What is a SQLTag?

A SQLTag, in many ways, is what is simply considered a "tag" in other systems. They are points of data, and may have static values or dynamic values that come from an OPC address, an expression, or a SQL query. They also offer scaling, alarming, and meta information facilities.

SQLTags provide a consistent data model throughout Ignition, and offer the easiest way to get up and running creating status, control, and simple history systems. Despite their low initial learning curve, however, SQLTags offer a great amount of power in system design and configuration. The ability to aggregate tags from a variety of installations in a central SQL database means that you can build widely distributed SCADA systems more easily than ever before, with a high level of performance and relatively easy configuration. SQLTag User Defined Types (UDTs) provide an object-oriented approach to tag building, allowing you to define parameterized data types, extend and override types, and then rapidly generate instances. A change to the type definition is then inherited by all instances, drastically saving time when making routine changes. The UDT data types are fully supported by Vision templates, which means you can configure templates for your custom data types and take advantage of drag-and-drop binding to rapidly build complex screens.

For more information about the benefits of SQLTags, see the SQLTags Overview in the Architecture chapter.

Tag Execution

SQLTags are executed by scan classes inside of a tag provider. In a typical system there will be one or two tag providers (the internal provider, which keeps the tag configuration in the project, and possibly an external tag provider in which tag configuration and values are stored in a database), and a number of scan classes.

SQLTags stored in an external provider will be available to all Ignition installations that have access to that database. One of the installations will be specified as the tag's driver. The driving system will have a copy of the scan class that it executes, which in turn evaluates the tag. The value will be stored to the database, and all of the other installations will be notified of the new value.

For more information about providers, see SQLTags in the gateway configuration section.

What can tags do?

Fundamentally, the primary goal of a tag is to represent a value in the Ignition system. The values can come from OPC, expressions, queries, etc., and can be used on screens, in transaction groups, and more. Additionally, SQLTags provide a core set of features above and beyond simple values:

Depending on the specific type of tag, even more options may be available. In general, SQLTags provide a common interface for tying together many types of data in Ignition.