Error converting data type bigint to varchar.

DECLARE @ID BIGINT
set @ID = 1323
UPDATE School 
SET R开发者_开发技巧egistrationFee = 'fee_' + @ID --<<<<error 
Where SchoolRegistrationId = 123

Error converting data type varchar to bigint.

You need to explicitly convert your bigint to varchar:

DECLARE @ID BIGINT
set @ID = 1323

UPDATE School 
SET RegistrationFee = 'fee_' + CAST(@ID AS VARCHAR(15)) 
WHERE SchoolRegistrationId = 123

T-SQL will not do this automatically for you - you need to be explicit and clear about it.

You can't concatenate a string to a number. You have to convert it:

SET RegistrationFee = 'fee_' + LTRIM(STR(@ID))

SQL Server automatically converts the data from one data type to another. This is Implicit conversion. Your script has a process with diffirent types (varchar and bigint). Bigint is an exact numeric type. "Character expressions that are being converted to an exact numeric data type must consist of digits, a decimal point, and an optional plus (+) or minus (-). Leading blanks are ignored. Comma separators, such as the thousands separator in 123,456.00, are not allowed in the string." (see. https://learn.microsoft.com/en-us/sql/t-sql/data-types/data-type-conversion-database-engine link.)

You should use explicit convertion to force It doesn't occur implicit convertion automaticly. Such as CAST function.

Choose one depend on RegistrationFee column data type in following expressions.

'fee_' + CAST(@ID AS NVARCHAR(25)) 
'fee_' + CAST(@ID AS VARCHAR(25)) 
'fee_' + CAST(@ID AS CHAR(25)) 
'fee_' + CAST(@ID AS NCHAR(25)) 

-- Bigint max value is '9,223,372,036,854,775,807' that has 25 chars.

It is a string of 20 characters in length (sign + 19 digits) since a bigint value does not have formatting.

DECLARE @b AS bigint = -9223372036854775807
SELECT CAST(@b AS varchar(20)) AS [Text]

Anything above 20 is excess.