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The typical operation is to look up a certain metadata value for a DSpace object with a given ID (UUID in DSpace 5+). First, we find the metadata field in the metadata registry. Say, we're looking for the metadata value (the list of authors) of an item (resource_type_id = 2) with a given UUID (dspace_object_id='f2cf9909-0357-4037-8305-4a426bf9a826') where the metadata field is "dc.contributor.author". First, we look up the ID of the "dc" schema in the metadataschemaregistry table:

SELECT metadata_schema_id
FROM metadataschemaregistry
WHERE short_id = 'dc';

This gives us metadata_schema_id of "1". Next, we look up the metadata field "dc.contributor.author" in the metadatafieldregistry table. We use the metadata_schema_id of the "dc" schema we just found, we query for element = 'contributor' and qualifier = 'author':

SELECT metadata_field_id
FROM metadatafieldregistry
WHERE metadata_schema_id = 1
AND element = 'contributor'
AND qualifier = 'author';

This gives us the metadata_field_id value of "9". Finally, we can use this value to ask for the metadata field containing authors in the metadatavalue table:

SELECT text_value
FROM metadatavalue
WHERE metadata_field_id = '9'
AND dspace_object_id='f2cf9909-0357-4037-8305-4a426bf9a826';

The same, written as a single query:

SELECT text_value
FROM metadatavalue
WHERE metadata_field_id = (
  SELECT metadata_field_id
  FROM metadatafieldregistry, metadataschemaregistry
  WHERE metadatafieldregistry.metadata_schema_id = metadataschemaregistry.metadata_schema_id
  AND short_id = 'dc'
  AND element = 'contributor'
  AND qualifier = 'author'
)
AND dspace_object_id='f2cf9909-0357-4037-8305-4a426bf9a826';

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If you use these queries often and in an ad-hoc manner, you may find it helpful to use these snippets of SQL as functions. You can find the definitions here: Helper SQL functions for DSpace 6

SELECT text_value
FROM metadatavalue
WHERE metadata_field_id = ds6_metadata_field2id('contributor', 'author')
AND dspace_object_id='f2cf9909-0357-4037-8305-4a426bf9a826';

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