How turn on sql_mode to stric in mysql

1) Edit my.cnf
For Debian :
located at /etc/mysql/my.cnf , add to my.cnf

    [mysqld]
    sql_mode="STRICT_ALL_TABLES"

 If you want strict mode with full group by use this

[mysqld]
sql_mode="STRICT_ALL_TABLES,ONLY_FULL_GROUP_BY"

2) Restart mysql

3) Check it by running this query

SELECT @@GLOBAL.sql_mode;

Note: If you want turn off strict mode on run time just run this query

SET @@global.sql_mode='STRICT_ALL_TABLES'
OR
SET @@session.sql_mode='STRICT_ALL_TABLES'

Thats it.