T-SQL Tuesday #151 : Controversial Coding Conventions

T-SQL Tuesday #151 : T-SQL Coding StandardsFor this month's T-SQL Tuesday, Mala Mahadevan asks us to talk about coding standards:

I decided to take it one step further, and talk about controversial coding standards. I could take the easy way and direct you at existing content expressing my opinions about tabs vs. spaces, table and column aliases, starting CTEs with ;WITH, or leading commas. I haven't talked about that last one, but Benn Stancil seems to have written exactly what I would have, albeit in a much more poetic way.

Instead, let's talk about…

CONVERT vs. CAST

I wrote recently about how T-SQL lets you write things very differently and still get the same answer – without compromising logic or performance.

I see a lot of people using CAST(col AS something) and it makes me wonder why they choose that syntax over CONVERT(something, col). In this twitter thread, I mentioned how CAST doesn't let you influence the conversion in any way, with vague hand-waving about this very specific example. Greg Low responded:

I use CAST always unless I need a style value

The thought that immediately came to mind is, "I wear two left shoes unless I need to dance."

Maybe there are reasons other than the loose "CAST is more standard" arguments we sometimes cling to, but I think that, for the most part, we cling to those artificially. More on that in a bit.

A realistic example

Imagine I have dozens or hundreds of procedures that say:

CREATE PROCEDURE dbo.something
  @date_string varchar(30)
AS
  SELECT CAST(@date_string AS datetime);

They're this way because they accept a string parameter, using yyyy-MM-dd format, from one of many datepickers or calendar controls in some JS or C# library I can’t change. The CAST is there because the author learned from some other part of the codebase (or from online examples or from a different platform) to use CAST instead of CONVERT. This works fine:

EXEC dbo.something @date_string = '2022-05-31';

Output:

2022-05-31 00:00:00.000

But yyyy-MM-dd isn't universally safe

A new user comes onboard, and starts writing unit tests against those procedures. They are based in Germany, or France, or England, or wherever, and have their language settings configured accordingly. So they run:

SET LANGUAGE British;
EXEC dbo.something @date_string = '2022-05-31';

Output:

Msg 242, Level 16, State 3, Line 4
The conversion of a varchar data type to a datetime data type resulted in an out-of-range value.

Whoops! This language happens to interpret yyyy-MM-dd as yyyy-dd-MM (as do more than half of the languages SQL Server supports).

No, it doesn't make any sense, because no human would ever look at, say, 2022-06-12 and think, "December 6th!" But this is how SQL Server works. And it could happen in other scenarios, too, like failing over to a different data center, building a new server, or someone adding a contradictory SET DATEFORMAT command somewhere.

Use datetime instead?

I bet you're thinking, "why not just change the parameter to datetime?" Arguably, sure, the argument never should have been a string in the first place:

ALTER PROCEDURE dbo.something
  @date_string datetime
AS
  SELECT CAST(@date_string AS datetime);

But remember, we can't change the caller; it's still going to pass its unfortunate yyyy-MM-dd string:

SET LANGUAGE British;
EXEC dbo.something @date_string = '2022-05-31';

Whoops! Now it just gets interpreted incorrectly at a different spot:

Msg 8114, Level 16, State 5, Line 0
Error converting data type varchar to datetime.

We could take advantage of the fact that date isn't subject to this bizarre interpretation, but that doesn't help us if the application ever passes in time along with the date. Or datetime2, but maybe the library doesn't support the newer types or just can't be updated to pass a different type. So we may be stuck with a string parameter anyway.

Enter CONVERT

Instead of these fixes, if I know the incoming format is yyyy-MM-dd, I can keep the incoming string parameter as is, and use the much more versatile CONVERT with a style number:

ALTER PROCEDURE dbo.something
  @date_string varchar(30)
AS
  SELECT CONVERT(datetime, @date_string, 23);

If the datepicker or library changes and passes a different format, I can just update the style number (see the full list here). For example, if time is included, I can use 120 instead of 23. If the input changes to MM/dd/yyyy, I can use 101. If it's dd/MM/yyyy, I can use 103. And so on.

In the meantime, I should be seeking out a new datepicker that uses an unambiguous string (or can use proper, strongly-typed parameters), or submitting a pull request to the one I'm using. Because ultimately I want to stop passing strings I have to convert at all.

CONVERT avoids chaos

Trusting CAST to interpret values consistently is a sloppy mess, and not very friendly to people who don’t yet know when they should use CONVERT and when they can get away with its pretty cousin. So, my suggestion: just always use CONVERT from the start, even in cases where you won't potentially need a style number later. Then your code is consistent and you're not suddenly changing styles on the fly or only in specific scenarios.

And about the artificial clinging…

Someone once told me that they preferred CAST because it is fewer characters to type. While generally I think these arguments are totally bogus anyway, I immediately reminded them they also have to type AS. There went that theory.

Greg in the thread above actually likes that it stands out when a style had to be used:

I also rarely use a style and I don't mind at all that the situations where I do so, stand out as different. Every time I use convert, I know a style was used.

Okay, fair enough; to each their own. Personally, I prefer consistency.

Everyone else claims portability:

"What if we switch to PostgreSQL or MySQL or Oracle or <NoSQL du jour>? I'll have to change all of my code that uses proprietary SQL Server syntax!"

Well:

  • CAST is also unsupported in several destination platforms anyway.
  • In my career I have only seen two true migrations away from SQL Server to something incompatible.
  • In both of those cases, syntax like CONVERT was the least of their worries; the application was entirely rewritten anyway.

I'd love to hear about a real scenario where you have avoided any and all proprietary syntax in SQL Server just in case you decided to switch platforms later (or where you used proprietary syntax and this was the blocker). It's just not that common, and when it happens, there's a lot to tackle in terms of updating all the queries (and metadata, and monitoring tools, and drivers, and so on). I feel like IDENTITY columns and GETDATE() and NEWID() and IIF and != and DATEADD/DATEDIFF/DATEPART/EOMONTH are much bigger and more widespread blockers than CONVERT or CAST. Changing CONVERT to CAST (or TO_VARCHAR or TO_CHAR) is probably the simplest among them, and avoiding that work simply isn't going to change the project timeline.

Just one example from recent memory: Have you ever tried to get WordPress up and running with SQL Server as the backend? I tried a while back and the experience convinced me I will never try again. Those are all allegedly simple CRUD-style queries, but even that is a middleware nightmare. Now imagine something even remotely complex. Shudder.

Aaron Bertrand

I am a passionate technologist with industry experience dating back to Classic ASP and SQL Server 6.5. I am a long-time Microsoft MVP, write at SQLPerformance and MSSQLTips, and have had the honor of speaking at more conferences than I can remember. In non-tech life, I am a father of two, a huge hockey and football fan, and my pronouns are he/him.

6 Responses

  1. Marc Noon says:

    There may also be a standard that is used such as ISO-8601. They may give it another name such as the International Date Time standard. I believe there may be some localized standards that just happen to match this international standard which can be a little confusing.

  2. Thom A says:

    Though I don't work with other RDBMS often, I realised years ago that "portable SQL" (SQL that runs on any RDBMS) is a dream that is never going to happen in my lifetime, and probably not my grandchildren's. Instead, I just use the tools I have available to me in the dialect I'm using, and yes that includes CONVERT, IIF, ISNULL etc. Writing for the tools you have is far more important, in my opinion, than writing for a tool you might use in the future (which will still require a massive rewrite).

  3. Erik Darling says:

    I've started using TRY_CAST in Query Store/Plan Cache scripts, specifically for the query plan XML column. TRY_CONVERT fails pretty often when databases aren't in a… compatible compatibility level 🙄

    • Thom A says:

      TRY_CONVERT is in all supported versions of SQL Server; you'd need to be in compatibility 100 or lower for TRY_CONVERT (and TRY_CAST) not to work; with 2008 being out of support for over 3 years now, I think that there are bigger problems afoot than TRY_CONVERT not being available.

      • Erik Darling says:

        Your comment is not only wrong (TRY_CAST works in compat level 90 on SQL Server 2012), but misses the point entirely (I'm a consultant and I need scripts to work on a variety of database configurations out of the box without error).

        Thanks, though.

      • Thom A says:

        I disagree, a client using completely unsupported version of SQL server (for the last 3 years) isn't a problem; it's your opinion, that's it not wrong, but I would say it is.