SQL生成的联接与VARCHAR小数转换

Question

I have a SQL Generator in C# which allows users to pass in parameters which specify a table to join to, and which keys to join on.

Given the flexibility, and the fact that SQL sometimes didn't cast the field appropriately, the query ended up like so:

SELECT * FROM [SourceTable] AS [src]
INNER JOIN [JoinTable] [join1] ON CAST([join1].[someField] AS VARCHAR(5000)) = CAST([src].[otherFIeld] AS VARCHAR(5000))
WHERE [blah] = 'blah'

This has worked for quite some time, until it was required that the someField column on JoinTable be changed from a float to a decimal. So now instead of '1234' = '1234' working as expected, we get '1234.00000000000' = '1234' which of course fails.

The biggest kink which is stopping me from a quick fix is that that fields don't have to be numeric. They could very well pass in fields that join on lastName or something, and so I can't just TRY_CONVERT to float or something.

The second kink is that this is a join, not just a select, so I don't know the repercussions of putting a bunch of logic in a join, or if that's even possible. Otherwise I wonder if I could do an IsNumeric type check and, if true, tryconvert to float, then cast to varchar, otherwise just cast to varchar?

We are using SQL Servers between 2014 and 2016

Answer 1

SELECT * FROM [SourceTable] AS [src] 
    INNER JOIN [JoinTable] [join1] 
    ON (CASE WHEN isnumeric('join1].[someField]')=1 
        then  CAST(CAST([join1].[someField] AS FLOAT) AS VARCHAR(50))
         ELSE 
             CAST([join1].[someField] AS varchar(50)) end) = 
      (CASE WHEN isnumeric([src].[otherFIeld])=1 then  CAST(CAST([src].
      [otherFIeld] AS FLOAT) AS VARCHAR(50)) 
       ELSE  CAST([src].[otherFIeld] AS varchar(50)) end)
      WHERE [blah] = 'blah'

Answer 2

Otherwise I wonder if I could do an IsNumeric type check and, if true , tryconvert to float, then cast to varchar, otherwise just cast to varchar?

Yes you have to do it exactly like this.cast both side of join to same data type.

SELECT * FROM [SourceTable] AS [src]
INNER JOIN [JoinTable] [join1] ON CAST([join1].[someField] AS VARCHAR(5000)) = CAST([src].[otherFIeld] AS VARCHAR(5000))
WHERE [blah] = 'blah' and ISNUMERIC([join1].[someField])=0 and ISNUMERIC([src].[otherFIeld])=0
union ALL
SELECT * FROM [SourceTable] AS [src]
INNER JOIN [JoinTable] [join1] ON CAST([join1].[someField] AS float) = CAST([src].[otherFIeld] AS float)
WHERE [blah] = 'blah' and ISNUMERIC([join1].[someField])=1 and ISNUMERIC([src].[otherFIeld])=1

declare @i decimal(10,4)=1234
declare @ii varchar(400)='1234'
--this won't work
select cast(@i as varchar(400)),@ii
-- this will work
select cast(@i as float),@ii