在SQL Server中转换数据类型

Question

I need to change some data types. I have tried using the CAST keyword, but it is not fixing the issue. Any suggestions on how to re-write these two lines with CAST statements?

1. Data loss might occur when casting from VarChar(100) to VarChar(20) .

Code:

SELECT TOP 1 @variableId = variableId
FROM @tempTable

2. Data loss might occur when casting from NVarChar(512) to NVarChar .

Code:

SELECT myVariable
FROM tableName

I tried doing something like the following, but still produces an error:

CAST((myVariable) as nvarchar)

Answer 1

CAST is used for converting between data types. It sounds more like you should use LEFT instead:

SELECT TOP 1 @variableId = LEFT(variableId,20)
FROM @tempTable

This won't give you any warning as the system assumes you already know you're going to lose the right 80 characters.

Answer 2

For using CONVERT instead of CAST :

CONVERT(nvarchar, myVariable)

Is a valid syntax and also:

CONVERT(varchar(20), myVariable)

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But for getting a part of a string we use SUBSTRING() or LEFT() or RIGHT() functions like this:

• SUBSTRING( value_expression , start_expression , length_expression )

  Returns part of a character, binary, text, or image expression.

• LEFT ( character_expression , integer_expression )

  Returns the left part of a character string with the specified number of characters.

• RIGHT ( character_expression , integer_expression )

  Returns the right part of a character string with the specified number of characters.