Always Use Semicolon Statement Terminators

ANSI-standard semicolon statement terminators are often omitted in T-SQL queries and many developers are unaware that this is syntax is deprecated.  Omitting statement terminators is a dangerous practice because, even if the batch compiles, you may get unexpected results.  Consider the insidious examples below pointed out by SQL Server MVP Erland Sommarskog:

Few of us will catch (no pun intended) the bug in the above script.  What results do you expect after running the above script under SQL Server 2012 or later versions?  Rather than leave this as an exercise for the reader, I’ll spoil the fun and mention that no run-time error is raised at all.  Instead, the THROW statement is interpreted as a column alias for the ERROR_MESSAGE() column.  This sort of coding error is especially nasty because catch blocks are rarely unit tested and this catch block coding mistake hides the run-time error entirely without raising an exception.

Similarly, the absence of statement terminators in the script below causes another problem.  Can you spot it?

At least an error is raised in this case, albeit not the one you might expect.  The resultant error is “Cannot roll back THROW. No transaction or savepoint of that name was found”.  This coding bug obfuscates the preceding divide by zero error and prevents the THROW statement from being executed.

Below is another example where the absence of the semi-colon terminator obfuscates the root cause of the error. As you may know, GO is not a T-SQL statement but a batch terminator command recognized by SSMS and other SQL Server tools and utilities. This script executes as expected from an SSMS query window because SSMS parses the script and executes each batch individually when GO commands are encountered:

However, running the same script with PowerShell (or any other client application) fails with the error “CREATE VIEW must be the first statement in a query batch”:

In this case, SQL Server interprets the GO as a column alias in the first SELECT query and the batch errs on the CREATE VIEW statement during compilation. If you a semi-colon is added to the end of the first SELECT statement, the correct error message results: “Incorrect syntax near ‘GO'”.

As a side note, one can execute scripts containing GO terminators programmatically using the SMO API, which is also used by some SQL Server tools. See this Stackoverflow answer. Another approach I’ve used is to parse scripts in code using the Transact-SQL script DOM and execute each batch individually. I’ll follow up with a separate article detailing that method and add the link here.

Semicolons Will Become Mandatory
Microsoft announced with the SQL Server 2008 release that semicolon statement terminators will become mandatory in a future version so statement terminators other than semicolons (whitespace) are currently deprecated.  This deprecation announcement means that you should always use semicolon terminators in new development.  I honestly don’t expect SQL Server to strictly enforce mandatory semicolons in the near future but it is still a best practice to use semicolon statement to avoid issues like those mentioned earlier as well as facilitate code maintainability.  I suggest specifying statement terminators in all new development and perhaps adding terminators to existing code as you perform maintenance.

Transact-SQL does not currently enforce the ANSI semicolon statement terminator requirement.  Instead, semicolon statement terminators are optional and any whitespace (spaces, tabs, newline) may be used instead.  The exception to this rule is that many of the statements introduced in SQL Server 2005 and later require the preceding statement to be properly terminated in order for the batch to compile.

Below are some guidelines I suggest on when to, and when not to, use semicolon statement terminators.

Suggested Guidelines
The Transact-SQL parser is quite lax, allowing any whitespace (e.g. space, tab, newline) to be used.  This laxness results in ambiguity like the examples at the beginning of this article demonstrate.  Similarly, statement terminators may not only be omitted, they may also be used in inappropriately.  I strongly suggest you adhere to the T-SQL syntax documented in the Books Online even if the parser allows otherwise.  This practice will help future-proof your code since relying on undocumented behavior is inherently risky.

Don’t precede a statement with a semicolon
Remember that the purpose of semicolons is to terminate SQL statements, not begin them.  A common mistake I see is throwing a semicolon in front of statements in order to get a batch of statements to compile, especially with newer statements like WITH (CTE expression) that require previous statement termination.  Although the T-SQL parser currently ignores extraneous and misplaced semi-colons, I suggest they be specified in the appropriate place according statement syntax documented in the SQL Server Books Online.

Specify semicolons at the end of each stand-alone SQL statement
Not only will this conform to the ANSI standard, your intent will be clearer and the code easier to read.

Terminate control-of-flow statement blocks at the end of the control-of-flow scope
Control-of-flow statements are not covered by the ANSI SQL standard because these are proprietary SQL extensions.  The SQL Server Books Online is sketchy on the subject and many of the examples (as of this writing) are inconsistent and do not always include statement terminators.  Furthermore, control-of-flow statement blocks are confusing due to the many variations, nesting, and optional BEGIN/END specifications.

Below are examples illustrating what I believe to be proper use of statement terminators control-of-flow block terminators using IF statements in SQL 2008 and later versions.  The same concepts apply to other control-of-flow constructs like WHILE and TRY/CATCH blocks.  I should add that this batch example will not compile under SQL 2005 because an explicit BEGIN/END block is required to execute a common table expression conditionally in that version.  T-SQL parser enhancements eliminated that requirement in SQL 2008 and later.

Summary
Consistent user of semicolons helps avoid bugs in code that might otherwise go undetected.  Code with statement terminators can also be more easily modified without introducing compile errors and make code easier to maintain because the end of each statement is readily apparent to subsequent developers.  Importantly, you’ll be better positioned for future SQL Server versions by consistently using semicolon statement terminators.

Use Caution with Explicit Transactions in Stored Procedures

Use Caution with Explicit Transactions in Stored Procedures

 

 

 

Explicit transactions are often used within stored procedures to guarantee all-or-nothing data integrity.  However, a little known fact is that a query timeout will leave the transaction open unless non-default session settings and/or special exception handling are used.  I’ll describe how to protect your application from problems following timeouts and other unexpected errors.

Consider the following stored procedure containing an explicit transaction:

You execute the script below from SQL Server Management Studio or Query Analyzer with the query timeout option set to 30 seconds and the second UPDATE statement in the proc times out. 

timeout option set to 30 seconds and the second UPDATE statement in the proc times out.

Assuming default session settings, check all that apply:

a)      Proc execution continues after the failed UPDATE

b)      @@ERROR is zero

c)       @@TRANCOUNT is zero

d)      The PRINT statement is executed

Let me first mention something important about timeouts before I provide the correct answer(s).  A command timeout occurs in the client application, not the SQL Server backend.  A timeout is basically just a cancel request that is sent by the client API when a command executes longer than the specified interval.  A timeout is very much like pressing the stop button in Query Analyzer or Management Studio because you feel a query has been running too long.  The only difference is that the stop is issued by the client API on behalf of the application.

Both “A” (proc continues) and “D” (PRINT executes) are false because the attention event from the client instructed SQL Server to cancel the currently executing batch in its entirety.  No code after the UPDATE executes, including the PRINT statement following the stored procedure execute.  This is logical since a query cancel or timeout wouldn’t be much use if SQL Server continued executing statements afterward.

“B” (zero @@ERROR) is true.  @@ERROR is zero because no error occurred on the backed; SQL Server successfully canceled the batch per the client cancel request after the timeout.  The timeout error is raised only on the client by the API to notify the application (SSMS/QA in this example) that the command timed out.  SSMS and QA simply catch the error and display the error message from the API.

“C” (zero @@TRANCOINT) is false because XACT_ABORT OFF is the default session setting.  With XACT_ABORT OFF, it is the client application’s responsibility to trap the timeout error and rollback the transaction if necessary.  The transaction is left open and uncommitted following the timeout error.  This can have serious and undesirable consequences if the application performs other work on the connection, unaware of the open transaction.

Using SET XACT_ABORT

SET XACT_ABORT specifies what action SQL Server should take following run-time errors.  The default session setting is SET XACT_ABORT OFF, which indicates that only the Transact-SQL statement that raised the error is rolled back and the transaction continues.  Depending on the severity of the error, the entire transaction may be rolled back and batch aborted, even with SET XACT_ABORT is OFF.   

A side effect of SET XACT_ABORT OFF is that a cancel/timeout error can leave an open transaction so it’s the client’s responsibility to cleanup following cancel/timeout.  To safeguard against leaving an open transaction, applications that execute transactions with SET XACT_ABORT OFF need to roll back transactions and perhaps close the connection following SQL exceptions.  

Note that with connection pooling, simply closing the connection without a rollback will only return the connection to the pool and the transaction will remain open until later reused or removed from the pool.  This can result in locks begin held unnecessary and cause other timeouts and rolling blocks.

SET XACT_ABORT ON instructs SQL Server to rollback the entire transaction and abort the batch when a run-time error occurs.  Compile errors (e.g. syntax errors) are not affected by SET XACT_ABORT. 

In my experience, SET XACT_ABORT ON provides the desired behavior in most cases.  I’ve never run into a situation where I wouldn’t want to rollback a transaction following a cancel or timeout.   I nearly always specify SET XACT_ABORT ON in stored procedures that contain explicit transactions to ensure that transactions are rolled back even if the application code doesn’t clean up properly.  The only time I don’t use XACT_ABORT is in rare cases where I need to trap and handle specific errors in Transact-SQL and continue.

I strongly recommend that SET XACT_ABORT ON be included in all stored procedures with explicit transactions unless you have a specific reason to do otherwise.  The consequences of an application unwittingly performing work on a connection with an open transaction are disastrous.

SQL Server error handling in general is a huge topic I focused on only on timeout errors and SET XACT_ABORT here.  For a thorough discussion of SQL Server error handling, I suggest perusing articles Implementing Error Handling with Stored Procedures and Error Handling in SQL Server – a Background by SQL Server MVP Erland Sommarskog.