UPPER and LOWER functions in SQL

Introduction

These functions change text to all uppercase or all lowercase letters. This helps to compare or display text in a consistent way.

When you want to make sure names are shown in all capital letters on a report.

When you need to compare two words without worrying about uppercase or lowercase differences.

When cleaning up user input to store all text in lowercase for easier searching.

When formatting email addresses to lowercase before saving them.

When sorting text data ignoring case differences.

Syntax

SQL

UPPER(text_expression)
LOWER(text_expression)

Replace text_expression with the column name or text you want to change.

These functions return a new text value; they do not change the original data in the table.

Examples

Converts the string 'hello world' to all uppercase letters.

SQL

SELECT UPPER('hello world');

Converts the string 'HELLO WORLD' to all lowercase letters.

SQL

SELECT LOWER('HELLO WORLD');

Shows all employee names in uppercase.

SQL

SELECT UPPER(name) FROM employees;

Shows all user emails in lowercase.

SQL

SELECT LOWER(email) FROM users;

Sample Program

This creates a table with names, then shows each name in uppercase and lowercase.

SQL

CREATE TABLE people (id INT, name VARCHAR(20));
INSERT INTO people VALUES (1, 'Alice'), (2, 'Bob'), (3, 'Charlie');

SELECT id, UPPER(name) AS name_upper, LOWER(name) AS name_lower FROM people ORDER BY id;

Important Notes

UPPER and LOWER work only on text data types like VARCHAR or CHAR.

They do not change the original data stored in the database, only the output of the query.

Use these functions to make text comparisons case-insensitive by converting both sides to the same case.

Summary

UPPER converts text to all uppercase letters.

LOWER converts text to all lowercase letters.

Use them to format or compare text without case differences.

Practice

1. What does the SQL function UPPER('Hello World') return?

easy

A. 'Hello World'

B. 'HELLO WORLD'

C. 'hello world'

D. 'HeLLo WoRLD'

Solution

1. Step 1: Understand the UPPER function

   The UPPER function converts all letters in the input string to uppercase letters.

2. Step 2: Apply UPPER to 'Hello World'

   Converting each letter to uppercase results in 'HELLO WORLD'.

3. Final Answer:

   'HELLO WORLD' -> Option B

4. Quick Check:

   UPPER('Hello World') = 'HELLO WORLD' [OK]

Hint: UPPER makes all letters capital [OK]

Common Mistakes:

• Confusing UPPER with LOWER
• Thinking it changes only first letter
• Assuming it reverses the string

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

easy

A. SELECT LOWER(name) FROM table;

B. SELECT LOWER name FROM table;

C. SELECT name.LOWER() FROM table;

D. SELECT LOWER[name] FROM table;

Solution

1. Step 1: Recall correct function syntax

   The LOWER function is called with parentheses around the column name: LOWER(column_name).

2. Step 2: Check each option

   SELECT LOWER(name) FROM table; uses correct syntax: LOWER(name). Others have syntax errors or invalid notation.

3. Final Answer:

   SELECT LOWER(name) FROM table; -> Option A

4. Quick Check:

   Use LOWER(column) with parentheses [OK]

Hint: Use LOWER(column_name) with parentheses [OK]

Common Mistakes:

• Omitting parentheses
• Using dot notation like name.LOWER()
• Using square brackets inside function

3. Given a table users with a column username containing values: 'Alice', 'BOB', 'charlie', what will the query SELECT LOWER(username) FROM users; return?

medium

A. ['ALICE', 'BOB', 'CHARLIE']

B. ['Alice', 'BOB', 'charlie']

C. ['alice', 'bob', 'charlie']

D. ['aLiCe', 'bOb', 'cHaRlIe']

Solution

1. Step 1: Understand LOWER function effect on each username

   LOWER converts all letters to lowercase regardless of original case.

2. Step 2: Apply LOWER to each username

   'Alice' -> 'alice', 'BOB' -> 'bob', 'charlie' -> 'charlie'.

3. Final Answer:

   ['alice', 'bob', 'charlie'] -> Option C

4. Quick Check:

   LOWER converts all letters to lowercase [OK]

Hint: LOWER makes all letters small [OK]

Common Mistakes:

• Assuming only first letter changes
• Confusing LOWER with UPPER
• Expecting mixed case output

4. Identify the error in this SQL query: SELECT LOWER username FROM users;

medium

A. No error, query is correct

B. LOWER function does not exist

C. Table name is incorrect

D. Missing parentheses around column name

Solution

1. Step 1: Check function syntax

   SQL functions require parentheses around arguments, so LOWER(username) is correct.

2. Step 2: Identify missing parentheses

   The query uses LOWER username without parentheses, causing syntax error.

3. Final Answer:

   Missing parentheses around column name -> Option D

4. Quick Check:

   Functions need parentheses [OK]

Hint: Always use parentheses with functions [OK]

Common Mistakes:

• Omitting parentheses for functions
• Assuming functions work like keywords
• Ignoring syntax errors

5. You want to find all rows in a table products where the category column equals 'Books', ignoring case differences. Which SQL WHERE clause correctly does this?

hard

A. WHERE LOWER(category) = 'books' AND UPPER(category) = 'BOOKS'

B. WHERE category = 'Books'

C. WHERE LOWER(category) = 'Books'

D. WHERE UPPER(category) = 'BOOKS'

Solution

1. Step 1: Understand case-insensitive comparison

   To ignore case, convert both sides to the same case before comparing.

2. Step 2: Apply LOWER and UPPER to category and compare both

   Using WHERE LOWER(category) = 'books' AND UPPER(category) = 'BOOKS' ensures the category matches 'Books' ignoring case by checking both lowercase and uppercase.

3. Final Answer:

   WHERE LOWER(category) = 'books' AND UPPER(category) = 'BOOKS' -> Option A

4. Quick Check:

   Use both LOWER and UPPER on the column to ensure case-insensitive match [OK]

Hint: Convert both sides to same case for comparison [OK]

Common Mistakes:

• Comparing without case conversion
• Mixing cases in comparison string
• Using only one case function