MySQL 8.0

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create table test (column1 int, column2 int, column3 int);
insert into test values (1,2,3);
insert into test values (1,NULL,NULL);
insert into test values (NULL,4,5);
insert into test values (NULL,NULL,6);
insert into test values (0,0,0);
insert into test values(NULL,NULL,NULL);
SELECT column1, column2, column3,
column1  + column2 + column3 AS sum,
COALESCE(column1,0)  + COALESCE(column2,0) + COALESCE(column3,0) AS sum2,
COALESCE(column1,column2,column3,100) AS value FROM test;


column1
column2
column3
sum
sum2
value


1
2
3
6
6
1

1
null
null
null
1
1

null
4
5
null
9
4

null
null
6
null
6
6

0
0
0
0
0
0

null
null
null
null
0
100


SELECT CASE WHEN column1 IS NULL THEN 0 ELSE column1 END AS column1,
COALESCE(column1,0) AS column1_b FROM test;


column1
column1_b


1
1

1
1

0
0

0
0

0
0

0
0

column1	column2	column3	sum	sum2	value
1	2	3	6	6	1
1	null	null	null	1	1
null	4	5	null	9	4
null	null	6	null	6	6
0	0	0	0	0	0
null	null	null	null	0	100