目錄
Basic Syntax of CAST
Converting Strings to Numbers
Converting Numbers to Strings
Converting to Date and Time Types
Handling Potential Errors
Common Use Cases
Supported Data Types by Database
Summary
首頁 資料庫 SQL 如何使用Cast函數轉換SQL中的數據類型

如何使用Cast函數轉換SQL中的數據類型

Aug 27, 2025 am 12:10 AM
sql CAST函数

CAST函數用於顯式轉換數據類型,確保數據兼容性,其基本語法為CAST(expression AS target_data_type),可將字符串轉為數字如SELECT CAST('123' AS INT),將數字轉為字符串以便拼接如SELECT 'Total: ' CAST(total AS VARCHAR),支持日期轉換如CAST('2023-10-01' AS DATE),但無效轉換會報錯,建議使用TRY_CAST等安全替代方法處理異常,常用於類型不匹配的查詢、格式化輸出和數據聚合,不同數據庫對數據類型命名略有差異,使用時需參考具體文檔,CAST在需要精確控制類型轉換時非常有效。

How to use the CAST function to convert data types in SQL

The CAST function in SQL is used to explicitly convert a value from one data type to another. It's helpful when you need to ensure compatibility between different data types during comparisons, arithmetic operations, or formatting output. Here's how to use it effectively.

Basic Syntax of CAST

The general syntax for the CAST function is:

 CAST(expression AS target_data_type)
  • expression : The value or column you want to convert.
  • target_data_type : The data type you want to convert to (eg, INT , VARCHAR , DATE , etc.).

Converting Strings to Numbers

Sometimes numeric data is stored as text (eg, VARCHAR ), and you need to perform calculations. Use CAST to convert it.

 SELECT CAST('123' AS INT) AS converted_number;

This converts the string '123' into an integer. You can also apply this to a column:

 SELECT CAST(price_text AS DECIMAL(10, 2)) AS price
FROM products;

This assumes price_text is a string column but contains numeric values.

Converting Numbers to Strings

To convert a number into a string (useful for concatenation or formatting), use:

 SELECT CAST(123.45 AS VARCHAR(10)) AS price_string;

Now you can combine it with other text:

 SELECT 'The total is: ' CAST(total_amount AS VARCHAR(20))
FROM orders;

Note : In some databases like PostgreSQL, use TEXT instead of VARCHAR . In MySQL, CHAR or VARCHAR works.

Converting to Date and Time Types

You can convert strings to dates when they follow a standard format:

 SELECT CAST('2023-10-01' AS DATE) AS event_date;

Or to a datetime:

 SELECT CAST('2023-10-01 14:30:00' AS DATETIME) AS event_datetime;

This is useful when importing data from text fields or comparing date strings.

Caution : If the string doesn't match a valid date format, the query will fail.

Handling Potential Errors

CAST will throw an error if the conversion isn't possible. For example:

 CAST('abc' AS INT) -- This causes an error

To avoid crashes, some databases offer safer alternatives:

  • In SQL Server , use TRY_CAST :
     SELECT TRY_CAST('abc' AS INT) -- Returns NULL instead of error
  • In PostgreSQL , CAST('abc' AS INT) fails, but you can use conditional logic or handle it in application code.
  • In MySQL , consider validating data before casting.

Common Use Cases

  • Joining tables on columns with mismatched types (eg, VARCHAR to INT )
  • Filtering with consistent types:
     SELECT * FROM users
    WHERE CAST(user_id AS VARCHAR) LIKE '12%';
  • Formatting output for reports or exports
  • Aggregating after converting text to numbers
  • Supported Data Types by Database

    While most databases support standard types, syntax and naming may vary:

    Data Type SQL Server MySQL PostgreSQL
    Integer INT SIGNED or INT INTEGER
    Decimal DECIMAL(10,2) DECIMAL(10,2) NUMERIC(10,2)
    String VARCHAR(50) VARCHAR(50) VARCHAR(50)
    Date DATE DATE DATE
    Datetime DATETIME DATETIME TIMESTAMP

    Always check your database's documentation for exact type names.

    Summary

    • Use CAST(value AS TYPE) to convert data types explicitly.
    • Ensure the original value is compatible with the target type.
    • Watch for errors with invalid conversions—consider TRY_CAST where available.
    • Useful for calculations, comparisons, formatting, and joins.

    Basically, CAST gives you control over data types when implicit conversion isn't enough or doesn't work. It's simple but powerful when used correctly.

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