CAST用于簡單跨數(shù)據(jù)庫類型轉(zhuǎn)換，CONVERT適用于SQL Server中帶格式的靈活轉(zhuǎn)換，優(yōu)先選CAST保證兼容性，用CONVERT需注意平臺限制和數(shù)據(jù)有效性。

When working with SQL, you often need to change data from one type to another—like turning a string into a date or a number into text. For this, SQL provides two key functions: CAST and CONVERT. Both do similar things but have slight differences in syntax and flexibility.

Using CAST for Simple Type Conversion

The CAST function is standard SQL and works across most database systems like PostgreSQL, SQL Server, MySQL (with limitations), and SQLite. It’s straightforward and ideal for basic conversions.

Syntax:

Examples:

• Convert a string to an integer: CAST('123' AS INT)
• Convert a date string to a DATE type: CAST('2024-05-20' AS DATE)
• Convert a number to decimal with precision: CAST(45 AS DECIMAL(5,2))

CAST is clean and portable. Use it when you want code that works across different databases.

Using CONVERT for Formatting and Flexibility (SQL Server & Sybase)

CONVERT is more flexible than CAST but is primarily used in SQL Server and Sybase. It allows formatting, especially useful for dates and strings.

Syntax:

The optional style parameter lets you control the output format—especially helpful for dates.

Examples:

• Convert a string to a date: CONVERT(DATE, '2024-05-20')
• Format a date as 'MM/DD/YYYY': CONVERT(VARCHAR, GETDATE(), 101)
• Convert a number to currency format: CONVERT(MONEY, 1234.56)

Style codes matter—like 103 for 'DD/MM/YYYY' or 120 for 'YYYY-MM-DD HH:MI:SS'. Refer to your database’s documentation for valid styles.

Key Differences and When to Use Which

CAST is part of the ANSI SQL standard. That means it's widely supported and best for simple, portable queries.

CONVERT is not standard SQL, so it may not work in PostgreSQL or MySQL the same way. But it shines when you need to format dates or handle special display needs in SQL Server.

If you're writing code for multiple databases, stick with CAST. If you're deep in SQL Server and need formatting, CONVERT gives you more control.

Handling Errors and Invalid Conversions

Both functions throw an error if the conversion isn’t possible—like trying to cast 'Hello' to an INT.

To avoid crashes, use safety checks:

• In SQL Server, wrap with TRY_CAST or TRY_CONVERT: TRY_CAST('abc' AS INT) returns NULL instead of failing.
• In other systems, use conditional logic or functions like ISNUMERIC, ISDATE where available.

Always validate input data when doing conversions, especially with user-provided or imported data.

Basically just pick CAST for simplicity and portability, or CONVERT when you’re on SQL Server and need formatting power. Watch out for bad data, and test your conversions.

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