MySQL CAST Function Explained: Syntax, Examples, Data Type Conversion & Best Practices

1. Basic Concept of the CAST Function

What Is the CAST Function?

The MySQL CAST function is an SQL function used to convert data types. It converts a specified expression into another data type. By using the CAST function, you can, for example, convert a string into an integer or convert a date type into a string. It is a useful tool that is frequently used in database operations to maintain data type consistency.

Main Uses of the CAST Function

The CAST function has a wide range of applications and is commonly used in situations such as the following:

• Data normalization: Used to standardize data stored in different formats. For example, it helps unify date formats stored in a table.
• Data type conversion: Converting integers to strings or vice versa to optimize data display and calculations.

SELECT CAST('2023-09-22' AS DATE);

Difference Between the CAST Function and Other Data Type Conversions

The CAST function is similar to other MySQL conversion functions (such as CONVERT), but the key difference is that CAST complies with the SQL standard and is supported by more database systems. CONVERT is specific to MySQL and is mainly used for special purposes such as character set conversion.

2. Syntax and Usage of the CAST Function

CAST Function Syntax

The basic syntax of the CAST function is as follows:

CAST(expression AS data_type)

In this syntax, specify the value to be converted in the expression part and the target data type in the data_type part.

Examples of Using the CAST Function

• Convert to an integer type: Used when converting a string into an integer.

SELECT CAST('123' AS SIGNED);

• Convert to a string type: Used when converting a numeric value into a string.

SELECT CAST(123 AS CHAR);

Error Handling

When using the CAST function, errors may occur if an invalid value is provided for the target data type. For example, attempting to convert 'abc' into a numeric value will result in an error. In such cases, it is recommended to handle errors by combining functions such as IFNULL.

3. Commonly Used Data Types and Conversion Examples

Common Scenarios for Data Type Conversion

With the CAST function, it is common to convert to the following data types:

• INT type: Converts a value into an integer type.
• VARCHAR type: Converts numeric or date values into strings.
• DATE type: Converts strings or numeric values into dates.

Converting to INT Type

SELECT CAST('456' AS SIGNED);

In this example, the string '456' is converted into an integer type.

Converting to VARCHAR Type

SELECT CAST(456 AS CHAR);

This is an example of converting an integer into a string. This operation is useful when you want to display data in a specific format.

Converting to DATE Type

SELECT CAST('2024-01-01' AS DATE);

This converts a string into a date type. In this way, the data is stored accurately and can later be used for date operations.

4. Important Notes and Best Practices for the CAST Function

Important Notes When Using the CAST Function

There are several important points to consider when using the CAST function:

1. Type compatibility: Invalid conversions may cause errors, so you must verify type compatibility in advance.
2. Loss of data precision: Especially when converting floating-point numbers, data precision may be lost.

Best Practices

• Use the DECIMAL type: When converting floating-point numbers, use the DECIMAL type to prevent precision loss.

SELECT CAST(123.456 AS DECIMAL(5,2));

• Error handling: If unexpected data types are mixed, it is recommended to handle errors using IFNULL or CASE statements.

5. Differences Between the CAST Function and the CONVERT Function

Comparison of CAST and CONVERT

Both CAST and CONVERT are used to convert data types, but they differ in syntax and purpose.

• CAST function: Widely used in standard SQL, with the syntax CAST(expression AS data_type).
• CONVERT function: A MySQL-specific function with the syntax CONVERT(expression, data_type).

Example of Using the CONVERT Function

The CONVERT function is mainly used for character set conversion.

SELECT CONVERT('abc' USING utf8);

In this example, the character set of the string is converted.

Which Should You Use?

In general, it is recommended to use the CAST function because it follows the SQL standard and is widely supported. However, when character set conversion is required, the CONVERT function is more appropriate.

6. Practical Examples: Data Manipulation Using the CAST Function

Real-World Data Manipulation Examples

Here are examples of performing actual data operations using the CAST function.

Sorting After Converting Numbers to Strings

For example, when converting numeric values into strings before sorting, you can write the query as follows:

SELECT CAST(column_name AS CHAR) FROM table ORDER BY column_name;

Filtering After Converting Strings to Numbers

You can also convert strings into numbers and filter based on a specific range.

SELECT * FROM table WHERE CAST(column_name AS SIGNED) > 100;

7. Conclusion

Article Summary

The CAST function is extremely useful for efficiently converting data types. In this article, we covered everything from basic usage to practical examples. When performing data type conversions, always pay attention to type compatibility and data precision, and make full use of the CAST function in your database operations.