以下SQL Server代码在你不能确定是否可以正常运行的话，建议您细读这篇文章，关于隐式数据转换。

• DECLARE @A CHAR(3) = 'DEF';
• SELECT CASE
•   WHEN @A = 'DEF' THEN 0
•   WHEN @A = 'GHI' THEN 1
•   ELSE 'DOES NOT APPLY'
• END;
• 
  
• GO
• DECLARE @A CHAR(3) = 'ABC';
• SELECT CASE
•   WHEN @A = 'DEF' THEN 0
•   WHEN @A = 'GHI' THEN 1
•   ELSE 'DOES NOT APPLY'
• END;
• 
  
• GO
• DECLARE
•   @A CHAR(3) = 'ABC',
•   @B CHAR(5) = 'DEFGH',
•   @C VARCHAR(10) = NULL,
•   @D INT = 1234;
• SELECT COALESCE(@C, @D, @A);
• 
  
• GO
• DECLARE
•   @A CHAR(3) = 'ABC',
•   @B CHAR(5) = 'DEFGH',
•   @C VARCHAR(10) = NULL,
•   @D INT = 1234;
• SELECT COALESCE(@C, @A, @D);
  

How to Get SQL Server Data-Conversion Horribly Wrong
One of the most certain ways of going wrong with any relational database is to get data conversion wrong. Implicit data conversion is a good feature for the expert but  can cause a lot of trouble to the unwary. These boil down to seven ways of  failing to get data conversion right. Rob Sheldon explains and gives sage advice on how to avoid the problems。


Failure #1: Not understanding data type precedence
Failure #2: Not taking performance into consideration
Failure #3: Assuming all numbers are created equal
Failure #4: Relying on the ISNUMERIC built-in function
Failure #5: Underestimating the world of silent truncation
Failure #6: Not understanding date/time data
Failure #7: Importing Excel data without thought to data types
Failure #8: Treating XML just like any other string
Failure #9: Failing to take portability into account


信息来源：redgate