Conversion rules for operations that combine strings

The operations that combine strings are concatenation, UNION, UNION ALL, EXCEPT, and INTERSECT. (These rules also apply to the MAX, MIN, VALUE, COALESCE, IFNULL, and CONCAT scalar functions and CASE expressions.) In each case, the CCSID of the result is determined at bind time, and the execution of the operation may involve conversion of strings to the coded character set identified by that CCSID.

The CCSID of the result is determined by the CCSIDs of the operands. The CCSIDs of the first two operands determine an intermediate result CCSID, this CCSID and the CCSID of the next operand determine a new intermediate result CCSID, and so on. The last intermediate result CCSID and the CCSID of the last operand determine the CCSID of the result string or column. For each pair of CCSIDs, the result CCSID is determined by the sequential application of the following rules:

• If the CCSIDs are equal, the result is that CCSID.
• If either CCSID is 65535, the result is 65535.²⁵
• If one CCSID denotes data in an encoding scheme different from the other CCSID, the result is determined by the following table:

• Otherwise, the resulting CCSID is determined by the following tables:

A variable containing data in a foreign encoding scheme is effectively converted to the native encoding scheme before it is used in any operation. The above rules are based on the assumption that this conversion has already occurred.

Note that an intermediate result is considered to be a derived value operand. For example, assume COLA, COLB, and COLC are columns with CCSIDs 37, 278, and 500, respectively. The result CCSID of COLA CONCAT COLB CONCAT COLC is determined as follows:

1. The result CCSID of COLA CONCAT COLB is first determined to be 37 because both operands are columns, so the CCSID of the first operand is chosen.
2. The result CCSID of "intermediate result" CONCAT COLC is determined to be 500, because the first operand is a derived value and the second operand is a column, so the CCSID of the second operand is chosen.

An operand of concatenation, or the result expression of the CASE expression, or the operands of the IN predicate, or the selected argument of the MAX, MIN, VALUE, COALESCE, IFNULL, or CONCAT scalar function is converted, if necessary, to the coded character set of the result string. Each string of an operand of UNION, UNION ALL, EXCEPT, or INTERSECT is converted, if necessary, to the coded character set of the result column. Character conversion is necessary only if all of the following are true:

• The CCSIDs are different.
• Neither CCSID is 65535.
• The string is neither null nor empty.
• The CCSID Conversion Selection Table (Coded character sets and CCSIDs) indicates that conversion is necessary.

An error occurs if a character of a string cannot be converted or if the CCSID Conversion Selection Table is used but does not contain any information about the CCSID pair. A warning occurs if a character of a string is converted to the substitution character.