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SQL: Does a Field Exist?

In the FoxPro world, checking to see if a field exists can be done a number of ways:

=AFIELDS(la)
IF ASCAN(la,"FIELDNAME")>0

ENDIF

or

IF TYPE("FIELDNAME") = "U"

(I'm sure there are lots of other ways especially if you're already working with a data dictionary tool)

In T-SQL, there is no real equivalent however, you can do the same by using the INFORMATION_SCHEMA table.

While there is no shortage of sites on the web that will show you similar code (like this one), I wanted to post it here for any VFP devs who may be switching between DBFs and SQL.

IF NOT EXISTS( SELECT * FROM INFORMATION_SCHEMA.COLUMNS
WHERE INFORMATION_SCHEMA.COLUMNS.TABLE_NAME = 'MyTable'
AND COLUMN_NAME='MyColumn')
ALTER TABLE myTable ADD MyColumn decimal(10,2) NULL

Of course, this assumes that the table already exists but you can use similar approaches for tables as well.

IF NOT EXISTS(SELECT TABLE_NAME FROM INFORMATION_SCHEMA.TABLES
WHERE TABLE_NAME = 'myTable')
CREATE TABLE .....

if NOT EXISTS(SELECT TABLE_NAME FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME = 'CFG_Fees'
AND COLUMN_NAME='minmFee')
ALTER TABLE .....


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Comments

Walt Krzystek said…
Very good stuff. I use the INFORMATION_SCHEMA views quite a bit for validation in my business objects. One of the nice parts is they are available to just about all users without security restrictions (except for firing the ALTER TABLE command). Another VFP native function I use for similar tasks is SQLCOLUMNS in which if you pass "NATIVE" as the 3rd parameter, you get the table structure information returned. SQLTABLES is another function to validate table existence.
April 15, 2009 at 1:16 PM
Andrew MacNeill said…
I agree about SQLCOLUMNS but that is specific to a SQL Server connection. My quest began when I was trying to find a column name just as easily as I do in VFP. In VFP, I could call SQLCOLUMNS without a problem - and I know there are a few stored procs in SQL that can do it - but I have found this approach works best.
April 15, 2009 at 2:45 PM
Unknown said…
How about using something like this?

SELECT * FROM table_name WHERE .f. INTO CURSOR junk

IF TYPE("junk.field_name") = "U"
* -- alter table here
ENDIF
USE IN SELECT("junk")
May 3, 2009 at 9:57 AM
Andrew MacNeill said…
Brian,

That would work in VFP - but not in raw SQL.

Andrew
May 3, 2009 at 1:33 PM
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