Here to note one way to replace the NULL values in SQL with other specified values.

Given 3 Tables A, B, C as follows:

Table A

id name
1 Mike
2 Cathy
3 Paul

Table B

id age
1 16
2 32

Table C

id height
1 180
3 168

As you can see, Table B and Table C have missing records for id = 2 and id = 3, if we LEFT JOIN them to Table A there will be 2 missing cells shown as NULL:

SELECT A.id,name,age,height
FROM (A LEFT JOIN B on A.id = b.id) LEFT JOIN C ON A.id=C.id;
id name age height
1 Mike 16 180
2 Cathy 32 NULL
3 Paul NULL 168

There is a way that we can replace the NULLs to other values, in our case 0s.

SELECT A.id,name,(CASE WHEN age IS NULL THEN '0' ELSE age END) AS age,
(CASE WHEN height IS NULL THEN '0' ELSE height END) AS height 
FROM (A LEFT JOIN B on A.id = b.id) LEFT JOIN C ON A.id=C.id;

Now we have replaced all NULL to 0

id name age height
1 Mike 16 180
2 Cathy 32 0
3 Paul 0 168