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Grammar Root Selection Guide

The guide below describes when it is suitable to select a segment, segment field, segment that repeats, segment group or the entire message as the grammar root for a table. In each case, a segment grammar example, description and example of its use are provided. In addition, the guide also explains when it would be useful to select a specific grammar root.

Grammar Root SelectedExample Segment GrammarDescriptionWhen is this Useful?Example
Segment

MSH PID OBR OBX

In this example PID is selected as the grammar root.

 This means that you cannot map data from any other segment (i.e. MSH, OBR or OBX).

This serves two purposes:

  • Keeps your mappings simple, and
  • You can re-use the same mapsets across different messages in the same VMD file. This means that if you have a message with different segment grammar but with the same PID segment (following the example), you can use the same mapset to map the data.
See Mapping a Table Automatically Using a Segment's Fields and Sub-Fields.
Message

MSH PID OBR OBX

In this example the entire Message is selected as the grammar root.

 This means you can map data from any segment.

This is useful if you want to insert some data from two or more segments into the same table. If your VMD file contains more than one message, it is not recommended that you choose Message as the grammar root.

For example, if you had two messages that each contained the PID segment, but the segment was in a different position for each message, it would not be possible to share mapsets. In such a case, you would choose the segment as the grammar root, instead of the entire message.

Another possibility would be to choose the Message as the grammar root and use different mappings.

See Setting the Maximum Repeat Mapping Limit.
Segment that repeats (also applies to a Segment Field that repeats)

MSH PID OBR {OBX}

In this example the repeating segment, OBX, is selected as the grammar root.

This means each repeat of the segment will be placed in a new row for that table.This is useful if you want to extract the repeating data and place it all in the same table.See Mapping Repeating Fields Using Table Groups.
Segment Group

MSH PID [{PV1}{INI}]

In this example the selected grammar root is the Segment Group itself.

This means you can map data from any segment inside the group. In this case, only the PV1 and INI segments are inside the segment group.

Note: You cannot map a repeating segment in the group without creating a table group whose grammar root is that segment group and a sub-table whose grammar root is the repeating segment. In the example, you could only map from the first (or first few) instances of PV1 or INI segments.

You would typically select the segment group to be the grammar root when the group itself repeats. When a segment or field repeats inside of that group, you need to create a table group.See Advanced: Repeated Segment Groups.
Segment with Python Considerations

MSH PID OBR {OBX}

In this example OBR is selected as the grammar root.

For this example, more context must be provided. The goal is to create one row for each OBR segment. The data from each repeat of the OBX segment will be inserted into the OBR segment row. This is useful when you want to combine fields in multiple segments of an incoming message into a single table row. This can be accomplished by using a global Python string variable and Python's string concatenation functionality.See Mapping Multiple Fields of an Incoming HL7 Message to a Single Table Column.
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