T-SQL:: 将 1 更改为 0.5 时将数据类型 varchar 转换为数字时出错

Question

I'm writing a complex T-SQL query with CASE that will help me calculate how much time it will take me to migrate databases based on the size of each database:

SELECT 
    ProductVersion
    ,CompatibilityLevel
    ,convert(numeric(10,2), substring([Size], 1, (len([Size]) - 3))) as [Size in MB]
    ,CASE
        when ProductVersion < '11%' THEN 1
        when ProductVersion = NULL then 1
        ELSE ''    
    END  + 
    CASE
        when CompatibilityLevel < '110' then 1
        when CompatibilityLevel = NULL then 1
        ELSE ''
    END  + 
    CASE
        when  convert(numeric(10,2), substring([Size], 1, (len([Size]) - 3))) < 100.00 THEN 1 -- Here is the problem
        ELSE ''
    END AS [Hours of work]
FROM MyDatabaseList

All good, it works when I set 1 hour of work for every database which has less then 100.00 MB.

Also all other values are summed and in the Hours of work column you can see numbers like 0 , 1 , 3 , 2 ... This means SQL Server is calculating values. All good.

But well, telling to my manager that I have to work 1 hour for a database that only has 100.00 MB of size is ridiculous.

Let's change the line:

    when  convert(numeric(10,2), substring([Size], 1, (len([Size]) - 3))) < 100.00 THEN 1

and let's put 0.5 instead of 1 :

    when  convert(numeric(10,2), substring([Size], 1, (len([Size]) - 3))) < 100.00 THEN 0.5

...and bang - I get this error:

Msg 8114, Level 16, State 5, Line 1
Error converting data type varchar to numeric.

Questions:

1. Why this error now?
2. Why it was successfully calculating before and now needs a datatype somewhere?
3. What change do I have to add to my query?
4. Why it says on Line 1 ? The error must be on Line 16 , right?

Answer 1

The error is telling you the conversion is happening in the statement that starts in line 1.

As for why, it's because '' cannot be converted to a numeric but it can be converted to an int .

SELECT CONVERT(decimal(2,1),'')
--Error converting data type varchar to numeric.
GO
SELECT CONVERT(int,'')
--
GO

You were previously converting it to an int (the literal 1 is an int ). Get out of the habit of declaring '' for a number; it is by definition not a number, it is a zero length string. If you want zero, then use 0 .