Sum of column values constraint MySQL - mysql

Question

I'm using MySQL with PhpMyAdmin. I'm looking for a way to put a constraint on a table so that:
column1(int) + column2(int) + column3(int) <= 100

Answer 1

In mysql as of now you can not have such constraints in the table definition. You may use the application layer to validate this. If you want to have it using mysql then you can use trigger and raise error using signal
https://dev.mysql.com/doc/refman/5.5/en/signal.html
Here is how the trigger would look like
delimiter //
create trigger check_col_sum before insert on test
for each row
begin
if (new.column1+new.column2+new.column3) > 100 then
signal sqlstate '45000' set message_text = 'Sum of column1,column2 and column3 must be less than equal to 100';
end if;
end;//
delimiter ;
Test case
mysql> create table test (id int auto_increment primary key, column1 int, column2 int, column3 int);
Query OK, 0 rows affected (0.14 sec)
mysql> delimiter //
mysql> create trigger check_col_sum before insert on test
-> for each row
-> begin
-> if (new.column1+new.column2+new.column3) > 100 then
-> signal sqlstate '45000' set message_text = 'Sum of column1,column2 and column3 must be less than equal to 100';
-> end if;
-> end;//
Query OK, 0 rows affected (0.09 sec)
mysql>
mysql> delimiter ;
mysql> insert into test (column1,column2,column3) values (10,20,30);
Query OK, 1 row affected (0.08 sec)
mysql> select * from test ;
+----+---------+---------+---------+
| id | column1 | column2 | column3 |
+----+---------+---------+---------+
| 1 | 10 | 20 | 30 |
+----+---------+---------+---------+
1 row in set (0.00 sec)
mysql> insert into test (column1,column2,column3) values (10,20,80);
ERROR 1644 (45000): Sum of column1,column2 and column3 must be less than equal to 100
mysql> select * from test ;
+----+---------+---------+---------+
| id | column1 | column2 | column3 |
+----+---------+---------+---------+
| 1 | 10 | 20 | 30 |
+----+---------+---------+---------+
1 row in set (0.00 sec)

Answer 2

You can use New feature of MySQL5.7 that is Generated columns, there you can define the sum of two columns in third column while creating table or can add the column by ALTER command. Example
CREATE TABLE triangle(
sidea DOUBLE,
sideb DOUBLE,
sidec DOUBLE AS (SQRT(sidea * sidea + sideb * sideb))
);
Refer the link:
MySQL Doc