SQL - Cast fields to data types

Hello Experts,

I need to convert fields in a query to particular datatypes.  But there's a lemon twist here ... the data types are part of the actual data.  That's confusing, so let me show you an example of the query results.

Data_Field      |      Data_Type
City                          varchar(25)
State                        varhcar(2)
Zip                           numeric
Date                        datetime

One field has the data, the other field has the expected data type.
So in my query, I need to do something like this.

SELECT CAST(Data_Field as Data_Type) as 'Data_Field'

But SQL doesn't like this.  Can anyone help me with syntax to make this work?

Many thanks,
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You need to use dynamic SQL.

DECLARE @T1 TABLE (Data_field VARCHAR(20), Data_Type VARCHAR(20))
INSERT INTO @T1 SELECT 'City','varchar(25)' UNION SELECT 'State','varchar(2)' UNION SELECT 'Zip','Numeric' UNION SELECT 'Date','datetime'
    @SQL + ',Cast([' + t1.Data_Field + '] AS ' + t1.Data_type + ') AS [' + t1.Data_Field + ']',
    'SELECT Cast([' + t1.Data_Field + '] AS ' + t1.Data_type + ') AS [' + t1.Data_Field + ']'
SET @Sql = @Sql + ' FROM MyTable'

--Print @SQL
SELECT CAST([Date] as datetime) AS 'Date'
You should always avoid naming your fields/tables the same as SQL functions/special words, though
You can also use MyTableName.MyFieldName nomenclature to avoid these situations. However, this won't avoid other issues. One I've seen recently was some here on EE that had a table with a field named Day. He then had a syntax error trying to perform a DateAdd(day,...,...) because SQL interpreted day as the field and not as the DateAdd argument.
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mauricerichardAuthor Commented:
Hi Cluskitt,

Sorry, this is very hard to explain.  Point taken with the naming conventions, but lets not worry about that for now.  Let me try to re-explain this.

The table has 2 fields: 1) Data_Field  2) Data_Type

Imagine you just created a table called Addresses with two fields, Data_Field & Data_Type

Now imagine you ran this statement for this table:
INSERT INTO Addresses(Data_Field, Data_Type) VALUES('City', 'varchar(25)')
INSERT INTO Addresses(Data_Field, Data_Type) VALUES('State', 'varchar(2)')
INSERT INTO Addresses(Data_Field, Data_Type) VALUES('Zipcode', 'numeric')
INSERT INTO Addresses(Data_Field, Data_Type) VALUES('Date', 'datetime')

You'd now have data that looked like I explained in my question.  But it's just data, the data_Type field isn't a SQL datatype ... it's a string of data.

So what I'd like do to is convert the Data_field to whatever Data_Type we entered for it.  So in the example above, when 'City' loads, I want it to be converted to varchar(25) ... when 'Date' loads up I want to convert it to datetime.

So my first thought was: SELECT CAST(Data_Field as Data_Type) as 'Data_Field'
But I get the message: Type c.Data_Type is not a defined system type.
I understand now. You have to use dynamic SQL as lludden posted, or some similar variations (like using Quotename functions). The way I handled a similar situation was to create a stored procedure that had @Field and @Type as arguments. You then create your query much like the example posted above (if you use Quotename, it surrounds the string with [])
mauricerichardAuthor Commented:
Great, thanks for all the help!
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