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Showing content from https://neon.com/postgresql/postgresql-tutorial/postgresql-nullif below:

PostgreSQL NULLIF function

Summary: in this tutorial, you will learn how to use the PostgreSQL NULLIF() function to handle null values.

The NULLIF() function is one of the most common conditional expressions provided by PostgreSQL.

Here’s the basic syntax of the NULLIF function:

NULLIF(argument_1,argument_2);

The NULLIF function returns a null value if argument_1 equals to argument_2, otherwise, it returns argument_1.

Let’s take some examples of using the NULLIF() function.

The following statements illustrate how to use the NULLIF() function:

SELECT NULLIF (1, 1); -- return NULL

Output:

nullif
--------
   null
(1 row)

It returns null because the two arguments are equal.

The following example returns the first argument because the two arguments are not equal:

SELECT NULLIF (1, 0); -- return 1

Output:

nullif
--------
      1
(1 row)

The following example uses the NULLIF() function with two unequal text arguments:

SELECT NULLIF ('A', 'B');

Output:

nullif
--------
 A
(1 row)

First, create a table called posts:

CREATE TABLE posts (
  id serial primary key,
  title VARCHAR (255) NOT NULL,
  excerpt VARCHAR (150),
  body TEXT,
  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
  updated_at TIMESTAMP
);

Second, insert some sample data into the posts table.

INSERT INTO posts (title, excerpt, body)
VALUES
      ('test post 1','test post excerpt 1','test post body 1'),
      ('test post 2','','test post body 2'),
      ('test post 3', null ,'test post body 3')
RETURNING *;

Output:

id |    title    |       excerpt       |       body       |         created_at         | updated_at
----+-------------+---------------------+------------------+----------------------------+------------
  1 | test post 1 | test post excerpt 1 | test post body 1 | 2024-02-01 11:28:38.779881 | null
  2 | test post 2 |                     | test post body 2 | 2024-02-01 11:28:38.779881 | null
  3 | test post 3 | null                | test post body 3 | 2024-02-01 11:28:38.779881 | null
(3 rows)

The goal is to retrieve data for displaying them on the post overview page that includes the title and excerpt of each post. To achieve this, you can use the first 40 characters of the post body as the excerpt.

Third, use the COALESCE function to handle NULL in the body column:

SELECT
  id,
  title,
  COALESCE (
    excerpt,
    LEFT(body, 40)
  )
FROM
  posts;

Output:

id |    title    |      coalesce
----+-------------+---------------------
  1 | test post 1 | test post excerpt 1
  2 | test post 2 |
  3 | test post 3 | test post body 3
(3 rows)

Unfortunately, there is a mix between null value and ” (empty) in the excerpt column. To address this issue, you can use the NULLIF function:

SELECT
  id,
  title,
  COALESCE (
    NULLIF (excerpt, ''),
    LEFT (body, 40)
  )
FROM
  posts;

Output:

id |    title    |      coalesce
----+-------------+---------------------
  1 | test post 1 | test post excerpt 1
  2 | test post 2 | test post body 2
  3 | test post 3 | test post body 3
(3 rows)

In this statement:

Another good example of using the NULLIF function is to prevent division-by-zero error.

First, create a new table named members:

CREATE TABLE members (
  id serial PRIMARY KEY,
  first_name VARCHAR (50) NOT NULL,
  last_name VARCHAR (50) NOT NULL,
  gender SMALLINT NOT NULL -- 1: male, 2 female
);

Second, insert some rows for testing:

INSERT INTO members (first_name, last_name, gender)
VALUES
  ('John', 'Doe', 1),
  ('David', 'Dave', 1),
  ('Bush', 'Lily', 2)
RETURNING *;

Output:

id | first_name | last_name | gender
----+------------+-----------+--------
  1 | John       | Doe       |      1
  2 | David      | Dave      |      1
  3 | Bush       | Lily      |      2
(3 rows)

Third, calculate the ratio between male and female members:

SELECT
  (
    SUM (CASE WHEN gender = 1 THEN 1 ELSE 0 END) / SUM (CASE WHEN gender = 2 THEN 1 ELSE 0 END)
  ) * 100 AS "Male/Female ratio"
FROM
  members;

In this example, we use the SUM function and CASE expression to calculate the total number of male members. Then we divide the total of male members by the total of female members to get the ratio. In this case, it returns 200%:

Male/Female ratio
-------------------
               200
(1 row)

Fourth, delete a female member:

DELETE FROM members
WHERE gender = 2;

And execute the query to calculate the male/female ratio again:

SELECT
  (
    SUM (CASE WHEN gender = 1 THEN 1 ELSE 0 END) / SUM (CASE WHEN gender = 2 THEN 1 ELSE 0 END)
  ) * 100 AS "Male/Female ratio"
FROM
  members;

We got the following error message:

ERROR:  division by zero

The reason is that the number of females is zero now. To prevent this division by zero error, you can use the NULLIF function as follows:

SELECT
  (
    SUM (CASE WHEN gender = 1 THEN 1 ELSE 0 END) / NULLIF (
      SUM (CASE WHEN gender = 2 THEN 1 ELSE 0 END),
      0
    )
  ) * 100 AS "Male/Female ratio"
FROM
  members;

Output:

Male/Female ratio
-------------------
              null
(1 row)

The NULLIF function checks if the number of female members is zero, it returns null. The total of male members is divided by NULL will return NULL .


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