Sql Server Filtered Index as a Unique Constraint on a Specific Value

Indexes often double as constraints. Each combination of values must only appear once among the columns of an index defined as UNIQUE. The “filtered” index allows some refinement. Suppose that in one column you won’t allow a specific value to appear more than once, but you don’t mind other values being duplicated.

A good example would be the IS_LATEST_VERSION column in a data warehouse table. Continue reading

Sql JOIN Cheat Sheet

Someone accidentally found my Sql Collation Cheat Sheet while searching for a “Sql JOIN Cheat Sheet” so I thought I’d knock out a post on the topic. Towards the end, you’ll find a script that demonstrates them. The script was run in Azure Sql. Some joins may be Microsoft extensions to the ANSI standard. Look at the documentation for fuller information.

When I was a beginner I had thought there was a deep meaning to the words “left” and “right”, as in LEFT OUTER JOIN. It was a mild let-down to find out that it only refers to placement of the table name in relation to the “JOIN” keyword; switching a LEFT OUTER to a RIGHT OUTER join defines the table named after the join keyword as the outer table. Continue reading

Sql Server CONTEXT_INFO with Numeric Values

A trigger takes no arguments but it is possible to use CONTEXT_INFO() to make 128 bytes of binary data values accessible to it. I wanted to provide an 8-byte (bigint) value from a sequence which the trigger would write to a primary key column.

Storing and retrieving a numeric value in CONTEXT_INFO is not difficult, but it’s another thing that doesn’t come up very often. For this type of problem I like to find concise explanations with source code so that I can get back to the main problem quickly. Once again my searches didn’t turn anything up so I’m trying to fill the gap with this post. Continue reading

Sql Server Identity and Membership Functions

On my current research project I’m looking at security. The number of different functions available (some of which are obsolescent) has always been an irritation, so I decided to create a Transact-Sql query that demonstrates the functions and provides links to the documentation. You can use EXECUTE AS LOGIN or EXECUTE AS USER to explore what’s returned for other security principals, always assuming you have the necessary permissions of course. Hope you find it useful. Continue reading

Agile Continuous Delivery of Databases (3)

A phase of my Agile Blue/Green deployment project is now complete, as I have a Proof of Concept which has been tested successfully. As you may recall from prior posts, the Blue/Green technique features two databases, the “green” live one that users connect to (I’ve made an arbitrary assignment of colours), and the offline “blue” one which has a different version of the schema (a previous or future release). Continue reading

Cost-Based (Design) Optimisation

No, not the Oracle Cost-Based Optimizer – this is about optimising your design for (financial) cost; and no, it doesn’t mean that we as developers need to be worrying about pennies while we’re coding, or even that we have much influence on costs.

It’s just an interesting remark from a podcast by Brent Ozar here. At 20 minutes 14 seconds in, Continue reading

Agile Continuous Delivery of Databases (2)

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Most applications include a database, and the goal of the project I’m working on is to make Agile continuous delivery possible in the database as well as in procedural code. The peculiar problem with databases is that when a change deployment fails we risk losing data that has been added since the release.

The “blue-green” deployment model is a way of dealing with this. We cut over to an “old version” database. The code and schema of this database pre-date the release, but it has received all the data changes since the release.

How can this be? Continue reading