Using COALESCE to handle NULL values in PostgreSQL

Question

I have the following query

SELECT  DISTINCT 
     pt.incentive_marketing, 
     pt.incentive_channel, 
     pt.incentive_advertising 
FROM test.pricing pt 
WHERE pt.contract_id = 90000 
group by 1,2,3 
order by pt.incentive_marketing;

The above query returns the o/p as shown in the attached image enter image description here

However I want to replace all null values by 0 using COALESCE Please let me know how this can be achieved in above SELECT query

Now I further modified the query using coalesce as below

SELECT  
     COALESCE( pt.incentive_marketing, '0' ), 
     COALESCE(pt.incentive_channel,'0'), 
     COALESCE( pt.incentive_advertising,'0') 
FROM test.pricing pt 
WHERE pt.contract_id = 90000 
group by 1,2,3

the result of which is as attached in image 2.

I still receive one row with blank values

Have you tried to replace null values with zeroes using COALESCE? What exactly went wrong? — НЛО
Then please try the coalesce() function and report any problem you have with it. Read the manual if you are unsure about how to use it: postgresql.org/docs/current/static/… - "No I haven't tried" isn't the way SO works. — a_horse_with_no_name
Additionally: using distinct and group by doesn't make sense. group by 1,2,3 already makes all columns distinct. So does the distinct operator. — a_horse_with_no_name
You may have empty strings in addition to null values there - these won't be altered by coalesce(), you need to use some "case" statement. Why your numbers are strings anyway? — Arvo

Andrea Ligios Andrea Ligios · Accepted Answer · 2014-12-15T13:56:57

You can use COALESCE in conjunction with NULLIF for a short, efficient solution:

COALESCE( NULLIF(yourField,'') , '0' )

The NULLIF function will return null if yourField is equal to the second value ('' in the example), making the COALESCE function fully working on all cases:

                 QUERY                     |                RESULT 
---------------------------------------------------------------------------------
SELECT COALESCE(NULLIF(null  ,''),'0')     |                 '0'
SELECT COALESCE(NULLIF(''    ,''),'0')     |                 '0'
SELECT COALESCE(NULLIF('foo' ,''),'0')     |                 'foo'

Using COALESCE to handle NULL values in PostgreSQL

2 Answers