Type casting and conversion in MySQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of CAST with string to integer conversion

What is the output of the following MySQL query?

SELECT CAST('123abc' AS UNSIGNED) AS result;

MySQL

SELECT CAST('123abc' AS UNSIGNED) AS result;

A 123abc

B 0

C Syntax error

D 123

💡 Hint

MySQL converts strings to numbers by reading digits from the start until a non-digit is found.

MySQL reads the string from left to right and converts the initial numeric part to an integer. Here, '123abc' converts to 123.

❓ query_result

intermediate

Result of CONVERT with incompatible type

What is the result of this MySQL query?

SELECT CONVERT('abc123', SIGNED) AS result;

MySQL

SELECT CONVERT('abc123', SIGNED) AS result;

A abc123

B 123

C 0

D NULL

💡 Hint

When the string does not start with digits, MySQL returns zero for numeric conversion.

Since 'abc123' starts with letters, MySQL cannot convert it to a number and returns 0.

📝 Syntax

advanced

Identify the correct syntax for casting to CHAR

Which of the following MySQL queries correctly casts the integer 100 to a CHAR type?

A SELECT CAST(100 AS CHAR);

B SELECT CAST(100 TO CHAR);

C SELECT CONVERT(100, CHAR);

D SELECT CONVERT(100 AS CHAR);

💡 Hint

CAST uses AS keyword, CONVERT uses comma to separate arguments.

CAST syntax requires AS keyword. CONVERT uses comma without AS. Option A uses correct CAST syntax.

❓ query_result

advanced

Effect of implicit conversion in arithmetic operation

What is the output of this MySQL query?

SELECT '10' + 5 AS result;

MySQL

SELECT '10' + 5 AS result;

A 15

B '105'

C 10 5

D Error

💡 Hint

MySQL converts strings to numbers automatically in arithmetic expressions.

The string '10' is implicitly converted to number 10, then added to 5, resulting in 15.

🧠 Conceptual

expert

Understanding behavior of CAST with floating point to integer

Consider the query:

SELECT CAST(123.987 AS SIGNED) AS result;

What is the value of result?

MySQL

SELECT CAST(123.987 AS SIGNED) AS result;

A 124

B 123

C 123.987

D Error

💡 Hint

Casting from float to integer truncates the decimal part in MySQL.

MySQL truncates the decimal part when casting float to integer, so 123.987 becomes 123.

Practice

1. What does the CAST() function do in MySQL?

easy

A. It changes the database user permissions.

B. It creates a new table in the database.

C. It converts a value from one data type to another.

D. It deletes rows from a table.

Solution

1. Step 1: Understand the purpose of CAST()

   The CAST() function is used to change the data type of a value, for example from string to integer.

2. Step 2: Compare with other options

   Options A, B, and C describe unrelated database operations, not type conversion.

3. Final Answer:

   It converts a value from one data type to another. -> Option C

4. Quick Check:

   CAST() = type conversion [OK]

Hint: CAST() changes data types, not structure or permissions [OK]

Common Mistakes:

• Thinking CAST() creates or deletes tables
• Confusing CAST() with permission commands
• Assuming CAST() modifies data values without type change

2. Which of the following is the correct syntax to convert a string '123' to an integer in MySQL?

easy

A. CAST('123' AS SIGNED)

B. CONVERT('123', TO INTEGER)

C. CAST('123' TO INTEGER)

D. CONVERT('123' AS INT)

Solution

1. Step 1: Recall correct CAST syntax

   In MySQL, the correct syntax is CAST(value AS type), so CAST('123' AS SIGNED) is valid.

2. Step 2: Check other options for syntax errors

   Options A, B, and D use incorrect syntax: A uses 'TO' instead of 'AS', B and D misuse CONVERT syntax, which is invalid in MySQL.

3. Final Answer:

   CAST('123' AS SIGNED) -> Option A

4. Quick Check:

   CAST(value AS type) = correct syntax [OK]

Hint: Use CAST(value AS type) format for conversions [OK]

Common Mistakes:

• Using TO instead of AS in CAST
• Using CONVERT with wrong syntax
• Mixing syntax from other SQL dialects

3. What is the result of this query?

SELECT CAST('2023-06-15' AS DATE) + INTERVAL 5 DAY AS new_date;

medium

A. 2023-06-20

B. 2023-06-10

C. 2023-06-15 00:00:00

D. Error: Invalid date operation

Solution

1. Step 1: Convert string to DATE type

   CAST('2023-06-15' AS DATE) converts the string to a date value representing June 15, 2023.

2. Step 2: Add 5 days interval

   Adding INTERVAL 5 DAY moves the date forward by 5 days, resulting in June 20, 2023.

3. Final Answer:

   2023-06-20 -> Option A

4. Quick Check:

   Date + 5 days = 2023-06-20 [OK]

Hint: Add intervals to dates after casting string to DATE [OK]

Common Mistakes:

• Forgetting to cast string to DATE before adding interval
• Adding interval to string without conversion
• Expecting time part in result

4. Identify the error in this query:

SELECT CONVERT('abc' AS UNSIGNED);

medium

A. Returns NULL

B. No error, returns 0

C. Runtime error: cannot convert 'abc' to number

D. Syntax error: wrong CONVERT syntax

Solution

1. Step 1: Check CONVERT syntax

   CONVERT(value, type) is correct syntax in MySQL, but 'AS' keyword is invalid here, so syntax error would occur.

2. Step 2: Analyze actual query

   The query uses CONVERT('abc' AS UNSIGNED), which is invalid syntax and causes syntax error, but if corrected to CONVERT('abc', UNSIGNED), it returns 0 because 'abc' cannot be converted to a number.

3. Final Answer:

   Syntax error: wrong CONVERT syntax -> Option D

4. Quick Check:

   CONVERT() syntax must be CONVERT(value, type) [OK]

Hint: Use CONVERT(value, type) without AS keyword [OK]

Common Mistakes:

• Using AS keyword inside CONVERT()
• Expecting NULL instead of 0 for invalid numeric conversion
• Confusing CONVERT syntax with CAST

5. You have a table sales with a column amount stored as VARCHAR. You want to calculate the total sales as a number. Which query correctly converts and sums the amounts?

hard

A. SELECT SUM(amount) FROM sales;

B. SELECT SUM(CAST(amount AS DECIMAL(10,2))) FROM sales;

C. SELECT SUM(CONVERT(amount AS CHAR)) FROM sales;

D. SELECT SUM(CAST(amount AS DATE)) FROM sales;

Solution

1. Step 1: Understand data type of amount

   amount is VARCHAR, so summing directly treats values as strings, causing wrong results or errors.

2. Step 2: Convert amount to numeric type before summing

   CAST(amount AS DECIMAL(10,2)) converts string to decimal number, allowing correct sum calculation.

3. Step 3: Check other options

   SELECT SUM(amount) FROM sales; sums strings (incorrect), C converts to CHAR (string again), D converts to DATE (invalid for amounts).

4. Final Answer:

   SELECT SUM(CAST(amount AS DECIMAL(10,2))) FROM sales; -> Option B

5. Quick Check:

   Convert strings to numbers before SUM [OK]

Hint: CAST strings to DECIMAL before SUM to avoid errors [OK]

Common Mistakes:

• Summing VARCHAR without conversion
• Casting to wrong type like DATE or CHAR
• Using CONVERT with wrong syntax