Why string functions matter in queries in SQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select the first 3 characters of the name column.

SQL

SELECT SUBSTR(name, 1, [1]) FROM employees;

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

A3

B5

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as length returns empty string.

Using 1 returns only the first character.

The SUBSTR function extracts a substring starting at position 1 with length 3, giving the first 3 characters.

2fill in blank

medium

Complete the code to convert the email column to lowercase.

SQL

SELECT [1](email) FROM users;

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

ALOWER

BLENGTH

CTRIM

DUPPER

Attempts:

3 left

💡 Hint

Common Mistakes

Using UPPER instead of LOWER.

Using LENGTH which returns number, not string.

The LOWER function converts all letters in the string to lowercase.

3fill in blank

hard

Fix the error in the code to find the position of '@' in the email column.

SQL

SELECT INSTR(email, [1]) FROM contacts;

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

A'at'

B@

C'@'

Dat

Attempts:

3 left

💡 Hint

Common Mistakes

Not using quotes around '@'.

Using the word 'at' instead of '@'.

The INSTR function needs the character '@' as a string, so it must be in quotes.

4fill in blank

hard

Fill both blanks to trim spaces and convert the username column to uppercase.

SQL

SELECT [1]([2](username)) FROM users;

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

AUPPER

BLOWER

CTRIM

DSUBSTR

Attempts:

3 left

💡 Hint

Common Mistakes

Reversing the order of functions.

Using LOWER instead of UPPER.

First TRIM removes spaces, then UPPER converts to uppercase.

5fill in blank

hard

Fill all three blanks to create a dictionary of usernames and their email domains (part after '@').

SQL

SELECT [1], SUBSTR(email, INSTR(email, [2]) + 1) AS domain FROM users WHERE email [3] '%@%';

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

Ausername

B'@'

CLIKE

Demail

Attempts:

3 left

💡 Hint

Common Mistakes

Using email instead of username.

Not quoting '@' in INSTR.

Using = instead of LIKE.

We select username, extract domain after '@', and filter emails containing '@' using LIKE.

Practice

1. Why do we use string functions like UPPER() or LOWER() in SQL queries?

easy

A. To perform mathematical calculations on numbers

B. To delete rows from a table

C. To create new tables in the database

D. To change text case for consistent comparison or display

Solution

1. Step 1: Understand the purpose of string functions

   Functions like UPPER() and LOWER() change the case of text data to uppercase or lowercase.

2. Step 2: Recognize their use in queries

   Changing case helps compare text consistently or display it uniformly, which is important for searching or reporting.

3. Final Answer:

   To change text case for consistent comparison or display -> Option D

4. Quick Check:

   String functions change text case = B [OK]

Hint: String functions change or check text, not numbers [OK]

Common Mistakes:

• Confusing string functions with numeric functions
• Thinking string functions create or delete tables
• Assuming string functions modify database structure

2. Which of the following is the correct syntax to get the first 3 characters of a column named name in SQL?

easy

A. SELECT MID(name, 3) FROM table;

B. SELECT LEFT(name, 3) FROM table;

C. SELECT RIGHT(name, 3) FROM table;

D. SELECT SUBSTRING(name FROM 3) FROM table;

Solution

1. Step 1: Identify the function to get characters from the start

   The LEFT() function extracts a specified number of characters from the beginning of a string.

2. Step 2: Check the syntax correctness

   LEFT(name, 3) correctly gets the first 3 characters. Other options either extract from the wrong side or have incorrect syntax.

3. Final Answer:

   SELECT LEFT(name, 3) FROM table; -> Option B

4. Quick Check:

   LEFT gets first chars = D [OK]

Hint: LEFT(string, n) extracts first n characters [OK]

Common Mistakes:

• Using RIGHT() instead of LEFT() for first characters
• Incorrect SUBSTRING syntax with FROM keyword
• Using MID() which is not standard SQL

3. Given a table users with a column email, what will this query return?

SELECT LENGTH(TRIM(email)) FROM users;

medium

A. Length of email excluding leading and trailing spaces

B. Length of email including spaces

C. Number of rows in the users table

D. Email addresses converted to uppercase

Solution

1. Step 1: Understand the TRIM() function

   TRIM(email) removes spaces at the start and end of the email string.

2. Step 2: Understand LENGTH() function

   LENGTH() returns the number of characters in the string after trimming spaces.

3. Final Answer:

   Length of email excluding leading and trailing spaces -> Option A

4. Quick Check:

   TRIM removes spaces, LENGTH counts chars = A [OK]

Hint: TRIM removes spaces, LENGTH counts characters [OK]

Common Mistakes:

• Thinking LENGTH counts spaces inside the string
• Confusing TRIM with UPPER or LOWER
• Assuming query counts rows, not string length

4. Identify the error in this SQL query that tries to find names starting with 'Jo':

SELECT * FROM employees WHERE name LIKE 'Jo%';

medium

A. LIKE is case-sensitive and may miss 'jo' lowercase

B. The wildcard % should be inside quotes

C. Missing a semicolon at the end

D. Using = instead of LIKE for pattern matching

Solution

1. Step 1: Check the LIKE pattern usage

   The pattern 'Jo%' correctly uses % wildcard inside quotes to match names starting with 'Jo'.

2. Step 2: Consider case sensitivity of LIKE

   LIKE is case-sensitive in many SQL systems, so it won't match 'jo' lowercase names, which may cause missed results.

3. Final Answer:

   LIKE is case-sensitive and may miss 'jo' lowercase -> Option A

4. Quick Check:

   LIKE case sensitivity can cause misses = A [OK]

Hint: LIKE may be case-sensitive; use UPPER() to fix [OK]

Common Mistakes:

• Thinking % wildcard must be outside quotes
• Ignoring case sensitivity in LIKE
• Assuming semicolon is mandatory for all SQL engines

5. You want to combine first name and last name columns into one full name column with a space between them. Which SQL expression correctly does this?

hard

A. SELECT first_name || ' ' || last_name AS full_name FROM users;

B. SELECT CONCAT(first_name, last_name) AS full_name FROM users;

C. SELECT CONCAT(first_name, ' ', last_name) AS full_name FROM users;

D. SELECT first_name + ' ' + last_name AS full_name FROM users;

Solution

1. Step 1: Understand string concatenation in SQL

   Different SQL systems use different ways to join strings: || or CONCAT(). The CONCAT() function can take multiple arguments.

2. Step 2: Check which option adds space between names

   SELECT CONCAT(first_name, ' ', last_name) AS full_name FROM users; uses CONCAT(first_name, ' ', last_name), correctly adding a space between first and last names. SELECT first_name || ' ' || last_name AS full_name FROM users; uses || which works in some systems but not all. SELECT CONCAT(first_name, last_name) AS full_name FROM users; misses the space. SELECT first_name + ' ' + last_name AS full_name FROM users; uses + which is invalid for strings in SQL.

3. Final Answer:

   SELECT CONCAT(first_name, ' ', last_name) AS full_name FROM users; -> Option C

4. Quick Check:

   CONCAT with space joins names correctly = C [OK]

Hint: Use CONCAT with space to join names safely [OK]

Common Mistakes:

• Forgetting space between names
• Using + operator for string concatenation
• Using CONCAT without space argument