7. Sorting
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## 7. Sorting

This chapter describes sorting and how it is supported in YAZ. Sorting applies to a result-set. The Z39.50 sorting facility takes one or more input result-sets and one result-set as output. The most simple case is that the input-set is the same as the output-set.

Z39.50 sorting has a separate APDU (service) that is, thus, performed following a search (two phases).

In SRU/Solr, however, the model is different. Here, sorting is specified during the search operation. Note, however, that SRU might perform sort as separate search, by referring to an existing result-set in the query (result-set reference).

### 7.1. Using the Z39.50 sort service

yaz-client and the ZOOM API support the Z39.50 sort facility. In any case the sort sequence or sort criteria is using a string notation. This notation is a one-line notation suitable for being manually entered or generated, and allows for easy logging (one liner). For the ZOOM API, the sort is specified in the call to ZOOM_query_sortby function. For yaz-client the sort is performed and specified using the sort and sort+ commands. For description of the sort criteria notation refer to the sort command in the yaz-client manual.

The ZOOM API might choose one of several sort strategies for sorting. Refer to Table 3.2, “ZOOM sort strategy”.

### 7.2. Type-7 sort

Type-7 sort is an extension to the Bib-1 based RPN query where the sort specification is embedded as an Attribute-Plus-Term.

The objectives for introducing Type-7 sorting is that it allows a client to perform sorting even if it does not implement/support Z39.50 sort. Virtually all Z39.50 client software supports RPN queries. It also may improve performance because the sort criteria is specified along with the search query.

The sort is triggered by the presence of type 7, and the value of type 7 specifies the sortRelation . The value for type 7 is 1 for ascending and 2 for descending. For the sortElement only the generic part is handled. If generic sortKey is of type sortField, then attribute type 1 is present and the value is sortField (InternationalString). If generic sortKey is of type sortAttributes, then the attributes in the list are used. Generic sortKey of type elementSpec is not supported.

The term in the sorting Attribute-Plus-Term combo should hold an integer. The value is 0 for primary sorting criteria, 1 for second criteria, etc.

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