Bad Idea Jeans Week: Dynamically Generating Long Queries

As part of an experiment, I needed to build a really long query. (Don’t ask.)

From another recent experiment , I know that SQL Server won’t let a queryreturn more than 65,535 columns. I set about writing a one-line query that would return 65,535. I’m a big fan of writing the simplest reproduction scripts possible I don’t want them to rely on tables if they don’t have to so we’ll starta CTE like this:

My favorite Thanksgiving side dish

That’s a simple CTE which gives me a fake table called Stuffing, with one field named Stuff.

I want to select that Stuffing field 65,535 times, butideally, I’d like to return that field with a different name each time. (I might use this to artificially insert 65,535 columns later.) I’ll need a trusty numbers table (like Method #7 from this StackOverflow answer ), which will give me 65,535 rows that I can use to build dynamic SQL:

Including 8, 6, 7, 5, 3, 0, and 9

Now we’ll use those to build dynamic SQL:

Using SQL to Build SQL

And I can just copy/paste that stuff into my SELECT statement and run it:

Hey, Mister DJ, I thought you said we had a deal

Alright, so we’ve learned that I can’t return more than 65,535 columns, AND I can only use 4,096 elements in my SELECT. I can think of several workarounds there 65,535 / 4096 = about 16, which means I could create a few tables or CTEs and do SELECT *’s from them, thereby returning all 65,535 columns with less than 4,096 things in my SELECT. But for now, we’ll just start with 4,096 things in my SELECT:

Lots of Stuffing

Presto! We’ve got stuffing. As long as we’re here, it’s interesting to see how SQL Server clips off queries in execution plans. If wehover themouse over the query in a plan, wecan see the query or at least, the beginning of it:

Hovercraft

Right-click on the plan and clickShow Execution Plan XML, and you can see exactly where SSMS clips it:

Cuts out at n156 (of 4096)

This is why I say you should never comment your code. You want to make sure you can see your entire query in its glorious glory. If you absolutely have to comment your code, avoid vowels at all costs, and type like a 14 year old texter.

Brent Ozar

I make Microsoft SQL Server faster and more reliable. I love teaching, travel, and laughing.