MinOccurs

MinOccurs is an XML Schema constraint that specifies the minimum number of times an element or particle can occur in an instance document. It applies to element declarations as well as to model groups such as sequences, choices, and alls. The default value is 1, meaning the item must appear at least once unless stated otherwise.

Values for minOccurs must be non-negative integers. A value of 0 makes the element optional, allowing it to be absent in the instance document. minOccurs is used together with maxOccurs, which may be a non-negative integer or the special value unbounded. The relationship is that the actual number of occurrences must fall within the range defined by minOccurs and maxOccurs. If maxOccurs is finite, minOccurs must not exceed it; if maxOccurs is unbounded, there is no upper limit beyond the lower bound.

Examples help illustrate typical usage. A declaration such as <xs:element name="phone" type="xs:string" minOccurs="0" maxOccurs="3"/> allows zero, one, two, or three phone elements. Another example, <xs:element name="author" type="xs:string" minOccurs="2" maxOccurs="unbounded"/>, requires at least two author elements, with no upper limit on repetitions.

In practice, minOccurs interacts with the surrounding compositor (sequence, choice, or all) and the containing complex type to enforce the occurrence constraints during document validation. It is a fundamental tool for expressing optionality and repetition in XML Schemas. See also XML Schema, maxOccurs, and occurrence constraints.