Question

I am using a SQL Server 2005 database, and I have an If statement that is behaving in two different manners against the same data set, depending on the actions to be taken in the begin…end block.

First, if I merely want to print to the console, the following code prints nothing, as expected:

if (not exists(select null from tblControls where Name = 'SOME_CONTROL_NAME'))
begin
  print 'control not found'
end

Whereas this code prints ‘control found’ as expected:

if (exists(select null from tblControls where Name = 'SOME_CONTROL_NAME'))
begin
  print 'control found'
end

However, if I change the code to this:

if (not exists(select null from tblControls where Name = 'SOME_CONTROL_NAME'))
begin
  insert into tblControls values (632, 'NEW_CONTROL_NAME', 'New Control', 1, 1, NULL, 1, 'DataControls.CheckBox', NULL, NULL, 1) 
end

The insert statement ALWAYS fires, even though a matching record exists in tblControls. Is there something special about insert statements in T-SQL 2005 that could be causing this behavior, or am I missing something obvious? I would understand if the logic of the code is wrong, but when I tested using the print statement it works as expected.

EDIT: Its generating an “Insert Error” message.

Any help with this frustrating problem is greatly appreciated.

Answer 1

It’s the NULL in the Exists..Select that’s confusing it. Try:

if (not exists(select * from tblControls where Name = 'SOME_CONTROL_NAME'))
begin
  insert into tblControls values (632, 'NEW_CONTROL_NAME', 'New Control', 1, 1, NULL, 1, 'DataControls.CheckBox', NULL, NULL, 1) 
end

Also, it can be rewritten without the parentheses and Begin/End:

IF NOT EXISTS (SELECT * FROM tblControls WHERE Name = 'SOME_CONTROL_NAME') 
  INSERT INTO tblControls VALUES (632, 'NEW_CONTROL_NAME', 'New Control', 1, 1, NULL, 1, 'DataControls.CheckBox', NULL, NULL, 1)