Solved

SQL - Cast fields to data types

Posted on 2012-03-26
6
322 Views
Last Modified: 2012-03-26
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,
Moe
0
Comment
Question by:mauricerichard
  • 3
  • 2
6 Comments
 
LVL 18

Expert Comment

by:Cluskitt
ID: 37766776
SELECT CAST([Date] as datetime) AS 'Date'
You should always avoid naming your fields/tables the same as SQL functions/special words, though
0
 
LVL 18

Expert Comment

by:Cluskitt
ID: 37766796
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.
0
 
LVL 18

Accepted Solution

by:
lludden earned 400 total points
ID: 37766854
You need to use dynamic SQL.

DECLARE @Sql VARCHAR(MAX)
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'
SELECT @SQL =
  COALESCE(
    @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 + ']'
  )
FROM @T1 T1
SET @Sql = @Sql + ' FROM MyTable'

EXECUTE (@Sql)
--Print @SQL
0
Complete VMware vSphere® ESX(i) & Hyper-V Backup

Capture your entire system, including the host, with patented disk imaging integrated with VMware VADP / Microsoft VSS and RCT. RTOs is as low as 15 seconds with Acronis Active Restore™. You can enjoy unlimited P2V/V2V migrations from any source (even from a different hypervisor)

 
LVL 1

Author Comment

by:mauricerichard
ID: 37767041
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.
0
 
LVL 18

Assisted Solution

by:Cluskitt
Cluskitt earned 100 total points
ID: 37767093
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 [])
0
 
LVL 1

Author Closing Comment

by:mauricerichard
ID: 37767392
Great, thanks for all the help!
0

Featured Post

What is SQL Server and how does it work?

The purpose of this paper is to provide you background on SQL Server. It’s your self-study guide for learning fundamentals. It includes both the history of SQL and its technical basics. Concepts and definitions will form the solid foundation of your future DBA expertise.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

Having an SQL database can be a big investment for a small company. Hardware, setup and of course, the price of software all add up to a big bill that some companies may not be able to absorb.  Luckily, there is a free version SQL Express, but does …
This article shows gives you an overview on SQL Server 2016 row level security. You will also get to know the usages of row-level-security and how it works
Via a live example, show how to extract insert data into a SQL Server database table using the Import/Export option and Bulk Insert.
Using examples as well as descriptions, and references to Books Online, show the documentation available for datatypes, explain the available data types and show how data can be passed into and out of variables.

810 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question