ÿ”น ORDER BY with Calculations

You can also sort using an expression.

SELECT product_name, quantity, price,
quantity * price AS total_value
FROM products
ORDER BY quantity * price DESC;


You can also use the alias in many SQL databases:

SELECT product_name,
quantity * price AS total_value
FROM products
ORDER BY total_value DESC;


๐Ÿ”น ORDER BY with DISTINCT

SELECT DISTINCT department
FROM employees
ORDER BY department ASC;


This returns each department once and sorts them alphabetically.

๐Ÿ”น ORDER BY and NULL Values

NULL represents a missing or unknown value. The position of NULL values when using ORDER BY can vary between SQL databases.

Some databases also support explicit control such as:

ORDER BY salary ASC NULLS LAST;


or

ORDER BY salary DESC NULLS FIRST;


Always check the syntax supported by your SQL database.

๐Ÿ”น ORDER BY with WHERE

WHERE filters the rows first, and ORDER BY sorts the resulting rows.

SELECT employee_name, salary
FROM employees
WHERE department = 'IT'
ORDER BY salary DESC;
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๐Ÿ”น Real-World Data Science Example

Suppose you are analyzing sales data. You want to identify the highest-value transactions.

SELECT customer_id, product_id, quantity, price,
quantity * price AS transaction_value
FROM sales
ORDER BY transaction_value DESC;


This can help with:

โ€ข Finding high-value transactions

โ€ข Identifying important customers

โ€ข Investigating unusually large purchases

โ€ข Preparing data for further analysis

๐Ÿ”น Common Mistakes

Mistake 1 โ€” Forgetting DESC

If you want the highest salary first: ORDER BY salary DESC; Not ORDER BY salary;

Mistake 2 โ€” Wrong column

Make sure the column used for sorting actually represents what you want to analyze.

Mistake 3 โ€” Incorrect multiple-column ordering

ORDER BY department, salary DESC; does NOT mean both columns are descending. It means: department โ†’ ASC, salary โ†’ DESC

If you want both descending: ORDER BY department DESC, salary DESC;

Mistake 4 โ€” Confusing WHERE and ORDER BY

WHERE filters rows. ORDER BY sorts rows. They perform different jobs.

๐Ÿ”น Interview Questions

1. What is ORDER BY?

ORDER BY sorts query results based on one or more columns.

2. What is the default sorting order?

Ascending order (ASC) is generally the default.

3. How do you find the highest salary?

SELECT * FROM employees ORDER BY salary DESC;


4. Can you sort using multiple columns?

Yes. ORDER BY department ASC, salary DESC;

5. What is the difference between ASC and DESC?

ASC sorts from low to high or Aโ€“Z. DESC sorts from high to low or Zโ€“A.

๐ŸŽฏ Practice Questions

1. Write a query to display all products from the cheapest to the most expensive.

2. Write a query to display employees from the highest salary to the lowest salary.

3. Write a query to display customers alphabetically by name.

4. Write a query to sort sales by date, showing the newest sales first.

5. Write a query to sort employees by department alphabetically and salary from highest to lowest within each department.

๐ŸŽฏ Key Takeaways

โ€ข ORDER BY sorts query results

โ€ข ASC means ascending

โ€ข DESC means descending

โ€ข Ascending is generally the default

โ€ข You can sort numbers, text and dates

โ€ข You can sort using multiple columns

โ€ข WHERE filters data; ORDER BY sorts data

โ€ข ORDER BY is especially useful when analyzing rankings and top/bottom records

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๐Ÿš€ Data Science Roadmap 2026

๐Ÿ“ Phase 3: SQL for Data Science

๐Ÿ“– Topic 4 โ€” LIMIT

LIMIT is used to control how many rows a SQL query returns.

It is especially useful when you want to see only the first few records, such as:

โ€ข Top 5 highest salaries

โ€ข Top 10 products by sales

โ€ข First 20 customers

โ€ข Latest 10 transactions

๐Ÿ”น Basic Syntax

SELECT *
FROM employees
LIMIT 5;


This returns only 5 rows from the result.

๐Ÿ”น LIMIT with ORDER BY

This is where LIMIT becomes especially powerful.

Suppose you want the 5 highest-paid employees:

SELECT employee_name, salary
FROM employees
ORDER BY salary DESC
LIMIT 5;


The logical idea is:

Filter โ†’ Sort โ†’ Take required number of rows

For example:

Salary
120000
110000
95000
90000
85000


Only the top 5 rows are returned.

๐Ÿ”น LIMIT Without ORDER BY

You can write:

SELECT *
FROM employees
LIMIT 5;


But without ORDER BY, you should not assume these are the "first" or "top" 5 meaningful records.

The database does not guarantee a particular order unless you explicitly specify one.

So for analysis, this is usually better:

SELECT *
FROM employees
ORDER BY salary DESC
LIMIT 5;


๐Ÿ”น LIMIT with WHERE

You can combine WHERE, ORDER BY, and LIMIT.

Example:

SELECT employee_name, salary
FROM employees
WHERE department = 'IT'
ORDER BY salary DESC
LIMIT 3;


This finds the 3 highest-paid employees in the IT department.

๐Ÿ”น LIMIT for Top-N Analysis

Top-N queries are extremely common in data analysis.

Top 10 products by sales

SELECT product_name, sales
FROM products
ORDER BY sales DESC
LIMIT 10;


5 most recent transactions

SELECT transaction_id, transaction_date, amount
FROM transactions
ORDER BY transaction_date DESC
LIMIT 5;


3 cheapest products

SELECT product_name, price
FROM products
ORDER BY price ASC
LIMIT 3;


๐Ÿ”น LIMIT with OFFSET

OFFSET allows you to skip a certain number of rows before returning results.

Example:

SELECT *
FROM employees
LIMIT 5 OFFSET 10;


This means:

Skip 10 rows

โ†“

Return the next 5 rows

This is commonly used for pagination.

For example:

โ€ข Page 1 โ†’ LIMIT 10 OFFSET 0

โ€ข Page 2 โ†’ LIMIT 10 OFFSET 10

โ€ข Page 3 โ†’ LIMIT 10 OFFSET 20

Syntax can vary between SQL databases, so always check the specific database you are using.

๐Ÿ”น LIMIT vs TOP vs FETCH

Different SQL databases may use different syntax.

MySQL/PostgreSQL/SQLite commonly use:

LIMIT 5;

SQL Server commonly uses:

SELECT TOP 5 *
FROM employees;


Standard SQL also supports:

FETCH FIRST 5 ROWS ONLY;

The concept is the same: restrict the number of returned rows.

๐Ÿ”น Real-World Data Science Example

Suppose an analyst wants to investigate the 10 largest customer transactions.

SELECT customer_id,
transaction_id,
amount
FROM transactions
ORDER BY amount DESC
LIMIT 10;


This can help identify:

โ€ข High-value transactions

โ€ข Potential anomalies

โ€ข Important customers

โ€ข Transactions requiring further investigation

๐Ÿ”น Common Mistakes

Mistake 1 โ€” Using LIMIT without sorting

SELECT *
FROM products
LIMIT 10;


This does not necessarily mean "10 best products."

For that:

ORDER BY sales DESC
LIMIT 10;


Mistake 2 โ€” Using ASC when you need the highest values

Wrong:

ORDER BY salary ASC
LIMIT 5;


This returns the lowest salaries.

For the highest salaries:
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ORDER BY salary DESC
LIMIT 5;


Mistake 3 โ€” Forgetting that LIMIT applies after sorting

For a top-N analysis, think:

ORDER BY โ†’ LIMIT

not simply:

LIMIT โ†’ assume top records

๐Ÿ”น Interview Questions

1. What is LIMIT?

LIMIT restricts the number of rows returned by a query.

2. How do you find the top 10 salaries?

SELECT *
FROM employees
ORDER BY salary DESC
LIMIT 10;


3. Can LIMIT be used with WHERE?

Yes.

SELECT *
FROM employees
WHERE department = 'Finance'
LIMIT 5;


4. Why is ORDER BY important when using LIMIT?

Because without sorting, the returned rows are not necessarily the highest, lowest, newest, or oldest records you want.

5. What is OFFSET used for?

OFFSET skips a specified number of rows before returning the requested rows, commonly for pagination.

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