Type casting and conversion in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is type casting in MySQL?

Type casting in MySQL means changing a value from one data type to another, like turning a string into a number or a number into a string.

beginner

How do you convert a string to an integer in MySQL?

You can use the CAST() function like this: CAST('123' AS UNSIGNED) to convert the string '123' to the number 123.

intermediate

What does the CONVERT() function do in MySQL?

CONVERT() changes a value from one type to another, similar to CAST(), for example CONVERT('2023-01-01', DATE) converts a string to a date.

intermediate

What happens if you cast a non-numeric string to a number in MySQL?

MySQL converts the string to 0 if it cannot find a valid number at the start of the string.

beginner

Why is type casting useful in SQL queries?

It helps compare or calculate values correctly when they are stored as different types, like comparing a number stored as text with a real number.

Which MySQL function converts a value to a different data type?

A ALTER()

B CHANGE()

C MODIFY()

D CAST()

CAST() is used to convert a value from one data type to another.

What is the result of CAST('abc' AS UNSIGNED) in MySQL?

A 0

B abc

C NULL

D Error

Non-numeric strings cast to numbers become 0 in MySQL.

How do you convert a string '2023-06-01' to a DATE type in MySQL?

A DATE('2023-06-01')

B CAST('2023-06-01' AS DATE)

C TO_DATE('2023-06-01')

D CONVERT('2023-06-01' TO DATE)

CAST('2023-06-01' AS DATE) correctly converts the string to a DATE.

Which data type would you cast to if you want a decimal number?

A DECIMAL

B CHAR

C DATE

D BLOB

DECIMAL is used for precise decimal numbers.

What does CONVERT('123', UNSIGNED) do in MySQL?

A Returns string '123' unchanged

B Changes string to uppercase

C Converts string '123' to number 123

D Throws an error

CONVERT('123', UNSIGNED) changes the string to the number 123.

Explain how to convert a string to a number in MySQL and what happens if the string is not numeric.

Think about how MySQL handles strings that don't start with digits.

Describe why type casting is important when working with different data types in SQL queries.

Consider what happens if you compare a number stored as text with a real number.

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