The World of MySQL & MariaDB

MySQL CAST Function Explained: Syntax, Use Cases, and Best Practices

Functions
目次

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 allows you to transform a given expression into another type. For example, you can convert a string into an integer, or change a date type into a string. The CAST function is a highly useful tool frequently applied in database operations to maintain data type consistency.

Main Uses of the CAST Function

The CAST function has many applications, commonly used in the following scenarios:

  • Data normalization: Used to unify data stored in different formats. For instance, it helps standardize date formats across a table.
  • Data type conversion: Converts integers to strings (or vice versa) to optimize data display and calculation.
SELECT CAST('2023-09-22' AS DATE);

Difference Between CAST 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 follows the SQL standard and is supported by more databases. CONVERT is MySQL-specific, mainly used for tasks like character set conversion.

2. Syntax and Usage of the CAST Function

CAST Function Syntax

The basic syntax of the CAST function is:

CAST(expression AS data_type)

Here, expression represents the value to be converted, and data_type specifies the target data type.

Examples of Using CAST

  • Converting to integer type: Used to transform a string into an integer.
SELECT CAST('123' AS SIGNED);
  • Converting to string type: Used to convert a number into a string.
SELECT CAST(123 AS CHAR);

Error Handling

When using the CAST function, errors can occur if invalid values are provided for the target type. For example, trying to convert 'abc' into a number will cause an error. In such cases, it is recommended to combine CAST with functions like IFNULL to handle errors gracefully.

3. Common Data Types and Conversion Examples

Typical Data Type Conversion Scenarios

The CAST function is frequently used for converting to the following data types:

  • INT: Converts numbers into integers.
  • VARCHAR: Converts numbers or dates into strings.
  • DATE: Converts strings or numbers into dates.

Converting to INT

SELECT CAST('456' AS SIGNED);

This example converts the string '456' into an integer.

Converting to VARCHAR

SELECT CAST(456 AS CHAR);

This example converts an integer into a string, useful when you need to display data in a specific format.

Converting to DATE

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

This converts a string into a date type, ensuring accurate storage and enabling later date operations.

4. Considerations and Best Practices for CAST

Key Considerations

When using the CAST function, keep these points in mind:

  1. Type compatibility: Invalid conversions may cause errors, so check type compatibility beforehand.
  2. Loss of precision: Converting floating-point numbers may result in a loss of precision.

Best Practices

  • Use DECIMAL for precision: To avoid precision loss, use DECIMAL when converting floating-point numbers.
SELECT CAST(123.456 AS DECIMAL(5,2));
  • Error handling: When dealing with mixed or unexpected data types, use IFNULL or CASE statements for safer conversions.

5. Difference Between CAST and CONVERT

Comparing CAST and CONVERT

Both CAST and CONVERT perform data type conversion, but their syntax and intended use differ:

  • CAST: Standard SQL syntax: CAST(expression AS data_type).
  • CONVERT: MySQL-specific syntax: CONVERT(expression, data_type).

Example of Using CONVERT

The CONVERT function is mainly used for character set conversion.

SELECT CONVERT('abc' USING utf8);

This example changes the character set of a string.

Which One Should You Use?

In general, it is best to use the SQL-standard CAST function for broader compatibility. However, if you need to perform character set conversions, CONVERT is the appropriate choice.

6. Practical Examples: Data Manipulation with CAST

Real-World Examples

Here are some practical examples of how to use the CAST function for data manipulation.

Sorting Numbers as Strings

For example, you can convert numbers to strings before sorting:

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

Filtering Strings as Numbers

You can also convert strings to numbers to filter specific ranges:

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

7. Conclusion

Summary

The CAST function is an essential tool for efficiently converting data types. This article covered its basic usage, best practices, and practical examples. When performing type conversions, always consider compatibility and precision to make the most of the CAST function in MySQL.