MySQL String and Numeric Type Conversion: CAST vs CONVERT Explained with Practical Examples

1. Introduction

When operating a database with MySQL, you will often encounter situations such as “data that should be treated as numbers is stored as a string type” or “calculations and aggregations cannot be performed while the data remains a string.” For example, data imported from Excel or CSV files is frequently stored as VARCHAR or CHAR, even though it represents numeric values. In such cases, operations like sum, average, and numeric comparison may not work as expected, making it difficult to write proper SQL queries.

On the other hand, there are scenarios where numeric data needs to be handled as strings. For example, when zero‑padding an ID or code, or when concatenating numeric data with other columns for display purposes. In such cases, converting numeric types to string types becomes necessary.

In this way, “type conversion between strings and numbers” is one of the fundamental skills required to manipulate data flexibly in MySQL. By using type conversion appropriately, you can maintain data integrity while enabling flexible aggregation and processing.

In this article, we will clearly explain how to convert between strings and numbers in MySQL, how to use these techniques effectively, and common pitfalls and best practices based on real‑world experience. Whether you are a beginner or an experienced SQL developer, this guide will provide practical value.

2. Types of Type Conversion in MySQL

MySQL provides several convenient features for converting data types. In this section, we will explain two major patterns: “explicit type conversion” and “implicit type conversion,” along with their representative methods.

2.1 Explicit Type Conversion

Explicit type conversion means clearly specifying in SQL that “this value should be converted to a specific type.” The following two functions are commonly used.

CAST Function
CAST() is a standard SQL function that converts a value to a specified type. It is widely used in MySQL.

SELECT CAST('123' AS SIGNED);

In this example, the string '123' is converted to a signed integer (SIGNED). Other convertible types include UNSIGNED (unsigned integer), DECIMAL (fixed‑point number), CHAR (string), and DATE (date).

CONVERT Function
CONVERT() is another function used to convert values to different data types. The syntax is as follows.

SELECT CONVERT('456', UNSIGNED);

In this example, the string '456' is converted to an unsigned integer (UNSIGNED). The main difference from CAST() is that CONVERT() can also be used for character set conversion.

2.2 Implicit Type Conversion

Implicit type conversion is a mechanism where MySQL automatically converts data types when performing operations or comparisons.

For example, when adding a number and a string, MySQL automatically converts the string to a numeric value.

SELECT 1 + '2';
-- Result: 3

Similarly, when concatenating numeric values as strings:

SELECT CONCAT(10, ' apples');
-- Result: '10 apples'

Implicit type conversion is convenient, but it can produce unintended results. Therefore, for complex logic or critical processes, using explicit type conversion is strongly recommended.

3. Practical Examples: Converting Strings to Numbers

In MySQL, when numeric values are stored as strings (such as CHAR or VARCHAR), you cannot reliably perform calculations or numeric comparisons as‑is. To aggregate and analyze such data correctly, you need to convert strings into numeric types. This section introduces commonly used conversion methods and key precautions.

3.1 Converting with the CAST Function

The most basic approach is using the CAST() function. For example, to convert the string '100' into an integer, write the following:

SELECT CAST('100' AS SIGNED) AS numeric_result;
-- Result: 100 (integer)

Use SIGNED for signed integers and UNSIGNED for unsigned integers. For decimal data, you can use DECIMAL or FLOAT as well.

SELECT CAST('123.45' AS DECIMAL(10,2)) AS decimal_result;
-- Result: 123.45

3.2 Converting with the CONVERT Function

The CONVERT() function can be used in almost the same way:

SELECT CONVERT('200', SIGNED) AS converted_result;
-- Result: 200

Both produce the same result, but since CAST() is standard SQL and offers better portability, it is generally recommended when in doubt.

3.3 Implicit Conversion via Arithmetic

When you perform arithmetic with numeric and string types in SQL expressions, MySQL automatically converts the string to a number. For example:

SELECT '50' + 25 AS total;
-- Result: 75

You can also apply this behavior to aggregation functions. For instance, if you pass a string-type column to SUM(), MySQL attempts numeric conversion automatically and sums the values:

SELECT SUM(amount) FROM sales_data;
-- Even if the amount column is VARCHAR, MySQL will attempt numeric summation

3.4 Watch Out for Zero-Padded Strings and Non-Numeric Values

Zero-padded strings (e.g., '000100') can also be converted to numbers:

SELECT CAST('000100' AS SIGNED) AS converted_result;
-- Result: 100

However, be careful if the string contains non-numeric characters. If you convert something like CAST('abc123' AS SIGNED), and there are no numeric characters at the start, MySQL returns 0. Depending on data quality, it is important to perform input validation before conversion.

3.5 Common Real-World Use Cases

• Aggregating sales or amount data that became strings due to Excel/CSV imports
• Sorting IDs stored as strings (even though they represent numbers) in numeric order
• Sorting date strings stored in YYYYMMDD format by date order (explained later)

4. Practical Examples: Converting Numbers to Strings

In MySQL, there are many cases where you want to treat numeric data as strings. Typical examples include displaying IDs or codes with zero padding, or building messages by concatenating numeric values with other text. This section introduces representative methods and practical use cases.

4.1 Converting with the CAST Function

To explicitly convert a numeric type (such as INT or DECIMAL) into a string type, use CAST():

SELECT CAST(123 AS CHAR) AS string_result;
-- Result: '123'

With this method, you can easily concatenate numeric columns with other strings.

4.2 Converting with the CONVERT Function

You can also do the same with CONVERT():

SELECT CONVERT(456, CHAR) AS converted_result;
-- Result: '456'

There is little difference from CAST(), but from a standard SQL perspective, CAST() is slightly preferred.

4.3 Implicit Conversion via Concatenation

When concatenating numbers and strings using functions like CONCAT(), MySQL automatically converts numbers into strings.

SELECT CONCAT(2024, ' year') AS fiscal_year_display;
-- Result: '2024 year'

Such implicit conversion is commonly used for routine reporting output and data formatting.

4.4 Common Real-World Use Cases

• Generating zero-padded IDs To display a numeric ID in 5 digits, combine it with LPAD() as follows:

  SELECT LPAD(CAST(id AS CHAR), 5, '0') AS zero_padded_id
  FROM users;
  -- If id=7, the result is '00007'
  

• Concatenating dates or amounts into strings

  SELECT CONCAT('The total amount is ', CAST(total AS CHAR), ' yen.') AS message
  FROM orders;
  -- If total=1500, the result is 'The total amount is 1500 yen.'
  

4.5 Notes

Even if the converted string looks the same visually, the behavior of “sorting” and “comparison operations” changes.
For example, when sorting as strings, '20' may come before '100' (lexicographical order). It is important to choose the approach depending on your purpose.

5. Advanced Use Cases with Type Conversion

Type conversion is not limited to simple numeric or string transformations. It can also be applied in various practical scenarios in real‑world systems. This section introduces commonly used advanced cases and their key considerations.

5.1 Comparing and Converting Date‑Like Strings

If your database stores dates in formats such as YYYYMMDD as numeric or string types, simple string comparison may not always behave as expected.

In such cases, converting the value to a numeric type using CAST() enables proper chronological sorting and comparison.

SELECT *
FROM events
ORDER BY CAST(event_date AS UNSIGNED);
-- Values such as '20240501', '20240502', etc. are sorted in date order

You can also combine this with the REPLACE() function to convert hyphen‑separated dates like '2024-05-01' into integers.

SELECT CAST(REPLACE('2024-05-01', '-', '') AS UNSIGNED);
-- Result: 20240501

5.2 Sorting ENUM Types or Code Values Numerically

For ENUM types or code values that represent numeric meaning, you may want to sort them in numeric order rather than lexicographical order. By converting them to numeric types with CAST() before sorting, you achieve intuitive ordering.

SELECT *
FROM products
ORDER BY CAST(product_code AS UNSIGNED);

5.3 Usage in Aggregation and Data Formatting

For example, if sales amounts are stored as VARCHAR, you can obtain an accurate total by converting them explicitly inside SUM().

SELECT SUM(CAST(sales_amount AS SIGNED)) AS total_sales
FROM sales_data;

5.4 Preventing Issues with Precision and Type Selection

For decimal or large numeric values, converting to the DECIMAL type helps preserve precision.

Choosing between SIGNED and UNSIGNED is also important, depending on whether negative values may exist in your data.

SELECT CAST('1234.567' AS DECIMAL(10, 3));
-- Result: 1234.567

5.5 Detecting Errors or Invalid Data

If conversion results in unexpected NULL or 0 values, this may indicate invalid data.

For example, if a string cannot be converted into a number, CAST() may return 0 or NULL depending on the SQL mode.

In such cases, checking conversion results can help identify data cleansing or validation needs.

SELECT original, CAST(original AS SIGNED) AS converted
FROM test_data
WHERE CAST(original AS SIGNED) = 0 AND original <> '0';
-- Extract only data that cannot be properly converted to numeric values

6. Differences Between CAST and CONVERT

When performing type conversion in MySQL, you typically use either the CAST function or the CONVERT function. Although they appear similar, there are differences in usage and characteristics. This section explains those differences and how to choose between them.

6.1 Basic Differences

• CAST Function Uses the syntax CAST(value AS type) to explicitly convert a value to the specified type. It is defined in standard SQL and works similarly across many database systems.

  SELECT CAST('123' AS SIGNED);
  

• CONVERT Function Uses the syntax CONVERT(value, type) to convert a value to the specified type. In MySQL, it can also be used for character set conversion.

  SELECT CONVERT('123', SIGNED);
  -- Type conversion
  SELECT CONVERT('hello' USING utf8mb4);
  -- Character set conversion
  

6.2 Compatibility with Standard SQL

Since CAST is part of the international SQL standard, it offers better portability to other databases (such as PostgreSQL, SQL Server, and Oracle). On the other hand, CONVERT includes MySQL‑specific extensions, particularly for character set conversion, using syntax like CONVERT(expr USING charset_name).

6.3 How to Choose Between Them

• For type conversion (numbers, strings, dates, etc.) In most cases, using CAST is the safest choice. It is standard SQL, highly portable, and suitable for future database migrations.
• For character set conversion (e.g., sjis to utf8mb4) You must use CONVERT(expr USING charset_name) .
• For MySQL-specific or special cases CONVERT may offer additional flexibility, but for general purposes, start with CAST and only use CONVERT when necessary.

6.4 Sample Comparison

-- Type conversion using CAST (convert to integer)
SELECT CAST('456' AS SIGNED);

-- Type conversion using CONVERT
SELECT CONVERT('456', SIGNED);

-- Character set conversion using CONVERT
SELECT CONVERT('Hello' USING utf8mb4);

6.5 Important Notes

• If conversion fails, both functions may return NULL or 0 .
• Character set conversion cannot be done with CAST .
• Behavior may differ depending on SQL mode or MySQL version, so always test in both development and production environments.

7. Important Notes and Best Practices

When using string and numeric type conversion in MySQL, there are several potential pitfalls to be aware of. This section introduces best practices to help prevent issues and ensure safe and accurate usage.

7.1 Errors or Unexpected NULL/0 from Invalid Conversions

When performing type conversion, if the source value is not in the correct format, MySQL may return unexpected NULL or 0 values.

SELECT CAST('abc' AS SIGNED) AS result;
-- Result: 0 (default MySQL behavior)

As shown above, converting a non-numeric string into a numeric type results in 0. Therefore, always verify that the original data does not contain invalid values before conversion.

Depending on SQL mode settings, conversion may instead produce an error or NULL. Always confirm the configuration of your production environment.

7.2 Choosing Precision and Signed/Unsigned Types

• For values containing decimals, convert to DECIMAL or FLOAT
• If negative values are possible, use SIGNED ; if values are strictly positive integers, use UNSIGNED

Always select the appropriate type according to your data characteristics.

7.3 Impact on Indexes

Using type conversion functions such as CAST or CONVERT in WHERE or ORDER BY clauses may prevent indexes from being used, resulting in performance degradation.

SELECT * FROM users WHERE CAST(user_id AS SIGNED) = 1000;
-- Even if user_id has an index, it is often not used

For large datasets or performance-critical queries, it is best to unify column types in the schema design to avoid unnecessary conversions.

7.4 Do Not Over-Rely on Implicit Conversion

Implicit type conversion in MySQL is convenient, but it can lead to unintended behavior. For important logic, always use explicit CAST or CONVERT.

SELECT '100a' + 20;
-- Result: 100 (only the leading numeric portion is used)

To avoid subtle bugs and data inconsistencies, make explicit conversion a habit.

7.5 Validate Types at Data Input Stage

When designing your database, store numeric data as numeric types and string data as string types. Minimizing the need for conversion through proper schema design is one of the most effective ways to prevent problems.

8. Summary

Type conversion between strings and numbers in MySQL is an unavoidable topic in daily data processing, aggregation, and system operations. In this article, we covered everything from basic usage to advanced techniques and important precautions.

For string-to-number conversion, explicit methods such as CAST and CONVERT are available. Although implicit conversion is frequently used in practice, prioritizing explicit conversion improves reliability and data quality. Conversely, number-to-string conversion is also widely used for formatting IDs, codes, and generating output messages.

In addition, techniques such as date-related applications, aggregation and sorting, and error detection demonstrate how powerful type conversion can be in real‑world scenarios. However, conversion also carries potential pitfalls and performance impacts. Always follow the best practices discussed above.

By mastering type conversion properly, you can significantly expand your ability to manipulate data effectively in MySQL.
We hope this article supports your daily work and learning.

9. FAQ (Frequently Asked Questions)

Q1. What happens if I CAST a string like ‘abc’ to a number?

In MySQL, when attempting to convert a string to a numeric type (such as SIGNED or UNSIGNED), MySQL returns the leading numeric portion if present; otherwise, it returns 0.

SELECT CAST('abc' AS SIGNED); -- Result: 0
SELECT CAST('123abc' AS SIGNED); -- Result: 123

However, depending on SQL mode (for example, STRICT_TRANS_TABLES), the result may be an error or NULL.

Q2. Should I use CAST or CONVERT?

For general type conversion (numbers, strings, dates, etc.), using CAST is recommended because it conforms to standard SQL. For character set conversion, you must use the CONVERT function with the USING clause. Choose based on your use case.

Q3. Is implicit type conversion sufficient?

Implicit conversion may work for small queries or testing, but for critical aggregation and production system development, explicit conversion (CAST or CONVERT) is strongly recommended. Writing explicit conversions helps prevent unintended behavior and bugs.

Q4. Does using type conversion in WHERE or ORDER BY disable indexes?

Yes. Applying functions such as CAST or CONVERT to a column may prevent MySQL from using indexes defined on that column.
If high‑performance querying of large datasets is required, unify column types in advance or consider using subqueries or generated columns.

Q5. Is type conversion safe for decimals or large numbers?

Using DECIMAL or FLOAT can preserve precision, but rounding errors, precision loss, or truncation may still occur. Always specify sufficient precision and scale for your requirements.

Q6. What is the difference between sorting as a string and sorting as a number?

When sorted as strings, values like '10' may come before '2' due to lexicographical ordering. If you want numeric order, always perform type conversion before sorting.