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update解析中getFieldValue()方法的具体内容是什么.COMMA参数代表什么？？？
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1. StringBuilder update_string=new StringBuilder();                  
2. Record.Type type=record.getOpt();
3. DataMessage.Record.Field field;
4. StringBuilder SetValue = new StringBuilder();
5. StringBuilder WhereCondition = new StringBuilder();
6. String ConditionStr;
7. boolean hasPk=false;
8. boolean pkMode=false;
9. boolean hasSet=false;
10. 
    
11. if(type.equals(Record.Type.UPDATE)){                        
12.     int i=0;
13.     DataMessage.Record.Field OldField = null;
14.     DataMessage.Record.Field NewField = null;
15.     List<DataMessage.Record.Field> fields = record.getFieldList();
16.     for (; i <fields.size() ; i++) {
17.         if (i % 2 == 0) {
18.             OldField = fields.get(i);
19.             continue;
20.         }
21. 
    
22.     NewField = fields.get(i);
23.     if (field.isPrimary()) {
24.         if (hasPk) {
25.             WhereCondition.append(" and ");
26.         }
27.         //where old value
28.         ConditionStr = getFieldValue(OldField);
29.         if(ConditionStr==null){
    
30.               WhereCondition.append("`"+field.getFieldname().toLowerCase()+"`" + " " + "is null");
31.         }else{
32.                WhereCondition.append("`"+field.getFieldname().toLowerCase()+"`"+" = "+ NewField.getValue());                      
33.          }
34.         hasPk = true;
35.     }
36.     if (hasSet) {
37.         SetValue.append(COMMA);
38.     }
39.       SetValue.append("`"+field.getFieldname().toLowerCase()+"`" + " = " + field.getValue());
40.     String setStr = getFieldValue(field);
41.     hasSet = true;
42.     }
43. 
    
44.     update_string.append("Update "+record.getTablename() +" Set " + SetValue + " Where "+WhereCondition +";");                        
45. }