SELECT with WHERE, ORDER BY, GROUP BY in DBMS Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from the table named 'employees'.

DBMS Theory

SELECT [1] FROM employees;

Drag options to blanks, or click blank then click option'

Acolumns

BALL

C*

Deverything

Attempts:

3 left

💡 Hint

Common Mistakes

Writing 'ALL' instead of '*'

Using 'columns' which is not valid SQL syntax

The asterisk (*) means select all columns from the table.

2fill in blank

medium

Complete the code to select employees with salary greater than 50000.

DBMS Theory

SELECT name, salary FROM employees WHERE salary [1] 50000;

Drag options to blanks, or click blank then click option'

A<

B>

C=

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' which selects salaries less than 50000

Using '=' which selects salaries exactly 50000

The '>' operator filters rows where salary is greater than 50000.

3fill in blank

hard

Fix the error in the code to order employees by their hire date in ascending order.

DBMS Theory

SELECT name, hire_date FROM employees ORDER BY hire_date [1];

Drag options to blanks, or click blank then click option'

AUP

BDESC

CDOWN

DASC

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'DESC' which sorts in descending order

Using 'UP' or 'DOWN' which are not valid SQL keywords

'ASC' means ascending order, which sorts from oldest to newest.

4fill in blank

hard

Fill both blanks to group employees by department and count how many are in each.

DBMS Theory

SELECT [1], COUNT(*) FROM employees GROUP BY [2];

Drag options to blanks, or click blank then click option'

Adepartment

Bsalary

Cdepartment_id

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Grouping by 'name' which is unique per employee

Selecting 'salary' which is not a grouping column

We select the department name and group by department name to count employees per department.

5fill in blank

hard

Fill all three blanks to select department names, average salary, and order results by average salary descending.

DBMS Theory

SELECT [1], AVG([2]) FROM employees GROUP BY [3] ORDER BY AVG(salary) DESC;

Drag options to blanks, or click blank then click option'

Adepartment_name

Bsalary

Demployee_id

Attempts:

3 left

💡 Hint

Common Mistakes

Grouping by employee_id which is unique per row

Averaging department_name which is not numeric

We select department_name, calculate average salary, group by department_name, and order by average salary descending.

Practice

1. What does the WHERE clause do in a SQL SELECT statement?

easy

A. Filters rows based on a condition

B. Groups rows with the same values

C. Sorts the result set

D. Joins two tables together

Solution

1. Step 1: Understand the role of WHERE clause

   The WHERE clause filters rows that meet a specific condition before any grouping or sorting.

2. Step 2: Differentiate from GROUP BY and ORDER BY

   GROUP BY groups rows, ORDER BY sorts rows, but WHERE filters rows.

3. Final Answer:

   Filters rows based on a condition -> Option A

4. Quick Check:

   WHERE = Filters rows [OK]

Hint: WHERE filters rows before grouping or sorting [OK]

Common Mistakes:

• Confusing WHERE with GROUP BY
• Thinking WHERE sorts data
• Assuming WHERE joins tables

2. Which of the following is the correct syntax to select all columns from a table named Employees where the Department is 'Sales'?

easy

A. SELECT * FROM Employees WHERE Department = 'Sales';

B. SELECT * Employees WHERE Department == 'Sales';

C. SELECT * FROM Employees WHERE Department := 'Sales';

D. SELECT * FROM Employees WHERE Department = Sales;

Solution

1. Step 1: Check correct SELECT syntax

   The correct syntax uses SELECT * FROM table WHERE condition with single equals and quotes around string values.

2. Step 2: Identify syntax errors in other options

   SELECT * Employees WHERE Department == 'Sales'; uses '==' which is invalid in SQL; SELECT * FROM Employees WHERE Department := 'Sales'; uses ':=' which is not standard; SELECT * FROM Employees WHERE Department = Sales; misses quotes around 'Sales'.

3. Final Answer:

   SELECT * FROM Employees WHERE Department = 'Sales'; -> Option A

4. Quick Check:

   Correct WHERE syntax uses = and quotes for strings [OK]

Hint: Use single = and quotes for string in WHERE [OK]

Common Mistakes:

• Using == instead of =
• Missing quotes around string values
• Wrong assignment operator :=

3. Given the table Sales with columns Region and Amount, what will this query return?

SELECT Region, SUM(Amount) FROM Sales WHERE Amount > 100 GROUP BY Region ORDER BY SUM(Amount) DESC;

medium

A. Sum of all sales amounts regardless of region

B. All sales records with amount over 100, sorted by region

C. Total sales amount per region for sales over 100, sorted highest to lowest

D. Syntax error due to ORDER BY on aggregate

Solution

1. Step 1: Analyze WHERE and GROUP BY

   WHERE filters rows with Amount > 100, GROUP BY groups rows by Region, SUM(Amount) calculates total per group.

2. Step 2: Understand ORDER BY with aggregate

   ORDER BY SUM(Amount) DESC sorts groups by total sales descending.

3. Final Answer:

   Total sales amount per region for sales over 100, sorted highest to lowest -> Option C

4. Quick Check:

   WHERE filters, GROUP BY groups, ORDER BY sorts aggregates [OK]

Hint: ORDER BY can sort aggregates after GROUP BY [OK]

Common Mistakes:

• Thinking ORDER BY on aggregate causes error
• Ignoring WHERE filter before grouping
• Confusing sorting by region instead of sum

4. Identify the error in this SQL query:

SELECT Department, COUNT(*) FROM Employees ORDER BY Department WHERE COUNT(*) > 5 GROUP BY Department;

medium

A. Missing semicolon at the end

B. ORDER BY appears before WHERE and GROUP BY, causing syntax error

C. WHERE cannot filter on aggregate functions like COUNT(*)

D. COUNT(*) cannot be used with GROUP BY

Solution

1. Step 1: Check SQL clause order

   The correct order is SELECT, FROM, WHERE, GROUP BY, HAVING, ORDER BY. ORDER BY before WHERE is invalid.

2. Step 2: Confirm other clauses

   COUNT(*) with GROUP BY is valid; filtering on aggregates requires HAVING, not WHERE; but main syntax error is clause order.

3. Final Answer:

   ORDER BY appears before WHERE and GROUP BY, causing syntax error -> Option B

4. Quick Check:

   Clause order matters: WHERE before GROUP BY, ORDER BY last [OK]

Hint: Remember SQL clause order: WHERE, GROUP BY, HAVING, ORDER BY [OK]

Common Mistakes:

• Placing ORDER BY before WHERE
• Using WHERE to filter aggregates instead of HAVING
• Ignoring clause order rules

5. You have a table Orders with columns CustomerID, OrderDate, and Total. Write a query to find customers who placed more than 3 orders in 2023, showing their CustomerID and total number of orders, sorted by number of orders descending.

hard

A. SELECT CustomerID, COUNT(*) FROM Orders WHERE YEAR(OrderDate) == 2023 GROUP BY CustomerID HAVING COUNT(*) > 3 ORDER BY COUNT(*) DESC;

B. SELECT CustomerID, COUNT(*) FROM Orders GROUP BY CustomerID WHERE YEAR(OrderDate) = 2023 HAVING COUNT(*) > 3 ORDER BY COUNT(*) DESC;

C. SELECT CustomerID, COUNT(*) FROM Orders WHERE OrderDate >= '2023-01-01' AND OrderDate <= '2023-12-31' GROUP BY CustomerID HAVING COUNT(*) >= 3 ORDER BY COUNT(*) DESC;

D. SELECT CustomerID, COUNT(*) FROM Orders WHERE OrderDate BETWEEN '2023-01-01' AND '2023-12-31' GROUP BY CustomerID HAVING COUNT(*) > 3 ORDER BY COUNT(*) DESC;

Solution

1. Step 1: Filter orders in 2023

   Use WHERE with OrderDate between '2023-01-01' and '2023-12-31' to select orders in 2023.

2. Step 2: Group by CustomerID and filter groups

   GROUP BY CustomerID groups orders per customer; HAVING COUNT(*) > 3 filters customers with more than 3 orders.

3. Step 3: Sort results

   ORDER BY COUNT(*) DESC sorts customers by number of orders from highest to lowest.

4. Final Answer:

   SELECT CustomerID, COUNT(*) FROM Orders WHERE OrderDate BETWEEN '2023-01-01' AND '2023-12-31' GROUP BY CustomerID HAVING COUNT(*) > 3 ORDER BY COUNT(*) DESC; -> Option D

5. Quick Check:

   Filter dates, group, HAVING filter, then ORDER BY count desc [OK]

Hint: Use HAVING to filter groups after GROUP BY [OK]

Common Mistakes:

• Using WHERE to filter aggregates instead of HAVING
• Incorrect date filtering syntax
• Using >= 3 instead of > 3 for count condition