Selection operation implementation in DBMS Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the SQL query 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'

A*

BALL

CEMPLOYEES

DCOLUMNS

Attempts:

3 left

💡 Hint

Common Mistakes

Using the table name instead of * to select all columns.

Using keywords like ALL or COLUMNS which are not valid in this context.

The asterisk (*) symbol in SQL means to select all columns from the specified table.

2fill in blank

medium

Complete the SQL query to select only the 'name' column from the 'employees' table.

DBMS Theory

SELECT [1] FROM employees;

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

Aage

Bsalary

Cname

Ddepartment

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting a column that does not exist or is unrelated.

Using * instead of a specific column name.

To select a specific column, write its exact name after SELECT.

3fill in blank

hard

Fix the error in the SQL query to select employees with salary greater than 50000.

DBMS Theory

SELECT * 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 means equal, not greater than.

Using '<' or '<=' which mean less than.

The operator '>' means greater than, which matches the requirement.

4fill in blank

hard

Fill both blanks to select employees whose age is between 25 and 40.

DBMS Theory

SELECT * FROM employees WHERE age [1] 25 AND age [2] 40;

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

A>=

B<

C<=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using > or < which exclude boundary values.

Mixing up the order of operators.

To select ages between 25 and 40 inclusive, use >= 25 and <= 40.

5fill in blank

hard

Fill all three blanks to select employee names and salaries where salary is above 60000 and department is 'Sales'.

DBMS Theory

SELECT [1], [2] FROM employees WHERE salary [3] 60000 AND department = 'Sales';

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

Aname

Bsalary

C>

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting wrong columns like 'age' instead of 'salary'.

Using '=' instead of '>' for salary condition.

Select the columns 'name' and 'salary', and use '>' to filter salaries above 60000.

Practice

1. What is the main purpose of the SELECT statement with a WHERE clause in a database?

easy

A. To change the structure of a table

B. To delete rows from a table

C. To add new columns to a table

D. To retrieve only rows that meet specific conditions

Solution

1. Step 1: Understand the role of SELECT

   The SELECT statement is used to get data from a table.

2. Step 2: Understand the role of WHERE clause

   The WHERE clause filters rows to include only those that meet given conditions.

3. Final Answer:

   To retrieve only rows that meet specific conditions -> Option D

4. Quick Check:

   SELECT + WHERE = filtered rows [OK]

Hint: WHERE filters rows; SELECT retrieves data [OK]

Common Mistakes:

• Confusing WHERE with DELETE
• Thinking WHERE adds columns
• Believing WHERE changes table structure

2. Which of the following is the correct syntax to select all columns from a table named Employees where the Age is greater than 30?

easy

A. SELECT * FROM Employees WHERE Age > 30;

B. SELECT * Employees WHERE Age > 30;

C. SELECT FROM Employees WHERE Age > 30;

D. SELECT * FROM Employees AGE > 30;

Solution

1. Step 1: Check SELECT syntax

   The correct syntax starts with SELECT, then columns or *, then FROM table name.

2. Step 2: Check WHERE clause syntax

   WHERE must be followed by a condition like Age > 30.

3. Final Answer:

   SELECT * FROM Employees WHERE Age > 30; -> Option A

4. Quick Check:

   Correct SELECT + FROM + WHERE syntax [OK]

Hint: SELECT * FROM table WHERE condition; [OK]

Common Mistakes:

• Omitting FROM keyword
• Placing WHERE before FROM
• Missing semicolon at end

3. Consider the table Products with columns ProductID, Name, and Price. What will be the result of this query?

SELECT Name FROM Products WHERE Price <= 50;

medium

A. All product names regardless of price

B. All product names with price less than or equal to 50

C. All product names with price greater than 50

D. An error because Price <= 50 is invalid

Solution

1. Step 1: Understand the SELECT clause

   The query selects only the Name column from the Products table.

2. Step 2: Understand the WHERE condition

   The condition Price <= 50 filters rows to those with price 50 or less.

3. Final Answer:

   All product names with price less than or equal to 50 -> Option B

4. Quick Check:

   WHERE Price <= 50 filters products [OK]

Hint: WHERE filters rows by condition; SELECT picks columns [OK]

Common Mistakes:

• Confusing <= with >= operator
• Expecting all products without filter
• Thinking query causes error

4. Identify the error in the following SQL query:

SELECT * FROM Customers WHERE City = 'New York'

medium

A. Missing FROM keyword

B. Incorrect use of single quotes around string

C. Missing semicolon at the end

D. WHERE clause should be after ORDER BY

Solution

1. Step 1: Check SQL syntax completeness

   SQL statements should end with a semicolon to mark the end.

2. Step 2: Verify other parts

   FROM keyword is present, single quotes around string are correct, WHERE comes before ORDER BY.

3. Final Answer:

   Missing semicolon at the end -> Option C

4. Quick Check:

   SQL statements end with ; [OK]

Hint: Always end SQL statements with a semicolon [OK]

Common Mistakes:

• Forgetting semicolon
• Misplacing WHERE clause
• Using double quotes instead of single quotes

5. You have a table Orders with columns OrderID, CustomerID, and Status. You want to select all orders that are either 'Pending' or 'Processing'. Which SQL query correctly implements this selection?

hard

A. SELECT * FROM Orders WHERE Status IN ('Pending', 'Processing');

B. SELECT * FROM Orders WHERE Status = 'Pending' AND 'Processing';

C. SELECT * FROM Orders WHERE Status = 'Pending' OR 'Processing';

D. SELECT * FROM Orders WHERE Status = 'Pending', 'Processing';

Solution

1. Step 1: Understand the condition for multiple values

   To select rows where Status matches multiple values, use IN or multiple OR conditions.

2. Step 2: Compare options

   IN ('Pending', 'Processing') is correct and concise. OR requires full conditions like Status = 'Pending' OR Status = 'Processing'. A lone string after OR like 'Processing' makes the condition always true, selecting extra rows. AND between values or commas cause syntax errors.

3. Final Answer:

   SELECT * FROM Orders WHERE Status IN ('Pending', 'Processing'); -> Option A

4. Quick Check:

   Use IN for multiple values in WHERE [OK]

Hint: Use IN for multiple OR conditions in WHERE [OK]

Common Mistakes:

• Using AND instead of OR
• Incorrect syntax with commas in WHERE
• Not using quotes around string values