Recursively Querying Row Groups

Published on: 2020-08-25

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Recursively Querying Related Rows

Recursive queries are fun to plan and write. They can be frustrating too depending on the complexity of the problem you are trying to solve.

This post shows one solution for finding all records that are related, either directly or via intermediate records, using recursive queries in SQL Server.

The Data

Here’s the data we’ll be working with:

DROP TABLE  IF EXISTS #relationships;

CREATE TABLE #relationships (
	Id int,
	FK1 varchar(10),
	FK2 varchar(10)
);

INSERT INTO #relationships
VALUES 
	/* This group of records are all related, directly or through intermediate nodes */
	(1,'A','B'),
	(2,'A','E'),
	(3,'A','G'),
	(4,'A','F'),
	(5,'B','F'),
	(6,'B','E'),
	(7,'E','F'),
	(8,'G','H'),

	(9,'B','A'),  /* This is  an identical relationship as Id=1 */
	(10,'B','F'), /* This is a straight up duplicate record to Id=5*/


	/* These records are related simply where FK2 of one row is FK1 of the subsequent row */
	(11,'I','J'),
	(12,'J','K'),
	(13,'K','L'),

	/* Some more related records */
	(14,'M','N'),
	(15,'O','M'),
	(16,'P','O'),
	(17,'M','N'), /* Duplicate of Id=14 */
	(18,'M','O'), /* Flipped duplicate of 15 */
	(19,'M','P'),

	/* These records are interesting because the FK2 values never appear in the FK1 column */
	(20,'Q','R'),
	(21,'S','R'),
	(22,'T','R');

Each row has values for Id,FK1,and FK2 columns. Id is a unique primary key that I included to make referencing individual rows easier. FK1 and FK2 are foreign keys that reference full records in another table. The point is that this table shows the relationships between records.

I used blank lines to visually indicate a group of records in the data above. The end result I want to achieve is to have each group of related records to share the same group_id. I also added some comments to point out some interesting scenarios in the data.

I programmed in some of the data edge cases I knew I would encounter into the test data; a sort of poor-man’s test driven development. Including these known edge cases helps me test my query so I know my final solution will handle them correctly. If you are applying this solution to your own data, make sure to add your own test cases into the data.

Query Transformations for Sorted Hashes

The first recursive query I wrote to solve this problem was ugly. I was creating sorted hashes to ensure that rows where the FK values were swapped were deduplicated (eg. I would want to dedupe where Id=1 and Id=9). This solution involved joining on CASE statements like this all throughout the set of queries: CASE WHEN FK1<=FK2 THEN '|'+FK1+','+FK2+'|' ELSE '|'+FK2+','+FK1+'|' END AS key_hash.

Yuck. The solution worked, but I know my future self would not want to maintain that type of code.

Rather than have the final query do all of the work, I decided to clean up the data first instead.

Initial Clean Up

I decided to transform the data to eliminate all duplicates and make sure my keys were always sorted so the value of FK1 < FK2. This allows for a simpler final query:

--Remove duplicates where keys are switched
DROP TABLE  IF EXISTS #deduped;
CREATE TABLE #deduped (
Id int IDENTITY(1,1),
FK1 varchar(10),
FK2 varchar(10),
key_hash varchar(100)
);
INSERT INTO #deduped (FK1,FK2,key_hash)
SELECT
	dupes.FK1,
	dupes.FK2,
    dupes.key_hash
FROM
	(
	SELECT
		hashes.*,
		ROW_NUMBER() OVER (PARTITION BY key_hash ORDER BY FK1) AS RN
	FROM
		(
		/* make sure FK1 is always smaller than FK2.  This eliminantes a lot of more complicated logic later on */
		SELECT
			CASE WHEN FK1 <= FK2 THEN FK1 ELSE FK2 END AS FK1,
			CASE WHEN FK1 <= FK2 THEN FK2 ELSE FK1 END AS FK2,
			CASE WHEN FK1 <= FK2 THEN '|'+FK1 +','+FK2+'|' ELSE '|'+FK2+','+FK1+'|' END AS key_hash
		
		FROM
			#relationships
		) hashes
	) dupes
WHERE
	dupes.RN = 1;

This still uses the key_hash CASE statement I mentioned previously, but it only creates it once initially to dedupe the entries regardless of their order.

With a deduped dataset, the recursive query for finding groups of related records is (relatively) straight-forward (and a refresher if you need to remember how recursive CTEs work in SQL Server):

WITH c AS (
	/* The initial query */
	SELECT
		1 as level,
		DENSE_RANK() OVER(ORDER BY FK1) group_id,
		key_hash,
		FK1
		,FK2
	FROM
		#deduped
	UNION ALL
	/* The recursive query that runs once for each of the above query's output and then once on every row from each subsequent result */
	SELECT 
		c.level+1,
		c.group_id,
		t.key_hash,
		t.FK1,
		t.FK2
	FROM 
		c
		INNER JOIN #deduped t
			ON c.FK2 = t.FK1
	WHERE
		/* don't include combinations already matched */
		c.key_hash not like '%'+t.key_hash+'%'
), 
/* regular CTE  */
output_with_dupes as (
SELECT
	level,
	key_hash,
	group_id,
	FK1,
	FK2,
	ROW_NUMBER() OVER (PARTITION BY key_hash ORDER BY group_id) AS  RN
FROM
	c
)

-- deduped output
SELECT
	group_id,
	FK1,
	FK2
FROM
	output_with_dupes
WHERE RN = 1
ORDER BY
	group_id,
	FK1
OPTION(MAXRECURSION 0)

Some records are duplicated after the initial join, so a subsequent expression is used to dedupe the final records. The result is a table of rows containing a group_id of records that are related to each other.

Couldn’t you have done this with hierarchyid?

Probably.

But I wanted a solution flexible enough to reuse for other relational databases that don’t support hierarchyid as well.

More than just recursive queries

If you’ve never written a recursive SQL query before, hopefully this post gives you an idea of how to do it.

The more important takeaway is that sometimes it’s easier to solve a data problem than a query problem. Like I said previously, my initial recursive query was hideous and unmaintainable because of the duplicates I had in my data. Rather than living with that query, it made sense to clean up the data first and then write the simpler query on the clean data.

Thanks for reading. You might also enjoy following me on Twitter.

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Should I Transform My Data In My SQL Query?

Published on: 2020-07-29

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Imagine you need to join two tables of data and filter the results. Perhaps you also need to convert some of the values for display as well (eg. 0 => “No”, 1 => “Yes”).

Do you choose to perform all of this in your SQL query? Or do you bring the data into your app and and handle it there with code?

Option 1: Write it in a SQL query

Usually I’m inclined to do as much in the SQL query as possible because:

Optimization

Relational SQL databases tend to be optimized for doing things like joins, filtering, aggregations, etc….

Software engineers have spent decades tuning their sorting algorithms and squashing bugs to make their relational databases handle these operations efficiently. Sure, you can probably find the occasional edge case where you could handcraft some app code to accomplish one of these things faster, but in most cases it’s not worth the additional time and effort.

Ordered Data

The types of operations in the example above (joins, filtering) benefit from ordered data.

Indexes in your database offer data stored in a predefined order, making all of those operations faster. If you need to perform a transformation that can utilize an index, it usually makes sense to let the database handle this operation in the SQL query rather than moving that data to your app and not have an index available.

Data Reduction

Joining and filtering your data in your database allows you to send a reduced number of records over the network to your application.

Network speeds tend to be one of the largest bottle necks in application systems, so eliminating the amount of data you need to pass through them up front in your SQL query can greatly improve the performance of your application.

This is especially true if you have switched to a work-from-home world where network speeds are even more of a bottleneck.

Hardware

This one can be a toss up. The SQL database servers I usually work with are beefy, having significantly more memory and CPU than my general purpose application servers (or my laptop running analytical code).

This means I can run most of my transformation logic faster on my database server than I can anywhere else. This comes at the cost of your performance hungry queries taking away resources from other queries that may be running on the database server at the same time, but on servers not running at capacity this trade off can be worth the speed. More on this in our app code option.

Portability

If you need to share your transformation logic, a SQL query is going to be easier to share with others or include in other processes than application code.

For example, SQL queries are like a universal language in most companies: programmers, analysts, data scientists, technical product managers, and anyone else who may be interested in understanding the business logic will be able to get a high-level understanding by looking at the SQL query. It will also be easier for them to incorporate into their processes.

Contrast that with some app code that is not easy to execute without installing dependencies and understanding language specific syntax; a SQL query will always be easier to share and reuse.

Option 2: Code it in the application

Everything you can write in a SQL query can be performed in most programming languages.

Let’s discuss when it makes sense to store transformation logic in app code.

Domain Specific Performance Improvements

Most SQL relational databases are built to be general purpose. That’s what makes them so powerful: they work well to help solve many different problems without needing domain specific optimizations.

Sometimes though, you may know something about your data that will allow you to work with it more efficiently than a relational database.

For example, maybe your data consists of mostly XML files, or you know you will be doing a lot of recursive processing of your data. Sure, most relational database engines have some way to accomplish these tasks, but performing this type of processing will most likely be faster to do in your application code.

Local Caching

Sometimes your application will need to process and reprocess the same data over and over again.

In these cases, it may make sense to transfer your data across the network a single time to your application, so it can locally process (and reprocess) that data as many times as needed.

Costs

Many enterprise database platforms cost money to license. Application servers usually don’t have those same kinds of costs. That means that running your transformation logic in a SQL query on your database server is often more expensive than in code on an application server.

If your database server is not at full capacity, then this likely isn’t an issue – after all you want to maximize the usage of the licenses you are paying for.

However, if your database environment is at or over capacity, offloading some transformation logic into your application code may make it run faster and will also likely be cheaper to run.

The cloud makes this case even more transparent, with database as a service offerings generally being more expensive than their application compute counterparts.

Conclusion: which is better?

It depends on your situation.

This post speaks to generalized scenarios. For specific uses, there will be times where it makes sense to store your logic in SQL queries, and other times in your application code.

The point is the next time before you dive in and start writing code, think through your restrictions and goals to figure out the best location to run things.

Thanks for reading. You might also enjoy following me on Twitter.

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SQL Server Converts Numbers to Asterisks

Published on: 2019-12-17

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Pop quiz: What will be the output of the below three statements?

DECLARE @val_varchar varchar(3) = '100';
DECLARE @val_bigint bigint = 100;
DECLARE @val_tinyint tinyint = 100;

PRINT('varchar to varchar:');
PRINT(CAST(@val_varchar AS VARCHAR(2)));

PRINT('bigint to varchar:');
PRINT(CAST(@val_bigint AS VARCHAR(2)));

PRINT('tinyint to varchar:');
PRINT(CAST(@val_tinyint AS VARCHAR(2)));

As you might have guessed, the first conversion truncates the value, leaving the answer as “10”:

varchar to varchar

The second response is also somewhat intuitive – a number can’t be converted into string that has fewer places than original digits:

bigint to varchar

However, the final answer returns:

tinyint to varchar

NOTE: if we change all of the conversions to VARCHAR(3) above, all three results return the string value ‘100’

What the ****?

Why does SQL Server sometimes error when converting a number into a string, but other times succeeds and returns an asterisk?

I don’t know.

The best (and logical) answer I could find online is from Robert Sheldon, who attributes it to poor error handling practices, “…before error handling got a more reputable foothold.”

My understanding is that the asterisk was originally the default truncation value. Later, when the SQL Server development team was adding the bigint datatype, they decided throwing an exception is a better way to handle truncation in errors. However, probably for backwards compatibility, they never went back to change the behavior of the asterisks for pre-existing datatypes.

The documentation is clear about this behavior as well: any int, smallint, or tiny int when converted to char or varchar with fewer characters will result in an asterisk.

This becomes a problem if you create strings out of tinyints, smallints, and ints and suddenly start receiving values larger than you originally expected:

DECLARE @Age int = 75;
PRINT(CAST(@Age AS VARCHAR(2)) + ' years old.');

SET @Age = 100;
PRINT(CAST(@Age AS VARCHAR(2)) + ' years old.');
* years old

Conversions are hard

So is SQL Server to blame for this issue? Sure. But I can empathize: breaking changes are not something people typically want to add to a product.

I’ve written before how implicit conversions can lead to funny results in SQL Server. This asterisk scenario is another case of SQL Server having to make a judgement call about handling an impossible situation you created for it.

While it’s easy to complain about whether SQL Server handles the conversion the correct way or not, at the end of the day the responsibility falls on you to prevent these types of impossible calculations from being requested of SQL Server in the first place.

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