无法启用约束。一行或多行包含违反非空，唯一或外键约束的值

我在 informix 数据库中建立外部连接并成功执行，但是我的代码中出现以下异常：

I make an outer join and executed successfully in the informix database but I get the following exception in my code:

DataTable dt = TeachingLoadDAL.GetCoursesWithEvalState(i, bat);

无法启用约束。一行或多行包含违反非空，唯一或外键约束的值
。

Failed to enable constraints. One or more rows contain values violating non-null, unique, or foreign-key constraints.

我知道问题，但我不知道如何解决它。

I know the problem, but I don't know how to fix it.

我做外连接的第二个表包含复合主键，在前面的外连接查询。

The second table I make the outer join on contains a composite primary key which are null in the previous outer join query.

编辑：

    SELECT UNIQUE a.crs_e,  a.crs_e  || '/ ' || a.crst crs_name, b.period,
           b.crscls, c.crsday, c.from_lect, c.to_lect,
           c.to_lect - c.from_lect + 1 Subtraction, c.lect_kind, e.eval, e.batch_no,
           e.crsnum, e.lect_code, e.prof_course
    FROM rlm1course a, rfc14crsgrp b, ckj1table c, mnltablelectev d,
         OUTER(cc1assiscrseval e)  
    WHERE a.crsnum = b.crsnum 
    AND b.crsnum = c.crsnum 
    AND b.crscls = c.crscls 
    AND b.batch_no = c.batch_no 
    AND c.serial_key = d.serial_key  
    AND c.crsnum = e.crsnum  
    AND c.batch_no = e.batch_no  
    AND d.lect_code= e.lect_code 
    AND d.lect_code = .... 
    AND b.batch_no = ....

发生与表 cc1assiscrseval 。主键是（batch_no，crsnum，lect_code）。

The problem happens with the table cc1assiscrseval. The primary key is (batch_no, crsnum, lect_code).

如何解决这个问题？

编辑：

根据 @PaulStock 建议：
我做他说的，我得到：

According to @PaulStock advice: I do what he said, and i get:

？ dt.GetErrors（）[0] {System.Data.DataRow} HasErrors：true ItemArray：
{object [10]} RowError：列'eval'不允许DBNull.Value。

? dt.GetErrors()[0] {System.Data.DataRow} HasErrors: true ItemArray: {object[10]} RowError: "Column 'eval' does not allow DBNull.Value."

所以我通过将 e.eval 替换为 NVL来解决我的问题（e.eval，''）eval 。这解决了我的问题。

So I solve my problem by replacing e.eval to ,NVL (e.eval,'') eval.and this solves my problem. Thanks a lot.