避免在 PostgreSQL 中被零除

Question

I'd like to perform division in a SELECT clause. When I join some tables and use aggregate function I often have either null or zero values as the dividers. As for now I only come up with this method of avoiding the division by zero and null values.

(CASE(COALESCE(COUNT(column_name),1)) WHEN 0 THEN 1
ELSE (COALESCE(COUNT(column_name),1)) END) 

I wonder if there is a better way of doing this?

Answer 1

You can use NULLIF function eg

something/NULLIF(column_name,0)

If the value of column_name is 0 - result of entire expression will be NULL

Answer 2

Since count() never returns NULL (unlike other aggregate functions), you only have to catch the 0 case (which is the only problematic case anyway). So, your query simplified:

CASE count(column_name)
   WHEN 0 THEN 1
   ELSE count(column_name)
END

Or simpler, yet, with NULLIF() , like Yuriy provided .

Quoting the manual about aggregate functions:

It should be noted that except for count , these functions return a null value when no rows are selected.

Answer 3

I realize this is an old question, but another solution would be to make use of the greatest function:

greatest( count(column_name), 1 )  -- NULL and 0 are valid argument values

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Note: My preference would be to either return a NULL, as in Erwin and Yuriy's answer, or to solve this logically by detecting the value is 0 before the division operation, and returning 0 . Otherwise, the data may be misrepresented by using 1 .

Answer 4

避免除以零的另一种解决方案，替换为1

select column + (column = 0)::integer;

Answer 5

If you want the divider to be 1 when the count is zero:

count(column_name) + 1 * (count(column_name) = 0)::integer

The cast from true to integer is 1.