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SQL - If Field Exists Or not

Posted on 2013-11-06
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Last Modified: 2013-11-21
I have 2 databases, one is a newer version with additional fields and the other doesn't have these new fields.

I just want to create one query for use with both database since it's same report.  I do not want to have 2 sets of query to toggle between one or the other.  Anyway I can check if field exists and if it does, use that field else put a "Null"?

Example:

New Database table (Employee)

SELECT ID, NAME, DEPT, PHONE, EMAIL, MOBILE FROM EMPLOYEE

Older Database table (Employee)

SELECT ID, NAME, DEPT, PHONE FROM EMPLOYEE

I am missing Email and Mobile from the older database.  When I query the older database, it would throw me an error stating that Email and Mobile doesn't exists.  How could I set it with one select statement to use a condition to check if field exists or not?

I found a few examples online to toggle, but would like to not have to toggle.  Can something like this be done?

SELECT ID, NAME, DEPT, PHONE, CASE WHEN COL_LENGTH('EMPLOYEE', 'EMAIL') IS NOT NULL THEN 'EMAIL' ELSE 'NULL' END AS EMAIL,  CASE WHEN COL_LENGTH('EMPLOYEE', 'MOBILE') IS NOT NULL THEN 'MOBILE' ELSE 'NULL' END AS MOBILE
FROM EMPLOYEE
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Question by:holemania
5 Comments
 

Expert Comment

by:sergiomn
ID: 39628803
The simplest solution would be just to add those 2 missing columns to the old database.

If you're using java, you could also use getMetaData() to get the column count, names and more, so you could have a function to retrieve results from any table, even if you don't know what columns are in it.

Also, if you're executing this query inside any sort of script, you could use a parameter so you can know if execute one or another query.

There are many other aproaches to this, it depends on your needs.
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Author Comment

by:holemania
ID: 39630176
Adding the 2 missing columns is a no no for us since when upgrading, it might create issue since the upgrade to the database will attempt to add those 2 fields and may fail.

Probably might have to create 2 queries and just toggle between the 2 depending on which database to connect to.
0
 
LVL 32

Expert Comment

by:Brendt Hess
ID: 39634794
Much depends on how you are running this.  However, it is fairly straightforward to get the number of columns for a table.  Here is one example:

DECLARE @tableName sysname
SET @tableName = 'MyTableName'

DECLARE @cols int

SELECT @cols = COUNT(*)
FROM sys.columns AS c
INNER JOIN sys.tables AS t
	ON c.object_id = t.object_id
WHERE t.object_id = OBJECT_ID(@tablename)

Open in new window

If this is a stored procedure, you can check the table, find the number of columns, and see if your version works when the COL_LENGTH check is replaced by  CASE @cols WHEN 6 THEN EMAIL END as email (for example)
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LVL 41

Accepted Solution

by:
Sharath earned 2000 total points
ID: 39638058
You can try dynamical query like this.
declare @sql nvarchar(200)
select @sql = 'SELECT ID, NAME, DEPT, PHONE' +
              case db_name() when 'OldDB' then ''
                             when 'NewDB' then 'EMAIL, MOBILE' end +
              ' FROM EMPLOYEE'
exec @sql
 

Open in new window

0
 

Author Closing Comment

by:holemania
ID: 39666882
Thanks that helped.
0

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