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The QID segment contains the information necessary to uniquely identify a query. Its primary use is in query cancellation or subscription cancellation.

HL7 Attribute Table - QID - query identification segment

Seq#	DataElement	Description	Must Implement	Flags	Cardinality	Length	C.LEN	Vocabulary	DataType
QID
1	00696	Query Tag	SHALL	=	[1..1]		32		ST
2	01375	Message Query Name	SHALL		[1..1]			0471	CWE

(Definition from QAK.1 in Ch. 5)

Definition: This field may be valued by the initiating system to identify the query, and may be used to match response messages to the originating query. If it is valued, the responding system is required to echo it back as the first field in the query acknowledgment segment (QAK). This field differs from MSA-2-message control ID in that its value remains constant for each message (i.e., all continuation messages) associated with the query, whereas MSA-2-Message control ID may vary with each continuation message, since it is associated with each individual message, not the query as a whole. QAK-1-Query tag is not conditional on the presence of the QRD-1-Query ID field in the original mode queries; in the original mode queries QAK-1-Query tag is not used.

(Definition from QID.1 in Ch. 5)

Definition: This field identifies the instance of a query.

(Definition from QPD.2 in Ch. 5)

Definition: This field may be valued by the initiating system to identify the query, and may be used to match response messages to the originating query. If this field is valued, the responding system is required to echo it back as the first field in the query acknowledgement segment (QAK).

This field differs from MSA-2-Message control ID in that its value remains constant for each message (i.e., all continuation messages) associated with the query, whereas MSA-2-Message control ID may vary with each continuation message, since it is associated with each individual message, not the query as a whole.

Implementation considerations: It is not necessary to value this field in implementations where the only return message on the socket will be the response to the query that was just sent. Conversely, in an "asynchronous" implementation where many queries, responses, and other messages may be communicated bidirectionally over the same socket, it is essential that this field be valued so that the Client knows to which query the Server is responding.

(Definition from QAK.3 in Ch. 5)

Definition: This field contains the name of the query. These names are assigned by the function-specific chapters of this specification. Site-specific event replay query names begin with the letter "Z." Refer to User defined table 0471 – Query name in Chapter 2C, Code Tables, for suggested values.

(Definition from QID.2 in Ch. 5)

Definition: This field contains the name of the query. These names are assigned by the function-specific chapters of this specification. Site-specific query names begin with the letter "Z." Refer to User defined table 0471 – Query name for suggested values.

(Definition from QPD.1 in Ch. 5)

Definition: This field contains the name of the query. These names are assigned by the function-specific chapters of this specification. It is one to one with the Query Profile for this query name, and it is in fact an identifier for that Query Profile. Site-specific query names begin with the letter "Z." Refer to User defined table 0471 – Query name in Chapter 2C, Code Tables, for suggested values.