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copied!<p>The fields used in the ORDER BY clause must be CMP fields, they cannot be entity identifiers or CMR fields. In addition, you must be careful which CMP fields you specify in the ORDER BY clause. If the query selects a collection of entities, the ORDER BY clause can be used only with CMP fields of the entity type selected. For example the following query is illegal, because the CMP field used in the ORDER BY clause is not a field of the entity type selected.</p> <p>SELECT OBJECT( c ) FROM Customer AS c ORDER BY c.address.city</p> <p>Because the city CMP field is not a direct CMP field of the Customer EJB, you cannot use it in the ORDER BY clause. The only CMP fields you can use in the ORDER BY clause are those that are direct CMP fields of the entity type being selected. This is an unreasonable restriction.</p> <p>A similar restriction applies to CMP results. The CMP field used in the ORDER BY clause must be the same as the CMP field identified in the SELECT clause. For example, the following query is illegal, because the CMP identified in the SELECT clause differs from the one used in the ORDER BY clause.</p> <p>SELECT c.address.city FROM Customer AS c ORDER BY c.address.state</p> <p>In the above query, we wanted a list of all the cities, ordered by state. Unfortunately, this is illegal. You can't order by the state CMP field if you are selecting using the city CMP field.</p> <p>I've heard that this second restriction may have been forced on us by the limitations of one of the major RDBS systems, which cannot order by any columns not present in the select clause - remember that EJB QL is compiled into a native query language, which in RDBS is SQL. The limitations of major vendors will be a factor in creating any abstraction - the least common denominator will prevail.</p>

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