SQL: Why does a CREATE TRIGGER need to be preceded by GO

Question

When making a SQL script to create a trigger on a table, I wanted to check that the trigger doesn't already exist before I create it. Otherwise the script cannot be run multiple times.

So I added a statement to first check whether the trigger exists. After adding that statement, the CREATE TRIGGER statement no longer works.

IF NOT EXISTS (SELECT name FROM sysobjects
               WHERE name = 'tr_MyTable1_INSERT' AND type = 'TR')
BEGIN
    CREATE TRIGGER tr_MyTable1_INSERT
        ON MyTable1
        AFTER INSERT
    AS
    BEGIN
        ...
    END
END
GO

This gives:

Msg 156, Level 15, State 1, Line 5
Incorrect syntax near the keyword 'TRIGGER'.

---

The solution would be to drop the existing trigger and then create the new one:

IF EXISTS (SELECT name FROM sysobjects
           WHERE name = 'tr_MyTable1_INSERT' AND type = 'TR')
    DROP TRIGGER tr_MyTable1_INSERT
GO
CREATE TRIGGER tr_MyTable1_INSERT
    ON MyTable1
    AFTER INSERT
AS
BEGIN
    ...
END
GO

---

My question is: why is the first example failing? What is so wrong with checking the trigger exists?

Answer 1

Certain statements need to be the first in a batch (as in, group of statements separated by GO ).

Quote:

CREATE DEFAULT, CREATE FUNCTION, CREATE PROCEDURE, CREATE RULE, CREATE SCHEMA, CREATE TRIGGER, and CREATE VIEW statements cannot be combined with other statements in a batch. The CREATE statement must start the batch. All other statements that follow in that batch will be interpreted as part of the definition of the first CREATE statement.

Answer 2

It's simply one of the rules for SQL Server batches (see):

http://msdn.microsoft.com/en-us/library/ms175502.aspx

Otherwise you could change an object, say a table, and then refer to the change in the same batch, before the change was actually made.