mysql - What are the benefits of only_full_group_by mode?

Question

I updated mysql and I went from MySQL Version 5.6.17 to version 5.7.14

Since I have errors on my sql queries

Indeed, many of my queries look like this:

SELECT count (id) as nbr, lic from prep WHERE key = '18'

And I have this error:

1140 - In aggregated query without GROUP BY, expression #2 of SELECT list contains nonaggregated column 'operator.preparation.orig_lic'; this is incompatible with sql_mode=only_full_group_by

After some research, I learn that Mysql 5.7.14 activates ONLY_FULL_GROUP_BY by default

Why is it enabled by default?

What is the best solution (for performance)? Disable ONLY_FULL_GROUP_BY or add a 'group by' on my query?

Thank you

Answer 1

only_full_group_by = on tells MySQL engine: Do not apply GROUP BY when you have doubt about what results to show and throw an error. only apply it if the command specifically tells you what to do. i.e. when the command is full and complete!

only_full_group_by = off tells MySQL engine: always apply GROUP BY and if you have doubt about what results to choose, just pick one randomly!

You will not turn it off if you use GROUP BY properly!

Example:

Table: users

 id   |  name
----------------
  1      ali
  2      john
  3      ali

When you use GROUP BY on the name column:

SELECT * FROM users GROUP BY name;

There are two possible results:

  1      ali
  2      john     

OR

  2      john
  3      ali

MYSQL does not know what result to choose! Because there are different ids but both have name=ali.

Solution1:

only selecting the name field:

SELECT name FROM users GROUP BY name;

result:

  ali
  john     

This is a perfect solution. removing columns that makes GROUP BY confused. This means you know what you're doing. Usually, you do not need
those columns, but if you need them, go to solution3!

Solution2:

Turning off only_full_group_by. MYSQL will show you one of the two possible results RANDOMLY!! (It's ok if you do not really care what id it will choose)

Solution3

Use an Aggregate function like MIN(), MAX() to help MYSQL to decide what it must choose.

For example, I want the minimum id:

SELECT MIN(id), name FROM users GROUP BY name;

result:

  1      ali
  2      john     

It will choose the ali row which has the minimum id.