Circular References

Kevin Feasel

2016-04-05

T-SQL

Daniel Hutmacher looks at circular references in recursive CTEs:

Suppose you have a tree hierarchy where (among other members) “3” is the parent of “8”, “8” is the parent of “B” and “B” is the parent of “E”. You could easily draw this as a branch structure where the members could be profit centers of a company, divisions of government, managers and employees, product lines, cell references in an Excel sheet or pretty much anything that can be described as a hierarchy.

3
--8 --B --E

Now, if we say that “E” is the parent of “3”, we’ve created a circular reference, and we end up with an infinite recursion. What that means is that if you follow the tree from the root to the leaf level, you’ll end up going round and round in circles. In terms of a database query, that means that the query will go on forever until it either fills up your log file or tempdb, or until the maximum number of recursions (OPTION MAXRECURSION) is reached, whichever happens first.

I did something similar a while back with foreign key dependencies.  Because foreign key relationships are potentially cyclical graphs, they can easily fall into the same scenario.

Don’t Use Select *

Kevin Feasel

2016-03-29

T-SQL

Kenneth Fisher warns against using SELECT * in queries:

Typing a * sounds a lot easier than actually listing all of the columns you need. However is that really a good idea? You’ll hear lots of people saying it isn’t, but why?

Well there are several reasons.

Read on for his reasons.

Using The OUTPUT Clause

Steve Jones shows how to use the OUTPUT clause with an INSERT statement:

I got asked a question about the OUTPUT clause recently and realized I didn’t remember the syntax. I’ve rarely used this, so I had to look it up and thought this would be a good basic post.

The idea with OUTPUT is that the data from the inserted and deleted tables can be output from the INSERT statement, outside of your triggers. This is the same data, but you can access it in the insert.

The format is

INSERT xxx OUTPUT yyyy INTO @zzz VALUES (or SELECT) mmmm

If I had one thing I could change about OUTPUT, I’d like to be able to output directly into variables for those cases in which I know I’m only going to get one result (or maybe I only care about one arbitrary result in a set).

String_Split Performance

Kevin Feasel

2016-03-21

T-SQL

Aaron Bertrand looks into how STRING_SPLIT performs compared to other string splitting methods:

So with those limitations out in the open, we can move on to some performance testing. Given Microsoft’s track record with built-in functions that leverage CLR under the covers (coughFORMAT() cough), I was skeptical about whether this new function could come close to the fastest methods I’d tested to date.

Let’s use string splitters to separate comma-separated strings of numbers, this way our new friend JSON can come along and play too. And we’ll say that no list can exceed 8,000 characters, so no MAX types are required, and since they’re numbers, we don’t have to deal with anything exotic like Unicode.

The results are surprising.  I expected it to be somewhere around CLR-level, but not way better.

Find Object Dependencies

Kevin Feasel

2016-03-18

T-SQL

Manoj Pandey has pulled out the code used in Management Studio to get dependencies:

And here is a very lengthy (~900 lines) T-SQL Code that I generated from SSMS & SQL Profiler to check the same Dependencies of a Table in SQL Server 2014. You can also create a Stored Procedure and apply the Table & Schema as parameters.

You can just replace the Table & Schema in the first 2 lines and execute the code to check table dependencies

You might be able to optimize this script, but it’s nice to have a starting point.

Using CHARINDEX To Find A String

Steve Jones shows us how to use CHARINDEX to find a string:

A quick one today, just looking for strings. I wrote an article on this, so there’s more detail there, but here’s a bit of code you can look through and see what it does.

He didn’t tag his post T-SQL Tuesday, but it certainly is apropos.

SELECT INTO With UNION

Jason Strate shows us that you can use a UNION (or UNION ALL) to insert into a temp table:

What makes this interesting is when I am using UNION to join the results. How do you place a final resultset from a UNION, EXCEPT, or INTERSECT into a temporary table using SELECT INTO? Where does the INTO portion of the query go?

This is actually a pretty simple thing to do. The INTO for the SELECT INTO goes into the first query of the set. An example of UNIONing the results from sys.dm_exec_query_stats and sys.dm_exec_query_stats into a temporary table is provided in listing 1.

No subqueries are necessary here.

Null Bytes In Text Strings

Kevin Feasel

2016-03-08

T-SQL

Jay Robinson has null bytes he wants to remove from Unicode strings:

As it turns out, when you have a character string in SQL Server that contains character 0x000, it really doesn’t know what to do with it most of the time, especially when you’re dealing with Unicode strings.

I did track down http://sqlsolace.blogspot.com/2014/07/function-dbostripunwantedcharacters.html, but I generally try to avoid calling UDF’s in my queries.

Jay’s got an answer which works, so check it out.  Also, I second the use of the #sqlhelp hashtag.  There’s a great community watching that hashtag.

Synonyms

Aaron Bertrand discusses synonyms:

Let’s say you have a table called dbo.BugReports, and you need to change it to dbo.SupportIncidents. This can be quite disruptive if you have references to the original name scattered throughout stored procedures, views, functions, and application code. Modern tools like SSDT can make a refactor relatively straightforward within the database (as long as queries aren’t constructed from user input and/or dynamic SQL), but for distributed applications, it can be a lot more complex.

A synonym can allow you to change the database now, and worry about the application later – even in phases. You just rename the table from the old name to the new name (or use ALTER TABLE ... SWITCH and then drop the original), and then create a synonym named with the old name that “points to” the new name

I’ve used synonyms once or twice, but they’re pretty low on my list, in part because of network effects:  if I create this great set of synonyms but the next guy doesn’t know about them, it makes maintenance that much harder.

Switching To Identities

Kevin Feasel

2016-03-02

T-SQL

James Anderson shows how to do a table switch to switch a table without an identity column to one with an identity column:

The SWITCH statement can instantly ‘move’ data from one table to another table. It does this by updating some meta data, to say that the new table is now the owner of the data instead of the old table. This is very useful as there is no physical data movement to cause the stresses mentioned earlier. There are a lot of rules enforced by SQL Server before it will allow this to work. Essentially each table must have the same columns with the same data types and NULL settings, they need to be in the same file group and  the new table must be empty. See here for a more detailed look at these rules.

If you can take a downtime, this is pretty easy.  Otherwise, making sure that the two tables are in sync until the switchover occurs is a key problem to keep in mind.

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