Voice of the DBA

Tag: T-SQL

  • I learned about the order of logical operations #SQLNewBlogger

    Another post for me that is simple and hopefully serves as an example for people trying to get blogging as #SQLNewBloggers.

    I had logic in a CS curriculum many years ago and I’ve worked with AND and OR statements for years. I’ve sometimes confused myself, but I usually ensure I have parenthesis included to clarify the code. Not just for me, but for anyone that might glance at the code later.

    As a side note, I also try to format code so a quick glance can reveal what happens.

    However, I learned something new this week. I saw a question about the order of logical operations in this form: a or b and c.

    I had somewhat assumed, like math, we’d use a left to right evaluation. However, that’s not correct. Look at this snippet:

    2018-03-22 10_06_22-SQLQuery1.sql - (local)_SQL2014.SimpleTalk_1_Development (PLATO_Steve (57))_ - M

    If we went left to write, we’d have two rows from the OR (n=1, n=2) and then an AND that produces no rows. So no results?

    That’s not correct. According to BOL for OR,  the AND operations occur first. So n=2 AND n > 3 occurs, with 0 rows. Then the OR with n=1 is evaluated to return 1 row.

    Fascinating.

    At least to me. I’ve never thought because I’d write

    WHERE (n = 1 OR n = 2) AND n > 3

    or

    WHERE n = 1 OR (n = 2 AND n > 3)

    and be sure that what I wanted to occur would occur.

    A quick lesson. While it’s good to know what the order or evaluation is for your platform, don’t count on this. If there is a chance for confusion or unintended consequences, use parenthesis. It’s simpler and easier, and I might argue, more elegant.

  • Enabling Guest in a Database–#SQLNewBlogger

    Another post for me that is simple and hopefully serves as an example for people trying to get blogging as #SQLNewBloggers.

    The guest account exists in all your databases. This is installed by default, and guest is used to map a login that doesn’t otherwise have access to a database.

    Sound scary?

    It should. This would be bad if any login could connect to any database, potentially reading data using the guest account. Fortunately Microsoft has done two things. First, guest is disabled in all user databases. This is because it’s disabled in model, which is our template.

    2018-02-08 08_51_24-SQLQuery8.sql - (local)_SQL2016.AdventureWorks2014 (PLATO_Steve (74))_ - Microso

    Second, guest is typically assigned no rights. It’s a member of the public role, which also has no rights by default.

    Enabling Guest

    If you want to allow anonymous access for logins through the guest account, it’s easy. Be wary and careful of doing this and be sure you understand what rights have been granted to public if you do this. In general, I’d expect auditors and any compliance/security officers to be against this, but you should check.

    The user exists already, and just needs the CONNECT permission to get enabled. You can do this with this code:

    GRANT CONNECT TO guest

    If you want to remove permission, use

    REVOKE CONNECT FROM guest

    That’s it. Remember, by default this user can’t access any objects. I would recommend you not grant rights to guest, but use roles. Either one of the built in ones, or better yet, create your own role and choose limited permissions.

    SQLNewBlogger

    One of the ways you can showcase your knowledge, show you’re learning, and show you’re motivated to enhance your career is blogging. This post is an example of what you could write, in your own words, about something you’ve learned.

    This one took my about 5 minutes after I’d spent a little time getting guest enabled for a test project.

  • Getting Your SET Options

    Have you ever used @@options? I haven’t typically needed this, but there are times that you might wonder what options are set for a session. You can check in SSMS, look for defaults, etc., but there’s an easy way.

    SELECT @@OPTIONS

    This uses the @@options function, which contains a bitmap of your session settings. There are defaults with the sp_configure user options, but these can be overridden.

    If you want to get the options, which are also stored as bitmaps in various places, you can code them with this BOL article.

    Or use a script like this:

    DECLARE @i INT;
SELECT @i = @@OPTIONS;
SELECT ConstraintChecking = CASE WHEN (@i & 1 = 1 ) THEN 'ON' ELSE 'OFF' end,
        ImplicitTransactions = CASE WHEN (@i & 2 = 2 ) THEN 'ON' ELSE 'OFF' end,
        CursorCloseonCommit = CASE WHEN (@i & 4 = 2 ) THEN 'ON' ELSE 'OFF' end,
        AnsiWarnings = CASE WHEN (@i & 8 = 8 ) THEN 'ON' ELSE 'OFF' end,
        AnsiPadding = CASE WHEN (@i & 16 = 16 ) THEN 'ON' ELSE 'OFF' end,
        AnsiNulls = CASE WHEN (@i & 32 = 32 ) THEN 'ON' ELSE 'OFF' end,
        ArithAbortSetting = CASE WHEN (@i & 64 = 64 ) THEN 'ON' ELSE 'OFF' end,
        ArithIgnoreSetting = CASE WHEN (@i & 128 = 128 ) THEN 'ON' ELSE 'OFF' end,
        QuotedIdentifer = CASE WHEN (@i & 256 = 256 ) THEN 'ON' ELSE 'OFF' end,
        NoCount = CASE WHEN (@i & 512 = 512 ) THEN 'ON' ELSE 'OFF' end,
        AnsiNullDefaultOn = CASE WHEN (@i & 1024 = 1024 ) THEN 'ON' ELSE 'OFF' end,
        AnsiNullDefaultOff = CASE WHEN (@i & 2048 = 2048 ) THEN 'ON' ELSE 'OFF' end,
        ConcatNullYieldsNull = CASE WHEN (@i & 4096 = 4096 ) THEN 'ON' ELSE 'OFF' end,
        NumericRoundAbort = CASE WHEN (@i & 8192 = 8192 ) THEN 'ON' ELSE 'OFF' end,
        XactAbort = CASE WHEN (@i & 16384 = 16384 ) THEN 'ON' ELSE 'OFF' END

    For the row-based people, how about this:

    DECLARE @i INT;
SELECT @i = @@OPTIONS;
SELECT 'ConstraintChecking', CASE WHEN (@i & 1 = 1 ) THEN 'ON' ELSE 'OFF' END
UNION 
SELECT  'ImplicitTransactions', CASE WHEN (@i & 2 = 2 ) THEN 'ON' ELSE 'OFF' end
UNION 
SELECT  'CursorCloseonCommit', CASE WHEN (@i & 4 = 4 ) THEN 'ON' ELSE 'OFF' end
UNION 
SELECT  'AnsiWarnings', CASE WHEN (@i & 8 = 8 ) THEN 'ON' ELSE 'OFF' end
UNION 
SELECT  'AnsiPadding', CASE WHEN (@i & 16 = 16 ) THEN 'ON' ELSE 'OFF' end
UNION 
SELECT  'AnsiNulls', CASE WHEN (@i & 32 = 32 ) THEN 'ON' ELSE 'OFF' end
UNION 
SELECT  'ArithAbortSetting', CASE WHEN (@i & 64 = 64 ) THEN 'ON' ELSE 'OFF' end
UNION 
SELECT  'ArithIgnoreSetting', CASE WHEN (@i & 128 = 128 ) THEN 'ON' ELSE 'OFF' end
UNION 
SELECT  'QuotedIdentifer', CASE WHEN (@i & 256 = 256 ) THEN 'ON' ELSE 'OFF' end
UNION 
SELECT  'NoCount', CASE WHEN (@i & 512 = 512 ) THEN 'ON' ELSE 'OFF' end
UNION 
SELECT  'AnsiNullDefaultOn', CASE WHEN (@i & 1024 = 1024 ) THEN 'ON' ELSE 'OFF' end
UNION 
SELECT  'AnsiNullDefaultOff', CASE WHEN (@i & 2048 = 2048 ) THEN 'ON' ELSE 'OFF' end
UNION 
SELECT  'ConcatNullYieldsNull', CASE WHEN (@i & 4096 = 4096 ) THEN 'ON' ELSE 'OFF' end
UNION 
SELECT  'NumericRoundAbort', CASE WHEN (@i & 8192 = 8192 ) THEN 'ON' ELSE 'OFF' end
UNION 
SELECT  'XactAbort', CASE WHEN (@i & 16384 = 16384 ) THEN 'ON' ELSE 'OFF' END

    UPDATE: From the comments, an UNPIVOT might be easier to read for some of you.

    DECLARE @i INT;
SELECT @i = @@OPTIONS;
SELECT [Option], [Value]
FROM(
SELECT ConstraintChecking = CASE WHEN (@i & 1 = 1 ) THEN ‘ON’ ELSE ‘OFF’ end,
ImplicitTransactions = CASE WHEN (@i & 2 = 2 ) THEN ‘ON’ ELSE ‘OFF’ end,
CursorCloseonCommit = CASE WHEN (@i & 4 = 2 ) THEN ‘ON’ ELSE ‘OFF’ end,
AnsiWarnings = CASE WHEN (@i & 8 = 8 ) THEN ‘ON’ ELSE ‘OFF’ end,
AnsiPadding = CASE WHEN (@i & 16 = 16 ) THEN ‘ON’ ELSE ‘OFF’ end,
AnsiNulls = CASE WHEN (@i & 32 = 32 ) THEN ‘ON’ ELSE ‘OFF’ end,
ArithAbortSetting = CASE WHEN (@i & 64 = 64 ) THEN ‘ON’ ELSE ‘OFF’ end,
ArithIgnoreSetting = CASE WHEN (@i & 128 = 128 ) THEN ‘ON’ ELSE ‘OFF’ end,
QuotedIdentifer = CASE WHEN (@i & 256 = 256 ) THEN ‘ON’ ELSE ‘OFF’ end,
NoCount = CASE WHEN (@i & 512 = 512 ) THEN ‘ON’ ELSE ‘OFF’ end,
AnsiNullDefaultOn = CASE WHEN (@i & 1024 = 1024 ) THEN ‘ON’ ELSE ‘OFF’ end,
AnsiNullDefaultOff = CASE WHEN (@i & 2048 = 2048 ) THEN ‘ON’ ELSE ‘OFF’ end,
ConcatNullYieldsNull = CASE WHEN (@i & 4096 = 4096 ) THEN ‘ON’ ELSE ‘OFF’ end,
NumericRoundAbort = CASE WHEN (@i & 8192 = 8192 ) THEN ‘ON’ ELSE ‘OFF’ end,
XactAbort = CASE WHEN (@i & 16384 = 16384 ) THEN ‘ON’ ELSE ‘OFF’ END) AS options
UNPIVOT (
[Value] FOR [OPTION] IN ([ConstraintChecking],[ImplicitTransactions],[CursorCloseonCommit],[AnsiWarnings],[AnsiPadding],[AnsiNulls],[ArithAbortSetting],[ArithIgnoreSetting],[QuotedIdentifer],[NoCount],[AnsiNullDefaultOn],[AnsiNullDefaultOff],[ConcatNullYieldsNull],[NumericRoundAbort],[XactAbort])
) AS T1
ORDER BY [Option]
GO

  • No Default CONTEXT_INFO()

    I was doing a little testing of Row-Level Security (RLS) for the stairway, and one of the ways that some people implement RLS is with CONTEXT_INFO().

    I haven’t every really used CONTEXT_INFO in production, though it’s been around for some time. This is a way of setting some session information, as this data is stored for the connection. However, since it can be reset by the connection, it’s value isn’t necessarily trustworthy from a system perspective.

    If you don’t set this with SET CONTEXT_INFO, then a NULL is stored there for on-premises systems. This makes sense, there’s no initialization there.

    In Azure SQL Database, however, you get a GUID that’s a unique value. That’s good to know, as if you’re checking if this is NULL, you might assume you have something stored there, and since you need to CAST this back to the original datatype, this might cause issues.

    This could be a good way to store data for a single session, but beware. If the session drops and reconnects, you’ll lose your data.