Why built-in functions matter in SQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select the current date using a built-in SQL function.

SQL

SELECT [1];

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

ACURRENT_DATE

BNOW

CGETDATE

DTODAY

Attempts:

3 left

💡 Hint

Common Mistakes

Using NOW() returns date and time, not just the date.

GETDATE() is specific to some SQL dialects like SQL Server.

TODAY() is not a standard SQL function.

The CURRENT_DATE function returns the current date in SQL.

2fill in blank

medium

Complete the code to count the number of rows in the 'employees' table using a built-in function.

SQL

SELECT [1](*) FROM employees;

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

ASUM

BNUMBER

CTOTAL

DCOUNT

Attempts:

3 left

💡 Hint

Common Mistakes

Using SUM(*) is invalid because SUM expects a numeric column.

TOTAL is not a standard SQL aggregate function.

NUMBER is not a SQL function.

The COUNT(*) function counts all rows in a table.

3fill in blank

hard

Fix the error in the code to convert the 'name' column to uppercase using a built-in function.

SQL

SELECT [1](name) FROM users;

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

Alower

Bcapitalize

CUPPER

Dtitle

Attempts:

3 left

💡 Hint

Common Mistakes

Using lower() converts text to lowercase, not uppercase.

capitalize and title are not standard SQL functions.

The UPPER() function converts text to uppercase in SQL.

4fill in blank

hard

Fill both blanks to select the average salary and round it to 2 decimal places.

SQL

SELECT ROUND(AVG([1]), [2]) FROM employees;

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

Asalary

B2

C3

Dbonus

Attempts:

3 left

💡 Hint

Common Mistakes

Using bonus instead of salary changes the meaning.

Rounding to 3 decimals when 2 is requested.

Use AVG(salary) to get the average salary and ROUND(..., 2) to round to 2 decimals.

5fill in blank

hard

Fill all three blanks to select the maximum age, minimum age, and calculate the age difference in the 'people' table.

SQL

SELECT MAX([1]) AS max_age, MIN([2]) AS min_age, MAX(age) - MIN([3]) AS age_diff FROM people;

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

Aage

Bbirth_year

Dheight

Attempts:

3 left

💡 Hint

Common Mistakes

Using birth_year or height instead of age causes wrong results.

Mixing different columns in the calculation.

Use age column for MAX and MIN functions to find max and min age, then subtract to get difference.

Practice

1. Why are built-in functions important in SQL?
COUNT(), SUM(), and UPPER() are examples of these functions.

easy

A. They simplify common tasks like counting and text changes.

B. They make the database slower by adding extra steps.

C. They require writing complex code every time.

D. They only work with numbers, not text or dates.

Solution

1. Step 1: Understand what built-in functions do

   Built-in functions perform common tasks like counting rows or changing text case without extra code.

2. Step 2: Identify the benefit in SQL queries

   Using these functions makes queries simpler and clearer, saving time and effort.

3. Final Answer:

   They simplify common tasks like counting and text changes. -> Option A

4. Quick Check:

   Built-in functions simplify tasks = C [OK]

Hint: Built-in functions do common tasks quickly [OK]

Common Mistakes:

• Thinking built-in functions slow down queries
• Believing built-in functions only work with numbers
• Assuming built-in functions require complex code

2. Which of the following is the correct syntax to convert a text column name to uppercase in SQL?

easy

A. SELECT UPPERCASE(name) FROM table;

B. SELECT TOUPPER(name) FROM table;

C. SELECT UPPER(name) FROM table;

D. SELECT UCASE(name) FROM table;

Solution

1. Step 1: Recall the standard SQL function for uppercase

   The standard function to convert text to uppercase is UPPER().

2. Step 2: Check the syntax correctness

   The correct syntax is SELECT UPPER(column) FROM table;. Other options are not standard SQL functions.

3. Final Answer:

   SELECT UPPER(name) FROM table; -> Option C

4. Quick Check:

   Uppercase function = UPPER() [OK]

Hint: Use UPPER() to convert text to uppercase [OK]

Common Mistakes:

• Using non-standard function names like TOUPPER or UCASE
• Adding extra letters to function names
• Forgetting parentheses after function name

3. Given the table sales with a column amount, what will this query return?

SELECT COUNT(amount), SUM(amount) FROM sales WHERE amount > 100;

medium

A. The count and sum of amounts greater than 100.

B. The total number of rows and sum of all amounts in the table.

C. Only the sum of amounts greater than 100.

D. An error because COUNT and SUM cannot be used together.

Solution

1. Step 1: Understand the WHERE clause effect

   The WHERE clause filters rows where amount > 100, so only those rows are counted and summed.

2. Step 2: Analyze COUNT and SUM functions

   COUNT(amount) counts rows with non-null amounts > 100, and SUM(amount) adds those amounts.

3. Final Answer:

   The count and sum of amounts greater than 100. -> Option A

4. Quick Check:

   WHERE filters rows, COUNT and SUM apply to filtered rows = A [OK]

Hint: WHERE filters rows before COUNT and SUM apply [OK]

Common Mistakes:

• Thinking COUNT counts all rows ignoring WHERE
• Believing SUM works without filtering
• Assuming COUNT and SUM cannot be used together

4. Identify the error in this SQL query:

SELECT SUM(amount FROM sales;

medium

A. SUM requires two arguments.

B. SUM cannot be used without GROUP BY.

C. Table name should come before the function.

D. Missing closing parenthesis after amount.

Solution

1. Step 1: Check function syntax

   The function SUM() requires parentheses enclosing the column name.

2. Step 2: Locate the syntax error

   The query misses the closing parenthesis after amount, causing a syntax error.

3. Final Answer:

   Missing closing parenthesis after amount. -> Option D

4. Quick Check:

   SUM syntax needs parentheses closed = A [OK]

Hint: Check all parentheses in functions carefully [OK]

Common Mistakes:

• Forgetting to close parentheses in functions
• Thinking GROUP BY is always needed with SUM
• Assuming SUM needs multiple arguments

5. You want to find the average sale amount but only for sales made in the last 30 days. Which SQL query correctly uses built-in functions to do this?

Options:
A) SELECT AVG(amount) FROM sales WHERE sale_date > CURRENT_DATE - INTERVAL '30' DAY;
B) SELECT AVG(amount) FROM sales WHERE sale_date > DATE_SUB(CURRENT_DATE, 30);
C) SELECT AVG(amount) FROM sales WHERE sale_date > CURRENT_DATE - 30;
D) SELECT AVG(amount) FROM sales WHERE sale_date > NOW() - INTERVAL 30 DAY;

hard

A. SELECT AVG(amount) FROM sales WHERE sale_date > DATE_SUB(CURRENT_DATE, 30);

B. SELECT AVG(amount) FROM sales WHERE sale_date > CURRENT_DATE - INTERVAL '30' DAY;

C. SELECT AVG(amount) FROM sales WHERE sale_date > CURRENT_DATE - 30;

D. SELECT AVG(amount) FROM sales WHERE sale_date > NOW() - INTERVAL 30 DAY;

Solution

1. Step 1: Understand date interval syntax

   Standard SQL uses CURRENT_DATE - INTERVAL '30' DAY to get the date 30 days ago.

2. Step 2: Check each option's correctness

   SELECT AVG(amount) FROM sales WHERE sale_date > CURRENT_DATE - INTERVAL '30' DAY; uses correct standard syntax. SELECT AVG(amount) FROM sales WHERE sale_date > DATE_SUB(CURRENT_DATE, 30); uses MySQL-specific DATE_SUB but missing interval unit. SELECT AVG(amount) FROM sales WHERE sale_date > CURRENT_DATE - 30; subtracts integer directly, invalid. SELECT AVG(amount) FROM sales WHERE sale_date > NOW() - INTERVAL 30 DAY; uses NOW() with interval but missing quotes around 30.

3. Final Answer:

   SELECT AVG(amount) FROM sales WHERE sale_date > CURRENT_DATE - INTERVAL '30' DAY; -> Option B

4. Quick Check:

   Standard SQL date interval syntax = A [OK]

Hint: Use INTERVAL with quotes for date calculations [OK]

Common Mistakes:

• Using incorrect or missing interval syntax
• Subtracting numbers directly from dates
• Mixing functions from different SQL dialects