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SQLite - Avoiding N...
 
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SQLite - Avoiding NULL returns (COALESCO)  

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Hans
 Hans
(@hans)
Noble Member Admin
Joined: 7 years ago
Posts: 1266
January 30, 2020 12:22 AM  

Sometimes a SELECT returns NULL values, for example when no records were found, for example in something like this:

SELECT  (SELECT someid FROM sometable WHERE somevalue=something) ...

The sub query COULD return a NULL value, and when trying to read this in an application that tries to convert this NULL to a number (integer for example) you will potentially run into an error - "" not an Integer;
To Avoid NULL values, initially I thought to define the fields as NOT NULL with a DEFAULT value. But that didn't work as I'd hoped.
After some searching, I found the function COALESCE which takes a variable number of arguments and returns the first value in the list that is not NULL (or NULL when all values are NULL).
So COALESCE(1,2,NULL) will return "1", and COALESCE(NULL,"a","b") will return "a".
So looking at out example query, we can make sure at least a number will be returned, say "0", by doing this:

SELECT COALESCE( (SELECT someid FROM sometable WHERE somevalue=something), 0)  ...

So if the subquery returns NULL, COALESCE will return '0'.


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