为什么exec参数中的串接有时会导致t-sql中的语法错误?

在MS SQL Server Management Studio 2005中,运行以下代码

EXEC('SELECT * FROM employees WHERE employeeID = ' + CAST(3 AS VARCHAR))

出现此错误: Incorrect syntax near 'CAST'

但是,如果我这样做,它会起作用:

DECLARE @temp VARCHAR(4000)
SET @temp = 'SELECT * FROM employees WHERE employeeID = ' + CAST(3 AS VARCHAR)
EXEC(@temp)

我在这里找到了一个解释: T-SQL: Cannot pass concatenated string as argument to stored procedure

根据公认的答案, EXEC 可以将局部变量或值作为其参数,但不能将其作为表达式。

但是,如果是这样,为什么这样做:

DECLARE @temp VARCHAR(4000)
SET @temp = CAST(3 AS VARCHAR)
EXEC('SELECT * FROM employees WHERE employeeID = ' + @temp)

'SELECT * FROM employees WHERE employeeID = ' + @temp 当然,在我看来这是一个表达式,但代码执行时没有错误。