How SQL statements execute in SQL Server Management Studio with GO and without GO statement?

Question

I have a simple query

CREATE TABLE #tempTable (id int)
DROP TABLE #tempTable

CREATE TABLE #tempTable (id int)
DROP TABLE #tempTable

From my understanding, in the second part, it should create the #tempTable.

But it shows the following error

Msg 2714, Level 16, State 1, Line 4
There is already an object named '#tempTable' in the database.

I have searched for the reason and found that it is because of a GO statement between the two part of the query. Therefore, the correct query is

CREATE TABLE #tempTable (id int)
DROP TABLE #tempTable
GO    

CREATE TABLE #tempTable (id int)
DROP TABLE #tempTable

I have also found that GO just tells SSMS to send the SQL statements between each GO in individual batches sequentially.

My question is, how are SQL statements executed? Is it not executed sequentially?

If it executes sequentially, then why does my first query cause an error?

Answer 1

The SQL Server documentation does a pretty good job of explaining this.

In your particular case, the issue is compile-time errors versus execution-time errors.

How does this work? Without a GO separating the statements, all are compiled at the same time. The problem is that the third statement is a CREATE TABLE statement and the table already exists. All that is happened is that the statements are parsed and compiled.

With the GO, the first two statements are compiled and executed. Voila! There is no table for the CREATE in the third statement.

Answer 2

The main reason of this error is that parsing the query occurs before executing the query.

It happens because the whole code is executed as one batch. And SQL Server has one task to parse and execute.

That's why a GO command (not a statement) allows you to avoid this problem. It signals the end of a batch.

Here is a good topic to read about it: Understanding how SQL Server executes a query