I have some code that looks like the following:
query = <<-EOF
select player_id, first_name, last_name,
max(case when site_id = 1 then salary end) fd_salary,
max(case when site_id = 2 then salary end) dd_salary,
max(case when site_id = 3 then salary end) ss_salary,
max(case when site_id = 4 then salary end) ds_salary,
max(case when site_id = 7 then salary end) dk_salary,
max(case when site_id = 8 then salary end) elite_salary
from player_salaries ps
where ps.gamedate = '2014-05-25'
and sport_id = #{$MLB_SPORT_ID}
group by player_id
EOF
salaries = PlayerSalary.connection.execute(query)
The problem is salaries in this case comes back as an array with values. These query is a bit complex and the names I'm using such as fd_salary, dd_salary and so forth aren't physical attributes in the PlayerSalary model. There's no way to do something like salaries.first.fd_salary. Is there a way to change the above in Rails 4 to get it access values by column name?
You could use find_by_sql for this:
find_by_sql(sql, binds = [])
Executes a custom SQL query against your database and returns all the results. The results will be returned as an array with columns requested encapsulated as attributes of the model you call this method from. If you call Product.find_by_sql then the results will be returned in a Product object with the attributes you specified in the SQL query.
So if you did:
salaries = PlayerSalary.find_by_sql(query)
then you could say things like salaries.first.fd_salary. Just don't try to use columns that weren't in your query or try to change the returned PlayerSalary objects and expect anything useful to happen.
If you want to retrieve your records as a hash, with column names you can use mysql2 adapter directly with:
ActiveRecord::Base.connection.instance_variable_get('#connection').query("SELECT * FROM `users`", as: :hash).first
I was searching for another way to do it but had no luck.
Related
I have a table illustrated on the attached screen:
And data is presented:
As you can see it contains all fields from lines (value, column_id - is field type). Each field has line_index and column_index of position and column_id.
I want to get back all lines with all field names like:
line_index column_id_1 column_name1 column_id_2 column_name2 column_id_3 column_name3
0 1 Age 2 Vasile 3 NY
More simply I need to build all fields(rows) to columns then to lines back.
Should I use Pivot and is it possible in MySQL?
Link to sqlfiddle
You will need to group the results by line_index and conditionally transform rows to columns.
SELECT l.line_index,
MAX(CASE WHEN ct.column_name = "NAME" THEN l.value ELSE NULL END) AS "NAME" ,
MAX(CASE WHEN ct.column_name = "AGE" THEN l.value ELSE NULL END) AS "AGE" ,
MAX(CASE WHEN ct.column_name = "ZUP" THEN l.value ELSE NULL END) AS "ZUP"
FROM columns_types ct
LEFT JOIN `lines` l ON l.column_id = ct.column_id
GROUP BY l.line_index;
Try it here. Also I did some changes in your schema which I felt didn't impact the data stored in the tables. You were adding redundant rows(perhaps) in your columns_types table. And by looking at the query, you know that you will have to build a MAX(CASE statement for each column, so its best if you first fetch it and then build the final query in some programming language.
I have a database that contains these values:
ID Value
1 Mail
2 Portal
3 Terrain
And in my program, I want to make a select, so when I pick a number, it shows the 'value' of that number. Something like:
SELECT ID as Mail, Portal, Terrain
FROM TABLE
You may be looking for conditional aggregation:
select max(case when id = 1 then value end) as mail,
max(case when id = 2 then portal end) as portal,
max(case when id = 3 then terrain end) as terrain
from ;
Make sure you're giving the ID by a textbox or something in your program
Select Value from TABLE where ID = #ID
I am working on a application where I need to write a query on a table, which will return multiple columns count in a single query.
After research I was able to develop a query for a single sourceId, but what will happen if i want result for multiple sourceIds.
select '3'as sourceId,
(select count(*) from event where sourceId = 3 and plateCategoryId = 3) as TotalNewCount,
(select count(*) from event where sourceId = 3 and plateCategoryId = 4) as TotalOldCount;
I need to get TotalNewCount and TotalOldCount for several source Ids, for example (3,4,5,6)
Can anyone help, how can I revise my query to return a result set of three columns including data of all sources in list (3,4,5,6)
Thanks
You can do all source ids at once:
select source_id
sum(case when plateCategoryId = 3 then 1 else 0 end) as TotalNewCount,
sum(case when plateCategoryId = 4 then 1 else 0 end) as TotalOldCount
from event
group by source_id;
Use a where (before the group by) if you want to limit the source ids.
Note: The above works in both Vertica and MySQL, and being standard SQL should work in any database.
I have a database with roughly 6 million entries - and will grow - where I'm running queries to return for a HighCharts charting functionality. I need to read longitudinally over years, so I'm running queries like this:
foreach($states as $state_id) { //php code
SELECT //mysql psuedocode
sum(case when mydatabase.Year = '2003' then 1 else 0 end) Year_2003,
sum(case when mydatabase.Year = '2004' then 1 else 0 end) Year_2004,
sum(case when mydatabase.Year = '2005' then 1 else 0 end) Year_2005,
sum(case when mydatabase.Year = '2006' then 1 else 0 end) Year_2006,
sum(case when mydatabase.Year = '2007' then 1 else 0 end) Year_2007,
sum(case when mydatabase.Year = '$more_years' then 1 else 0 end) Year_$whatever_year,
FROM mytable
WHERE State='$state_id'
AND Sex IN (0,1)
AND Age_segment IN (5,4,3,2,1)
AND "other_filters IN (etc, etc, etc)
} //end php code
But for various state at once... So returning lets say 5 states, each with the above statement but a state ID is substituted. Meanwhile the years can be any number of years, the Sex (male/female/other) and Age segment and other modifiers keep changing based on filters. The queries are long (at minimum 30-40seconds) a piece. So a thought I had - unless I'm totally doing it wrong - is to actually store the above query in a second table with the results, and first check that "meta query" and see if it was "cached" and then return the results without reading the db (which won't be updated very often).
Is this a good method or are there potential problems I'm not seeing?
EDIT: changed to table, not db (duh).
Table structure is:
id | Year | Sex | Age_segment | Another_filter | Etc
Nothing more complicated than that and no joining anything else. There are keys on id, Year, Sex, and Age_segment right now.
Proper indexing is what is needed to speed up the query. Start by doing an "EXPLAIN" on the query and post the results here.
I would suggest the following to start off. This way avoids the for loop and returns the data in 1 query. Not knowing the number of rows and cardinality of each column I suggest a composite index on State and Year.
SELECT mytable.State,mytable.Year,count(*)
FROM mytable
AND Sex IN (0,1)
AND Age_segment IN (5,4,3,2,1)
AND "other_filters IN (etc, etc, etc)
GROUP BY mytable.State,mytable.Year
The above query can be further optimised by checking the cardinality of some of the columns. Run the following to get the cardinality:
SELECT Age_segment FROM mytable GROUP BY Age_segment;
Pseudo code...
SELECT Year
, COUNT(*) total
FROM my_its_not_a_database_its_a_table
WHERE State = $state_id
AND Sex IN (0,1)
AND Age_segment IN (5,4,3,2,1)
GROUP
BY Year;
userid data_type, timespentaday
1 League of Legends 500
1 Hearthstone 1500
1 Hearthstone 1400
2 World of Warcraft 1200
1 Dota 2 100
2 Final Fantasy 500
1 Dota 2 700
Given this data. I would like to query the most time each user has spent on every.
Output desired:
User League Of Legends Hearthstone World of Warcraft Dota 2
1 500 1500 0 700
2 0 0 1200 0
Something along the lines of this is something I've tried
SELECT t1.* FROM user_info GROUP BY userid JOIN(
SELECT(
(SELECT max(timespentaday) where data_type='League of Legends'),
(SELECT max(timespentaday) where data_type='Hearhstone'),
(SELECT max(timespentaday) where data_type='Dota 2)'
FROM socialcount AS t2
) as t2
ON t1.userid = t2.userid
basically to do this you need the greatest n per group.. there is a good article on it but the gist is in mysql you have to use variables to even get close to this.. especially with doing a pivot on the table (a fake pivot since MySQL doesn't have native support for that).
SELECT userid,
MAX(CASE WHEN data_type = "League of Legends" THEN timespentaday ELSE 0 END) as "League of Legends",
MAX(CASE WHEN data_type = "Hearthstone" THEN timespentaday ELSE 0 END) as "Hearthstone",
MAX(CASE WHEN data_type = "Dota 2" THEN timespentaday ELSE 0 END) as "Dota 2",
MAX(CASE WHEN data_type = "World of Warcraft" THEN timespentaday ELSE 0 END) as "World of Warcraft",
MAX(CASE WHEN data_type = "Final Fantasy" THEN timespentaday ELSE 0 END) as "Final Fantasy"
FROM
( SELECT *, #A := if(#B = userid, if(#C = data_type, #A + 1, 1), 1) as count_to_use, #B := userid, #C := data_type
FROM
( SELECT userid, timespentaday, data_type
FROM gamers
CROSS JOIN(SELECT #A := 0, #B := 0, #C := '') temp
ORDER BY userid ASC, data_type ASC, timespentaday DESC
) t
HAVING count_to_use = 1
)t1
GROUP BY userid
DEMO
NOTE:
MySQL DOCS is quite clear on warnings about using user defined variables:
As a general rule, you should never assign a value to a user variable
and read the value within the same statement. You might get the
results you expect, but this is not guaranteed. The order of
evaluation for expressions involving user variables is undefined and
may change based on the elements contained within a given statement;
in addition, this order is not guaranteed to be the same between
releases of the MySQL Server. In SELECT #a, #a:=#a+1, ..., you might
think that MySQL will evaluate #a first and then do an assignment
second. However, changing the statement (for example, by adding a
GROUP BY, HAVING, or ORDER BY clause) may cause MySQL to select an
execution plan with a different order of evaluation.
I am not going to give you a query with the output format you desire, as implementing that pivot table is going to be a very ugly and poorly performing query, as well as something that is not scalable as the number of distinct games increases.
Instead, I will focus on how to query the data in the most straightforward manner and how to read it into a data structure that would be used by application logic to create the pivot view as desired.
First the query:
SELECT
userid,
data_type,
MAX(timespentaday) AS max_timespent
FROM social_count
GROUP BY userid, data_type
This would give results like
userid data_type max_timespent
------ --------- -------------
1 League of Legends 500
1 Hearthstone 1500
1 Dota 2 700
2 World of Warcraft 1200
2 Final Fantasy 500
Now when reading the results out of the database, you just read it into a structure that is useful. I will use PHP as example language, but this should be pretty easily portable to any langauge
// will hold distinct list of all available games
$games_array = array();
// will hold user data from DB
$user_data = array();
while ($row = /* your database row fetch mechanism here */) {
// update games array as necessary
if (!in_array($row['data_type'], $games_array)) {
// add this game to $games_array as it does not exist there yet
$games_array[] = $row['data_type'];
}
// update users array
$users[$row['userid']][$row['data_type']] = $row['max_timespent'];
}
// build pivot table
foreach($users as $id => $game_times) {
// echo table row start
// echo out user id in first element
// then iterate through available games
foreach($games_array as $game) {
if(!empty($game_times[$game])) {
// echo $game_times['game'] into table element
} else {
// echo 0 into table element
}
}
// echo table row end
}
You will not be able to build a query with a dynamic number of columns. You can do this query if you already know the game list, which I guess is not what you need.
BUT you can always post-process your results with any programming language, so you only have to retrieve the data.
The SQL query would look like this:
SELECT
userid AS User,
data_type AS Game,
max(timespentaday) AS TimeSpentADay
FROM
my_table
GROUP BY
userid
data_type
Then iterate over the results to fill any interface you want
OR
If and only if you can't afford any post-processing of any kind, you can retrieve the list of games first THEN you can build a query like the query below. Please bear in mind that this query is a lot less maintainable than the previous (beside being more difficult to build) and can and will cause you a lot of pain later in debugging.
SELECT
userid AS User,
max(CASE
WHEN data_type = 'Hearthstone' THEN timespentaday
ELSE NULL
END) AS Hearthstone,
max(CASE
WHEN data_type = 'League Of Legends' THEN timespentaday
ELSE NULL
END) AS `League Of Legends`,
...
FROM
my_table
GROUP BY
userid
The CASE contstruction is like an if in a procedural programming language, the following
CASE
WHEN data_type = 'League Of Legends' THEN timespentaday
ELSE NULL
END
Is evaluated to the value of timespentaday if the game is League Of Legends, and to NULL otherwise. The max aggregator simply ignore the NULL values.
Edit: added warning on the second query to explain the caveat of using a generated query thanks to Mike Brant's comment