Category: T-SQL

Izik Ben-Gan highlights a reader submission from his last post:

Last month I covered a Special Islands challenge. The task was to identify periods of activity for each service ID, tolerating a gap of up to an input number of seconds (@allowedgap). The caveat was that the solution had to be pre-2012 compatible, so you couldn’t use functions like LAG and LEAD, or aggregate window functions with a frame. I got a number of very interesting solutions posted in the comments by Toby Ovod-Everett, Peter Larsson, and Kamil Kosno. Make sure to go over their solutions since they’re all quite creative.

Curiously, a number of the solutions ran slower with the recommended index than without it. In this article I propose an explanation for this.

Even though all solutions were interesting, here I wanted to focus on the solution by Kamil Kosno, who’s an ETL developer with Zopa. In his solution, Kamil used a very creative technique to emulate LAG and LEAD without LAG and LEAD. You will probably find the technique handy if you need to perform LAG/LEAD-like calculations using code that is pre-2012 compatible.

Kamil’s solution was quite clever.

Pedro Lopes shows us a new use hint introduced in SQL Server 2017 CU10:

In this scenario, you only have this one query that apparently does better in SQL Server 2014 than 2017. That’s all “New CE” – there’s no CE70 vs CE 120+ at issue here. Using any known trace flag, the FORCE_LEGACY_CARDINALITY_ESTIMATION hint or the FORCE_DEFAULT_CARDINALITY_ESTIMATION hint doesn’t help. Rewriting the query is an option, but in the interim, I need a quick fix. How?

In SQL Server 2017 CU10, we have introduced a few new USE HINTs: the QUERY_OPTIMIZER_COMPATIBILITY_LEVEL_n, where n is a supported database compatibility level. This forces the query optimizer behavior at a query level, as if the query was compiled with database compatibility level. You can refer to sys.dm_exec_valid_use_hints for a list of currently supported values for n.

So to be clear, the new hint is not forcing only a specific CE model, it’s forcing the equivalent of the specific database compatibility level’s query optimizer behavior, including any query optimizer fixes that are enabled by default in that database compatibility level.

Something to keep in mind, though ideally not something you’d want to use regularly.

Bert Wagner investigates an issue he found where his long JSON strings were becoming NULL in SQL Server:

After a little bit more research, I discovered that the return type for JSON_VALUE is limited to 4000 characters.   Since JSON_VALUE is in lax mode by default, if the output has more than 4000 characters, it fails silently.

To force an error in future code I could use SELECT JSON_VALUE(@json, ‘strict $.FiveThousandAs’)  so at least I would be notified immediately of an problem with my  query/data (via failure).

Although strict mode will notify me of issues sooner, it still doesn’t help me extract all of the data from my JSON property.

Read on for the answer.

Kenneth Fisher shows that deleting the top N records with an ORDER BY clause is not straightforward:

Did you know you can’t do this?

DELETE TOP (10)
FROM SalesOrderDetail
ORDER BY SalesOrderID DESC;

Msg 156, Level 15, State 1, Line 8
Incorrect syntax near the keyword ‘ORDER’.

I didn’t. Until I tried it anyway. Turns out, it says so right in the limitations section of BOL. Fortunately, that same section does have a recommendation on how to move forward.

Read on for a couple of methods to do this.

Itzik Ben-Gan shares with us a special case of the islands problem:

In your database you keep track of services your company supports in a table called CompanyServices, and each service normally reports about once a minute that it’s online in a table called EventLog. The following code creates these tables and populates them with small sets of sample data:

[…]

The special islands task is to identify the availability periods (serviced, starttime, endtime). One catch is that there’s no assurance that a service will report that it’s online exactly every minute; you’re supposed to tolerate an interval of up to, say, 66 seconds from the previous log entry and still consider it part of the same availability period (island). Beyond 66 seconds, the new log entry starts a new availability period. So, for the input sample data above, your solution is supposed to return the following result set (not necessarily in this order):

It’s a neat twist on an old problem.

Riley Major shows a couple techniques for including ad hoc functions in T-SQL, namely Common Table Expressions and the APPLY operator:

It’s helpful to think of each APPLY as a pipe operation, taking the values from the previous derived table and passing them into the next to be manipulated. Programming T-SQL in this manner (loosely) approximates modern functional programming techniques.

It keeps each step of the logic smaller, so that it’s easier to understand. And you can expose the intermediary columns to help with debugging.

This is one of my favorite uses of the APPLY operator, as it lets you think through a problem step-by-step while still allowing the optimizer to create a set-based solution for you.

Kenneth Fisher shows how to include table-valued parameters in a dynamic SQL query:

Recently I did a presentation on dynamic SQL. In the presentation I pointed out the similarity of using sp_executesql to creating a stored procedure to do the same task. After the session I was asked: If that’s the case, can I pass a TVP (table valued parameter) into sp_executesql?

Awesome question! Let’s give it a shot.

Read on to see how to do this.