MySQL dynamic crosstab query: Selecting child records as additional columns

问题

I have three tables in a MySQL database:

users(user_settings,setting_types)

setting_types(id,name)

user_settings(value,user_id,setting_type_id)

I would like to denormalize these so that when I query the DB I get a result like this:

User.id    User.username    setting_name_1    setting_name_2    etc...
1          Admin            true              false
2          User             false             false

The issue at hand is that the setting type system is extensible: I don't necessarily know which settings a user is going to have ahead of time, so I can't hardcode them directly into the query.

Is it possible to query this in such a way that the query returns user records with additional columns for each child user_setting by setting_type? Or is this beyond MySQL?

回答1:

plan

write pivot source query to get all information required for pivot

generate dynamic sql to repeatedly join and pivot source

setup

create table users
(
  id integer primary key not null,
  username varchar(23) not null
  -- some user data..
);

create table setting_types
(
  id integer primary key not null,
  name varchar(23) not null
);

create table user_settings
(
  id integer primary key not null,
  user_id integer not null,
  setting_type_id integer not null,
  value varchar(13) not null,
  foreign key ( user_id ) references users( id ),
  foreign key ( setting_type_id ) references setting_types ( id )
);

insert into users
( id, username )
values
( 1, 'Admin' ),
( 2, 'heresjonny' )
;

insert into setting_types
( id, name )
values
( 1, 'setting_type_1' ),
( 2, 'setting_type_2' ),
( 3, 'setting_type_3' ),
( 4, 'setting_type_4' ),
( 5, 'setting_type_5' ),
( 6, 'setting_type_6' ),
( 7, 'setting_type_7' ),
( 8, 'setting_type_8' )
;

insert into user_settings
( id, user_id, setting_type_id, value )
values
( 1, 1, 1, 'true' ),
( 2, 1, 2, 'false' ),
( 3, 1, 3, 'false' ),
( 4, 1, 4, 'false' ),
( 5, 2, 3, 'true' ),
( 6, 2, 4, 'true' ),
( 7, 2, 5, 'false' ),
( 8, 2, 6, 'true' ),
( 9, 2, 7, 'true' ),
( 10, 2, 8, 'true' )
;

pivot

set @pivot_source = '(
select st.id as setting_id, st.name, users.id as user_id, users.username, coalesce(us.value, ''false'') as value
from setting_types st
cross join
(
  select id, username
  from users
) users
left join user_settings us
on  users.id = us.user_id
and st.id    = us.setting_type_id
)';

set @pivot_sql := replace('
select user_id, username,
#setting_aliases#
from
(
select #first_user_dets#,
#settings_fields#
from 
#pivot_source# #first_alias#
inner join
#all_joins#
) q
order by user_id
;', '#pivot_source#', @pivot_source);

set @pivot_block := replace('
#pivot_source# #alias# 
on  #last_alias#.user_id = #alias#.user_id
and #last_alias#.setting_id < #alias#.setting_id 
inner join #all_joins#', '#pivot_source#', @pivot_source)
;

select count(*) into @ignore
from
(
select 
@pivot_sql := replace(@pivot_sql, '#all_joins#', replace(replace(@pivot_block, '#alias#', concat('sett', right_id)), '#last_alias#', concat('sett', left_id)))
from
(
select `left`.id as left_id, min(`right`.id) as right_id
from setting_types `left`
inner join setting_types `right`
on `left`.id < `right`.id
group by 1
) t
order by left_id
) `ignore`
;

select concat('sett', id) into @first_alias
from setting_types
order by id
limit 1
;

select concat(@first_alias, '.user_id,',@first_alias,'.username') into @first_user_dets;

select group_concat(concat('sett', id, '.value ', name) SEPARATOR ',') into @settings_fields
from setting_types
;

select group_concat(name SEPARATOR ',') into @setting_aliases
from setting_types
;

select count(*) into @ignore
from
(
select
@pivot_sql := replace(@pivot_sql, '#first_user_dets#', @first_user_dets),
@pivot_sql := replace(@pivot_sql, '#settings_fields#', @settings_fields),
@pivot_sql := replace(@pivot_sql, '#setting_aliases#', @setting_aliases),
@pivot_sql := replace(@pivot_sql, '#first_alias#', @first_alias),
@pivot_sql := replace(@pivot_sql, 'inner join #all_joins#', '')
) `ignore`
;

select @pivot_sql;

prepare pivot_sql from @pivot_sql;
EXECUTE pivot_sql;
deallocate prepare pivot_sql;

output

+---------+------------+----------------+----------------+----------------+----------------+----------------+----------------+----------------+----------------+
| user_id |  username  | setting_type_1 | setting_type_2 | setting_type_3 | setting_type_4 | setting_type_5 | setting_type_6 | setting_type_7 | setting_type_8 |
+---------+------------+----------------+----------------+----------------+----------------+----------------+----------------+----------------+----------------+
|       1 | Admin      | true           | false          | false          | false          | false          | false          | false          | false          |
|       2 | heresjonny | false          | false          | true           | true           | false          | true           | true           | true           |
+---------+------------+----------------+----------------+----------------+----------------+----------------+----------------+----------------+----------------+

sqlfiddle

note

its more common to do this pivoting in application code. if your reason for doing this is for performance, should benchmark this against analogous pivoting in php to test if is actually significantly better..

might find my earlier answer on pivoting with dynamic columns useful for developing your php code for benchmarking performance

来源：https://stackoverflow.com/questions/34206766/mysql-dynamic-crosstab-query-selecting-child-records-as-additional-columns

标签

mysql

sql

denormalization