# 如何将DWS联结查询的null结果替换成0？
在执行outer join（left join、right join、full join）联结查询时，outer join在匹配失败的情况下结果集会补空，产生大量NULL值， 可以在联结查询时将这部分null值替换为0。
可使用coalesce函数，它的作用是返回参数列表中第一个非NULL的参数值。例如：
```
SELECT coalesce(NULL,'hello');
 coalesce
----------
 hello
(1 row)
```
有表course1和表course2，使用left join对两表进行联结查询：
```
SELECT * FROM course1;
  stu_id  |  stu_name  |     cour_name
----------+------------+--------------------
 20110103 | ALLEN      | Math
 20110102 | JACK       | Programming Design
 20110101 | MAX        | Science
(3 rows)
SELECT * FROM course2;
 cour_id |     cour_name      | teacher_name
---------+--------------------+--------------
    1002 | Programming Design | Mark
    1001 | Science            | Anne
(2 rows)
SELECT course1.stu_name,course2.cour_id,course2.cour_name,course2.teacher_name FROM course1 LEFT JOIN course2 ON course1.cour_name = course2.cour_name ORDER BY 1;
  stu_name  | cour_id |     cour_name      | teacher_name
------------+---------+--------------------+--------------
 ALLEN      |         |                    |
 JACK       |    1002 | Programming Design | Mark
 MAX        |    1001 | Science            | Anne
(3 rows)
```
使用coalesce函数将查询结果中的空值替换为0或其他非0值：
```
SELECT course1.stu_name,
 coalesce(course2.cour_id,0) AS cour_id,
 coalesce(course2.cour_name,'NA') AS cour_name,
 coalesce(course2.teacher_name,'NA') AS teacher_name
 FROM course1
 LEFT JOIN course2 ON course1.cour_name = course2.cour_name
 ORDER BY 1;
  stu_name  | cour_id |     cour_name      | teacher_name
------------+---------+--------------------+--------------
 ALLEN      |       0 | NA                 | NA
 JACK       |    1002 | Programming Design | Mark
 MAX        |    1001 | Science            | Anne
(3 rows)
```