What does this condition return?
WHERE age BETWEEN 25 AND 30
WHERE age BETWEEN 25 AND 30
Anonymous Quiz
47%
Ages greater than 25 and less than 30 only
46%
Ages from 25 through 30, including both 25 and 30
5%
Only ages exactly 25 and 30
2%
Ages below 25 or above 30
โค1
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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๐ Data Science Roadmap 2026
๐ Phase 3: SQL for Data Science
๐ 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:
โข Highest salary to lowest salary
โข Lowest price to highest price
โข Newest date to oldest date
โข AโZ or ZโA
โข Highest sales to lowest sales
๐น Basic Syntax
By default, SQL sorts in ascending order.
This generally returns employees from the lowest salary to the highest salary.
๐น ASC โ Ascending Order
ASC means ascending.
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
๐น DESC โ Descending Order
DESC means descending.
Example: 100000, 75000, 40000, 25000, 10000
This is especially useful when you want to find:
โข Highest-paid employees
โข Top-selling products
โข Most expensive products
โข Most recent transactions
โข Highest-performing regions
๐น Sorting Text
You can sort text columns alphabetically.
๐น Sorting Dates
To find the most recent transactions:
To see transactions from oldest to newest:
๐น 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
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
๐น ORDER BY with Calculations
You can also sort using an expression.
You can also use the alias in many SQL databases:
๐น ORDER BY with DISTINCT
This returns each department once and sorts them alphabetically.
๐น 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:
or
Always check the syntax supported by your SQL database.
๐น ORDER BY with WHERE
WHERE filters the rows first, and ORDER BY sorts the resulting rows.
๐ Phase 3: SQL for Data Science
๐ 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:
โข Highest salary to lowest salary
โข Lowest price to highest price
โข Newest date to oldest date
โข AโZ or ZโA
โข Highest sales to lowest sales
๐น 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.
๐น 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;๐น 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:
โข Highest-paid employees
โข Top-selling products
โข Most expensive products
โข Most recent transactions
โข Highest-performing regions
๐น 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;
๐น 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;
๐น 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
๐น 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;
๐น ORDER BY with DISTINCT
SELECT DISTINCT department
FROM employees
ORDER BY department ASC;
This returns each department once and sorts them alphabetically.
๐น 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.
๐น 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;
โค4
๐น Real-World Data Science Example
Suppose you are analyzing sales data. You want to identify the highest-value transactions.
This can help with:
โข Finding high-value transactions
โข Identifying important customers
โข Investigating unusually large purchases
โข Preparing data for further analysis
๐น Common Mistakes
Mistake 1 โ Forgetting DESC
If you want the highest salary first:
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
If you want both descending:
Mistake 4 โ Confusing WHERE and ORDER BY
WHERE filters rows. ORDER BY sorts rows. They perform different jobs.
๐น 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?
4. Can you sort using multiple columns?
Yes.
5. What is the difference between ASC and DESC?
ASC sorts from low to high or AโZ. DESC sorts from high to low or ZโA.
๐ฏ 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.
๐ฏ Key Takeaways
โข ORDER BY sorts query results
โข ASC means ascending
โข DESC means descending
โข Ascending is generally the default
โข You can sort numbers, text and dates
โข You can sort using multiple columns
โข WHERE filters data; ORDER BY sorts data
โข ORDER BY is especially useful when analyzing rankings and top/bottom records
๐ Double Tap โค๏ธ For More
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:
โข Finding high-value transactions
โข Identifying important customers
โข Investigating unusually large purchases
โข Preparing data for further analysis
๐น 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 DESCIf 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.
๐น 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 AโZ. DESC sorts from high to low or ZโA.
๐ฏ 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.
๐ฏ Key Takeaways
โข ORDER BY sorts query results
โข ASC means ascending
โข DESC means descending
โข Ascending is generally the default
โข You can sort numbers, text and dates
โข You can sort using multiple columns
โข WHERE filters data; ORDER BY sorts data
โข ORDER BY is especially useful when analyzing rankings and top/bottom records
๐ Double Tap โค๏ธ For More
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๐ Data Science Roadmap 2026
๐ Phase 3: SQL for Data Science
๐ 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:
โข Top 5 highest salaries
โข Top 10 products by sales
โข First 20 customers
โข Latest 10 transactions
๐น Basic Syntax
This returns only 5 rows from the result.
๐น LIMIT with ORDER BY
This is where LIMIT becomes especially powerful.
Suppose you want the 5 highest-paid employees:
The logical idea is:
Filter โ Sort โ Take required number of rows
For example:
Only the top 5 rows are returned.
๐น LIMIT Without ORDER BY
You can write:
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:
๐น LIMIT with WHERE
You can combine WHERE, ORDER BY, and LIMIT.
Example:
This finds the 3 highest-paid employees in the IT department.
๐น LIMIT for Top-N Analysis
Top-N queries are extremely common in data analysis.
Top 10 products by sales
5 most recent transactions
3 cheapest products
๐น LIMIT with OFFSET
OFFSET allows you to skip a certain number of rows before returning results.
Example:
This means:
Skip 10 rows
โ
Return the next 5 rows
This is commonly used for pagination.
For example:
โข Page 1 โ
โข Page 2 โ
โข Page 3 โ
Syntax can vary between SQL databases, so always check the specific database you are using.
๐น LIMIT vs TOP vs FETCH
Different SQL databases may use different syntax.
MySQL/PostgreSQL/SQLite commonly use:
SQL Server commonly uses:
Standard SQL also supports:
The concept is the same: restrict the number of returned rows.
๐น Real-World Data Science Example
Suppose an analyst wants to investigate the 10 largest customer transactions.
This can help identify:
โข High-value transactions
โข Potential anomalies
โข Important customers
โข Transactions requiring further investigation
๐น Common Mistakes
Mistake 1 โ Using LIMIT without sorting
This does not necessarily mean "10 best products."
For that:
Mistake 2 โ Using ASC when you need the highest values
Wrong:
This returns the lowest salaries.
For the highest salaries:
๐ Phase 3: SQL for Data Science
๐ 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:
โข Top 5 highest salaries
โข Top 10 products by sales
โข First 20 customers
โข Latest 10 transactions
๐น Basic Syntax
SELECT *
FROM employees
LIMIT 5;
This returns only 5 rows from the result.
๐น 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.
๐น 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;
๐น 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.
๐น 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;
๐น 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:
โข Page 1 โ
LIMIT 10 OFFSET 0โข Page 2 โ
LIMIT 10 OFFSET 10โข Page 3 โ
LIMIT 10 OFFSET 20Syntax can vary between SQL databases, so always check the specific database you are using.
๐น 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.
๐น 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:
โข High-value transactions
โข Potential anomalies
โข Important customers
โข Transactions requiring further investigation
๐น 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:
โค1
ORDER BY salary DESC
LIMIT 5;
Mistake 3 โ Forgetting that LIMIT applies after sorting
For a top-N analysis, think:
ORDER BY โ LIMITnot simply:
LIMIT โ assume top records๐น 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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