Only the values of like types are permitted to be compared. A type is like another type if they correspond to the same Java language type, or if one is a primitive Java language type and the other is the wrapped Java class type equivalent (e.g., int and Integer are like types in this sense).

There is one exception to this rule: it is valid to compare numeric values for which the rules of numeric promotion apply. Conditional expressions attempting to compare non-like type values are disallowed except for this numeric case.

Note that the arithmetic operators and comparison operators are permitted to be applied to state-fields and input parameters of the wrapped Java class equivalents to the primitive numeric Java types. Two entities of the same abstract schema type are equal if and only if they have the same primary key value. Only equality/inequality comparisons over enumeration constants are required to be supported.

JPA 1.0 - BNF:

comparison_expression ::= string_expression comparison_operator {string_expression | all_or_any_expression} |
                         boolean_expression {=|<>} {boolean_expression | all_or_any_expression} |
                         enum_expression {=|<>} {enum_expression | all_or_any_expression} |
                         datetime_expression comparison_operator {datetime_expression | all_or_any_expression} |
                         entity_expression {=|<>} {entity_expression | all_or_any_expression} |
                         arithmetic_expression comparison_operator {arithmetic_expression | all_or_any_expression}

JPA 2.0 - BNF:

comparison_expression ::= string_expression comparison_operator {string_expression | all_or_any_expression} |
                         boolean_expression {=|<>} {boolean_expression | all_or_any_expression} |
                         enum_expression {=|<>} {enum_expression | all_or_any_expression} |
                         datetime_expression comparison_operator {datetime_expression | all_or_any_expression} |
                         entity_expression {=|<>} {entity_expression | all_or_any_expression} |
                         arithmetic_expression comparison_operator {arithmetic_expression | all_or_any_expression} |
                         entity_type_expression {=|<>} entity_type_expression}