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Updated on 2023-03-08 GMT+08:00
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How Do I Replace the Null Result with 0?

When OUTER JOIN (LEFT JOIN, RIGHT JOIN, and FULL JOIN) is executed, the match failure in the outer join generates a large number of NULL values. You can replace these null values with 0.

You can use the COALESCE function to do that. This function returns the first non-null parameter value in the parameter list. For example:

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SELECT coalesce(NULL,'hello');
 coalesce
----------
 hello
(1 row)

Use left join to join the tables course1 and course2.

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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)

Use the COALESCE function to replace null values in the query result with 0 or other non-zero values:

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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)
Parent topic: Database Usage