How to convert a varchar column to bit column in SQL SERVER

问题

Flag1 is a varchar column with values "true" and "false". I need to convert this into bit column.

When I try to do this:

Convert(Bit,Flag1)

it shows an error

Msg 245, Level 16, State 1, Line 2
Syntax error converting the varchar value 'False' to a column of data type bit.

回答1:

I suspect that there are other values in addition to 'true' and 'false' in the field 'Flag1'. So check for the values in Flag1.

select distinct Flag1 from YouTable.

Here is my proof:

declare @Flag varchar(25) = 'False'
select CONVERT(Bit, @Flag)

It works fine.

However, this will give the same error.

declare @Flag varchar(25) = '  False' -- Pay attention to the the space in '  False'!
select CONVERT(Bit, @Flag)

-> Msg 245, Level 16, State 1, Line 2 Conversion failed when converting the varchar value ' False' to data type bit.

Pay attention to the the space in ' False' in the error message!

回答2:

While selecting from table, you can do this:

SELECT CASE Flag1 WHEN 'true' THEN 1 ELSE 0 END AS FlagVal

Syntax:

CASE input_expression 
     WHEN when_expression THEN result_expression [ ...n ] 
     [ ELSE else_result_expression ] 
END 
Searched CASE expression:
CASE
     WHEN Boolean_expression THEN result_expression [ ...n ] 
     [ ELSE else_result_expression ] 
END

回答3:

I do not think it is to do with if you have other values in your column. Its to do with how you've defined "true" or "false". SQL thinks it's a string rather than a bit. In your column I'd suggest using a Case Statement like:

select ...., case when ColumnName = "True" then 1 else 0 end as Flag1

Make sure you do not have any spaces in true or false. For that you could use:

rtrim(ltrim(ColumnName)) 

To remove any spaces.

来源：https://stackoverflow.com/questions/22382997/how-to-convert-a-varchar-column-to-bit-column-in-sql-server