MySQL 8.0

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select version();


version()


8.0.22


create table table_a (column_a int, column_b int, `group` varchar(50));

insert into table_a values 
(888, 999, 'marketing'),
(888, 77, 'marketing'),
(999, 777, 'marketing'),
(999, 777, 'blog'),
(999, 999, 'store');
/*when sql_mode "only_full_group_by" is in effect*/
SELECT * FROM table_a 
WHERE (column_a=999) OR (column_b=999) GROUP BY `group`;
Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'db_2139386257.table_a.column_a' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by

/*when sql_mode "only_full_group_by" is in effect and uses any_value() function
 on columns non in group by. I added count(*) and you can see there's a row having 
 more than 1 count.*/
SELECT any_value(column_a), any_value(column_b), `group`, count(*) FROM table_a 
WHERE (column_a=999) OR (column_b=999) GROUP BY `group`;


any_value(column_a)
any_value(column_b)
group
count(*)


888
999
marketing
2

999
777
blog
1

999
999
store
1


/*using MAX() or MIN() can return result but still it might not be the correct 
result, especially for any row(s) that have more than 1 count*/
SELECT MAX(column_a), MAX(column_b), `group`, count(*)
FROM table_a 
WHERE (column_a=999) OR (column_b=999) GROUP BY `group`;


MAX(column_a)
MAX(column_b)
group
count(*)


999
999
marketing
2

999
777
blog
1

999
999
store
1


/*using group_concat to see what are the values in each column 
 that has been grouped. As you can see, there's one row having two values. 
 In that row, which one you actually want to show? Compare it to the result above, 
 does the above result returning according to what you want?*/
SELECT group_concat(column_a), group_concat(column_b), group_concat(`group`), count(*)
FROM table_a 
WHERE (column_a=999) OR (column_b=999) GROUP BY `group`;


group_concat(column_a)
group_concat(column_b)
group_concat(`group`)
count(*)


999
777
blog
1

888,999
999,777
marketing,marketing
2

999
999
store
1


/*when sql_mode "only_full_group_by" is in effect and group by consists of all
  columns in select. you can see there's a difference. The group by combination 
  is treated as unique in this set of result.*/
SELECT column_a, column_b, `group`, count(*) FROM table_a 
WHERE (column_a=999) OR (column_b=999) GROUP BY column_a, column_b,`group`;


column_a
column_b
group
count(*)


888
999
marketing
1

999
777
marketing
1

999
777
blog
1

999
999
store
1


/*Temporarily set sql_mode="" (or none). This gives you same result as the first
  query. It works but again, does it really make sense?*/
set sql_mode="";
SELECT *, count(*) FROM table_a 
WHERE (column_a=999) OR (column_b=999) GROUP BY `group`;


column_a
column_b
group
count(*)


888
999
marketing
2

999
777
blog
1

999
999
store
1