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Category: T-SQL

TOP And UNIONs

Published 2017-11-27 by Kevin Feasel

Andy Levy points out that the TOP operator doesn’t always apply to each element in a UNION:

This system uses TOP every now and then trying to limit the number of records it gets back (and the developers always seem to use the arbitrary 301 – I’m guessing some degree of cargo cult programming going on). I’d prefer a well-constructed WHERE clause to limit the result set but beggars can’t be choosers when working with legacy vendor code.

What I found odd was that the Ledger1 table didn’t get a lot of traffic – with the WHEREclause in use (omitted here for brevity), you’d only get a handful of records, maybe a dozen at most.

Click through to see the rest of the problem, as well as Andy’s solution.

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Fun With Dynamic SQL: Implicit Casting Can Allow SQL Injection

Published 2017-11-27 by Kevin Feasel

Remus Rusanu shows an example where implicit casting from NVARCHAR to VARCHAR can introduce a SQL injection vulnerability that you otherwise wouldn’t expect:

In both examples above the SQL executed apparently should had been safe from SQL injection, but it isn’t. Neither REPLACE nor QUOTENAME were able to properly protect and the injected division by zero was executed. The problem is the Unicode MODIFIER LETTER APOSTROPHE(NCHAR(0x02bc)) character that I used in constructing the NVARCHAR value, which is then implicitly cast to VARCHAR. This cast is converting the special ‘modifier letter apostrophe’ character to a plain single quote. This introduces new single quotes in the value, after the supposedly safe escaping occurred. The result is plain old SQL Injection.

Click through for the script.  The upside of this is that it’s entirely under your control and you should be able to get it right by using NVARCHAR consistently.

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Using The GROUPING SETS Operator

Published 2017-11-16 by Kevin Feasel

Alfonso Hernandez goes into detail with what you can do with GROUPING SETS:

In T-SQL, you summarize data by using the GROUP BY clause within an aggregate query. This clause creates groupings which are defined by a set of expressions. One row per unique combination of the expressions in the GROUP BY clause is returned, and aggregate functions such as COUNT or SUMmay be used on any columns in the query. However, if you want to group the data by multiple combinations of group by expressions, you may take one of two approaches. The first approach is to create one grouped query per combination of expressions and merge the results using the UNION ALLoperator. The other approach is to use the GROUPING SETS operator along with the GROUP BY clause and define each grouping set within a single query.

In this article I’ll demonstrate how to achieve the same results using each method.

Mastering GROUPING SETS makes reporting queries in T-SQL so much more effective.

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Searching Stored Procedures And Ad Hoc Queries

Published 2017-11-15 by Kevin Feasel

Bert Wagner has a couple queries to help you find references in T-SQL objects, as well as ad hoc statements which are currently in the plan cache:

Have you ever wanted to find something that was referenced in the body of a SQL query?

Maybe you need to know what queries you will have to modify for an upcoming table rename. Or maybe you want to see how many queries on your server are running SELECT *

Below are two templates you can use to search across the text of SQL queries on your server.

Click through for the scripts.  Finding references in T-SQL objects (views, procedures, functions, triggers, etc.) is a fairly straightforward process.  Finding references in ad hoc statements is much more hit-or-miss.

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Grouping Data With ROLLUP

Published 2017-11-09 by Kevin Feasel

Steve Jones shows how easy it is to use the ROLLUP function:

I was editing an article recently that talked about ROLLUP, and I wanted to play with this a bit more. I hadn’t used this much in my career, but it’s a handy operator that’s worth including in your toolkit.

ROLLUP is used to provide additional totals for your aggregates while using GROUP BY. Here’s a little example. I’ve got some data for American Football quaterbacks. In this case, I’ve extracted some stats for a few noteworthy players today.

I’ll probably end up using ROLLUP about once every three months and be very pleased when I remember that it exists.  I use GROUPING SETS more often and almost never use CUBE.

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String Splitting With Multi-Character Delimiters

Published 2017-11-01 by Kevin Feasel

Derek Colley shows a way of using the STRING_SPLIT function when your delimiter is more than one character:

But what happens if we try to use STRING_SPLIT with a multi-character separator, like this?

SELECT * FROM STRING_SPLIT(@GodawfulString, ‘,,’)

Msg 214, Level 16, State 11, Line 67
Procedure expects parameter ‘separator’ of type ‘nchar(1)/nvarchar(1)’.

Ah.  That’s a no, then.

Click through for the solution.

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Getting SQL Server Context Settings

Published 2017-11-01 by Kevin Feasel

Frank Gill has a script to find current Query Store context settings:

The following query will return the hex and integer value for each row in the table (NOTE: Query store must be enabled for the database to return values):

USE YourQueryStoreDatabase;

SELECT set_options, CONVERT(INT, set_options) AS IntSetOptions
FROM sys.query_context_settings;

The set_options value represents a bit mask, with each binary digit representing a specific set option. The full list of values can be found here. I created stored procedure ReturnSetOptions to take the IntSetOptions from the query above and return the set options represented. The code for the procedure is listed below.

Read on to get a script which breaks the bitmask field into human-readable results.

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Finding Objects Relating To A Schema

Published 2017-10-30 by Kevin Feasel

Jason Brimhall has a script to help you find which objects are tied to a particular schema:

I have run into this very issue where there are far too many objects in the schema to be able to drop one by one. Add to the problem that I am looking to do this via script. Due to the need to drop the schema and the (albeit self imposed) requirement of doing it via script, I came up with the following that will cover most cases that I have encountered.

Click through for the script.

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Persisting Computed Columns

Published 2017-10-30 by Kevin Feasel

Greg Low describes persisted computed columns:

Each time the value from that column is queried, the calculation is performed so the result can be returned. This makes sense when the value is changing regularly and the value is queried infrequently.

However, according to my completely subjective statistics, most computed columns are queried much more than they are ever changed. So why work the value out each and every time?

One really nice thing about persisted computed columns is that you can then build non-clustered indexes using these columns.  It’s a great way of pre-computing work that you need to do often but which would violate rules of database normalization.

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Line Continuation In T-SQL

Published 2017-10-30 by Kevin Feasel

Solomon Rutzky shows how line continuation works with SQL Server:

While it is not widely used (at least I have never seen anyone besides myself use it), T-SQL does actually have a line-continuation character: \ (backslash). Placing a backslash at the end of a line within a string literal (or constant as the MSDN documentation refers to it) or binary string will ignore the newline after the backslash. For example:

PRINT N'Same
Line';

displays the following in the “Messages” tab:

Same
Line

But, add in the backslash (well, a space and then a backslash so that it looks right):

PRINT N'Same \
Line';

and now the following is displayed in the “Messages” tab:

Same Line

Read on for more details.

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