EJB query compatibility issues with SQL
Because an EJB query is compiled into structured query language (SQL), be aware of compatibility issues between the Java language and SQL. The two languages differ along the following points that can be critical to correct EJB query formulation:
- The comparison semantics of SQL strings do not exactly match those of the Java language. For example: 'A' (the letter A) and'A ' (the letter A plus a blank space) are considered equal in SQL, but not in the Java language.
- Comparisons and collating order depend on the underlying database. For example, if you are using DB2 with an EBCDIC code page, the collating order is not the same as doing the sort in a Java program. Some databases sort the NULL value low while others sort the NULL value high.
- An arithmetic overflow causes an exception in SQL, but not in the Java language.
- SQL databases have differing minimum and maximum ranges for floating point values, which can differ from floating point value ranges in the Java language. Values near the range limits of Java Double may fail to translate into SQL.
- Java methods do not translate into SQL; therefore standard EJB queries cannot include Java methods.
Only with the dynamic EJB query service can you use functions that do not translate into SQL. Such functions include Java methods and converters or composers that are used in mapping enterprise beans to relational databases (RDBs). A standard finder or select query that uses any of these functions fails at deployment time with the message "Cannot push down query". (You can resolve this problem by changing either the query or the mapping.) The dynamic query run time, however, processes the query by performing the operation involving the function in the appserver.
Related concepts
EJB query language
Rules for data type manipulation in EJB query
Database restrictions for EJB query
Related tasks
Use EJB query