Added api/classifications endpoint, requiring a new query

internal/store/queries.go

@@ -75,7 +75,9 @@ SELECT t.* FROM opportunities t WHERE t.uuid = ?`
 
 // ListOpportunities is assembled by listOpportunitiesQuery as:
 //
-//	listOpportunitiesSelect [WHERE filter AND filter ...] listOpportunitiesOrderAndPage
+//	listOpportunitiesSelect
+//	[WHERE filter AND filter ...]
+//	listOpportunitiesOrderAndPage
 //
 // Each filterOpportunity* fragment takes exactly one "?" argument, except
 // filterOpportunityMark* fragments, which take none. A mark fragment is true
@@ -213,6 +215,17 @@ INSERT INTO opportunity_classifications (opportunity_uuid, classification_id)
 VALUES (?, ?)
 ON CONFLICT DO NOTHING`
 
+// selectClassificationsInUse reads every classification code that at least
+// one opportunity has, once each. c.* must map entirely onto
+// types.ClassificationCode.
+const selectClassificationsInUse = `
+SELECT c.* FROM classification_codes c
+WHERE EXISTS (
+	SELECT 1 FROM opportunity_classifications tc
+	WHERE tc.classification_id = c.id
+)
+ORDER BY c.code, c.id`
+
 // selectClassificationsForOpportunities reads the classifications of several
 // opportunities. "IN (?)" is expanded by sqlx.In, which must be followed by
 // Rebind. c.* must map entirely onto types.ClassificationCode.