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Category: Syntax

DATEDIFF

Published 2018-11-23 by Kevin Feasel

Randolph West continues a series on covering dates and times, looking at DATEDIFF and DATEDIFF_BIG:

The only functional difference between them is that the DATEDIFF_BIG() returns values as a BIGINT, for results that exceed the boundary of an INT. Keep this in mind when deciding which one to use. For example, the maximum number of seconds an INT can hold is 68 years, while a BIGINT can comfortably store the number of seconds in 10,000 years. This becomes especially important when dealing with microseconds and nanoseconds.

The rest of the post will use DATEDIFF() to refer to both functions.

I think this might be the first time I’d read about DATEDIFF_BIG()and I’m not aware of ever having used it.  But hey, it could make sense if you need to track more than 2 billion microseconds.

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Casting Constants And POWER()

Published 2018-11-23 by Kevin Feasel

Steve Jones walks us through a case of an unexpected error:

I ran into an interesting problem while working with the POWER() function. I was trying to do some binary conversions and had a statement like this to process powers of 2.

SELECT POWER(2, n)

This was designed to take a value and return a power of 2. I then used a different value to determine if this was added to my conversion factor or not. In trying to work with some larger numbers, I ran into this error:

Msg 232, Level 16, State 3, Line 3
Arithmetic overflow error for type int, value = 2147483648.000000.

Click through for a description of the problem as well as Steve’s solution.

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Cannot Rollback TRUNCATE In Redshift

Published 2018-10-30 by Kevin Feasel

Derik Hammer notes that you cannot rollback a TRUNCATE TABLE operation in Redshift:

In SQL Server, or PostgreSQL for that matter, the TRUNCATE command is allowed in a transaction and it will commit or rollback like any other DML operation. In all of the scripts, below, I will do the following.

  1. Check my row counts

  2. Begin a transaction

  3. TRUNCATE the table

  4. INSERT one row

  5. Check my row counts from within the transaction

  6. ROLLBACK

  7. Check to see if  my row checks revert or not

That’s an important behavioral difference when loading data using a truncate-and-reload pattern.

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Valid WAITFOR Data Types

Published 2018-10-29 by Kevin Feasel

Dave Mason investigates the valid data types you can use as inputs for WAITFOR:

There are certain design patterns in T-SQL that give me pause. They may not be “code smells” per se, but when I encounter them I find myself thinking “there’s got to be a more sensible way to accomplish this”. WAITFOR DELAY is one example. I’ve used it a few times here and there, mostly in one-off scripts. If I wrote some code that used it regularly in production, I’d be paranoid about putting my SPID to sleep forever. Maybe a little bit of paranoia is a good thing. But I digress.

A recent task found its way to me, and I’ve decided to use WAITFOR DELAY as part of my solution. (It hasn’t been tested or implemented yet–perhaps more on this in another post.) My usage this time has been more complex than in the past. What I already knew is that you can use a string literal for the time_to_pass argument. For example, this will delay for 3½ seconds:

WAITFOR DELAY '00:00:03.500'

Click through for a bunch of testing.

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Understanding ANY And ALL In SQL

Published 2018-10-23 by Kevin Feasel

Doug Kline explains the ANY and ALL operators in SQL:

-- note that this creates a single column of values
-- which could be used in something like IN
-- for example
SELECT   1
WHERE    12 IN    (  SELECT   tempField
                     FROM     (VALUES(11),(12),(7)) tempTable(tempField))

-- I could rephrase this as:
SELECT   1
WHERE    12 = ANY (  SELECT   tempField
                     FROM     (VALUES(11),(12),(7)) tempTable(tempField))

I rarely see these operators in the wild and might have used them in production code a couple of times if that.

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What’s In SQL Server 2019 CTP 2.0?

Published 2018-09-25 by Kevin Feasel

Aaron Bertrand gives us the highlights:

  • Certificate Management in Config Manager View and validate all of your certificates from a single interface, and manage and deploy certificate changes across all of the replicas in an Availability Group or all of the nodes in a Failover Cluster Instance.

  • Built-in data classification A new ADD SENSITIVITY CLASSIFICATION statement helps you identify and automatically audit sensitive data, a huge step up from the previous SSMS wizard (which just used extended properties).

Aaron also digs into the engine a bit:

APPROX_COUNT_DISTINCT

This new aggregate function is designed for data warehouse scenarios, and is an equivalent for COUNT(DISTINCT()). Instead of performing expensive distinct sort operations to determine actual counts, it relies instead on statistics to get something relatively accurate. You should find that the margin of error is within 2% of the precise count, 97% of the time, which is usually fine for high-level analytics, values that populate a dashboard, or quick estimates.

On my system I created a table with integer columns ranging from 100 to 1,000,000 unique values, and string columns ranging from 100 to 100,000 unique values. There were no indexes other than a clustered primary key on the leading integer column. Here are the results of COUNT(DISTINCT()) vs. APPROX_COUNT_DISTINCT() against those columns, so you can see where it is off by a bit (but always well within 2%):

By the way, APPROX_COUNT_DISTINCT() is a really good idea, and I’m glad it’s here.

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Using AT TIME ZONE In SQL Server

Published 2018-09-06 by Kevin Feasel

Randolph West looks at the AT TIME ZONE clause when working with a specific time zone in SQL Server:

The time zone name is taken from a list maintained in the following Windows registry hive: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones

Note: For SQL Server on Linux (and Docker containers), a registry shim performs the same function as the Windows registry by intercepting the API calls and returning the expected value(s).

We can also use a Transact-SQL (T-SQL) query against the system view sys.time_zone_info, which uses the information from the registry hive. This is the recommended method if you do not have access to the registry hive.

Click through for a couple of examples.

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Finding The First Non-NULL Value In A Window

Published 2018-08-29 by Kevin Feasel

Bert Wagner shows off the FIRST_VALUE window function and walks us through a case it struggles with:

The SQL Server FIRST_VALUE function makes it easy to return the “first value in an ordered set of values.”

The problem is that if that first value happens to be a NULL, there is no easy, built-in way to skip it.

While a UserVoice item exists to add the ability to ignore nulls (go vote!), today, we’re going accomplish that end result with some alternative queries.

Click through for the demo, as well as a video version of the post.

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GOTO And Labels In T-SQL

Published 2018-08-28 by Kevin Feasel

Ryan Desmond demonstrates the purpose of GOTO in T-SQL:

So I was playing around at work today and decided for whatever reason to see how I could get the code I was writing to fire off only in certain situations.

If it’s Sunday maybe, or if this is in a particular environment, or if a record in an admin table was something specific.  I’m not sure how I’ll use this but I stumbled on Labels and decided to play with them.

Ok, so how to get to know labels.  Well, in order to get them to work sometimes I have to create labels that are based on some criteria.

I do try to avoid these as much as possible, but they are valid syntax and I’ve seen a couple of cases where it makes sense to use GOTO.

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