I have this sqlite database:
And I need to get the set from AssignmentMetric for a certain Athlete_id AND for Assignments which include a skill with Value X.
I've looked all over JOIN, LEFT JOIN but cannot find any examples that seems to match exactly this. I can query to get Assignment metrics for a particular Athlete ID And Assignment ID, but I am trying to narrow the Assignments to ones that are for a specific skill "of X value". How do I "reach through" the Assignment table to grab the value from the Skill table?
You could do:
SELECT ...
FROM
AssignmentMetric
INNER JOIN Assignment
ON AssignmentMetric.Assignment_id =
Assignment.Assignment_id
INNER JOIN
Skill
ON Skill.Skill_id = Assignment.Skill_id
WHERE ...
In the WHERE you can refer to the colums of the tables using the tablename.columnname notation.
Related
I'm generating a table which will in turn be used to format several different statistics and graphs.
Some columns of this table, are a result of subqueries which use a nearly identical structure. My query works, but it is very inefficient even in a simplified example like the following one.
SELECT
o.order,
o.date,
c.clienttype,
o.producttype,
(SELECT date FROM orders_interactions LEFT JOIN categories WHERE order=o.order AND category=3) as completiondate,
(SELECT amount FROM orders_interactions LEFT JOIN categories WHERE order=o.order AND category=3) as amount,
DATEDIFF((select date from orders_interactions LEFT JOIN categories where order=o.order AND category=3),o.date) as elapseddays
FROM orders o
LEFT JOIN clients c ON c.idClient=o.idClient
Being this a simplified example of a much more complex query, I would like to know the recommended approaches for a query like this one, taking into account query times, and readability.
As the example shows, I had to repeat a subquery (the one with date), just to calculate a datediff, since I cannot directly reference the column 'completiondate'
Thank you
You can try a left join.
SELECT o.order,
o.date,
o.producttype,
oi.date completiondate,
oi.amount,
datediff(oi.date, o.date) completiondate
FROM orders o
LEFT JOIN orders_interactions oi
ON oi.order = o.order
AND oi.category = 3;
That doesn't necessarily perform better but there are good chances. For performance an index on order_interactions (order, category) might help in any case.
And if you consider it more readable is up to you. But at least it's less repetitive (Which doesn't necessarily translates to more performance. Just because an expression is repeated in a query doesn't necessarily mean it repeatedly calculated.)
It seems I might have found the answer.
In my opinion, it improves readability quite a bit, and in my real usage scenario, both profile and execution plans are way more efficient, and results are returned in less than 1/3 of the time.
My answer relies on using a SELECT inside the LEFT JOIN, hence, using a subquery as the JOINs 'input'.
SELECT
o.order,
o.date,
c.clienttype,
o.producttype,
tmp.date,
tmp.amount,
DATEDIFF(tmp.date,o.date) as elapseddays
FROM orders o
LEFT JOIN clients c ON c.idClient=o.idClient
LEFT JOIN (SELECT order,date,amount FROM orders_interactions oi LEFT JOIN categories ct ON ct.order=oi.order AND category=3) AS tmp ON tmp.order=o.order
The answer idea, and the explanation about how and why it works, came from this post: Mysql Reference subquery result in parent where clause
I have 3 tables in a SQLite database for an Android app. This picture below shows the relevant tables that I'm working with.
Tables
I'm trying to get two fields, value and name, from measurement_lines and competences respectively, tied to a specific person_id in measurements. I'm trying to make a query that returns these fields but I'm having little luck. The best I've got so far is the following query:
SELECT name, value
FROM measurements, measurement_lines, competences
WHERE measurements.id = measurement_lines.measurements_id
AND measurement_lines.competences_id = competences.id
AND measurements.persons_id = 1
This, however, has one issue. This query won't return any records when a person has no entries in measurements (and subsequently, nothing in measurement_lines). What I want is to always get a list of competence names, even if the value column is empty. I'm guessing I need a Left Outer Join for this but I can't seem to make it work. The following query just returns no records:
SELECT name, value
FROM measurements AS m, competences AS c
LEFT OUTER JOIN measurement_lines AS ml ON c._id = ml.competence_id
WHERE ml.measurement_id = m._id AND m.persons_id = 1
For inner joins, you can be sloppy with the distinction between join conditions and selection predicates, but when outer joins are involved that makes a difference. Any criterion appearing in the WHERE clause filters your result rows after all joins are performed (logically, at least), which can remove result rows associated with outer tables.
In addition, if you're ever uncertain about join order, you can use parentheses to make your intent clear. At least in many DBMSs. It lokos like SQLite doesn't support them.
It looks like you may want this: (edited to avoid use of parentheses)
SELECT c.name, pm.value
FROM competences c
LEFT OUTER JOIN (
SELECT ml.competences_id AS cid,
ml.value AS value
FROM measurement_lines ml
INNER JOIN measurements m
ON m.id = ml.measurements_id
WHERE m.person_id = 1
) pm
ON pm.cid = c.id
Im working with the oracle pdf's to learn pl/sql.
There is an exercise where i have to create a new table with data out
of two other tables already existing. I thought this would do the trick:
CREATE TABLE new_depts
AS SELECT d.department_id, d.department_name, sum(e.salary) dept_sal
FROM employees e, departments d
WHERE e.department_id = d.department_id;
But this raises the following error:
SQL-Fehler: ORA-00937: not a single-group group function
00937. 00000 - "not a single-group group function"
I cant find something usefull about this error. From what i know yet
about SQL my code should work fine!
Am i wrong?
Try adding group by clause :
CREATE TABLE new_depts
AS SELECT d.department_id, d.department_name, sum(e.salary) dept_sal
FROM employees e, departments d
WHERE e.department_id = d.department_id
group by d.department_id,d.department_name
Update 1
You need to use group by clause in your select query because you are using aggregate function: sum(e.salary). If you are using aggregate function then you need to have group by clause. Please see here for more information about group by clause.
The main concept to understanding why aggregate functions or columns that are specified in the GROUP BY clause cannot be mixed with other non aggregate expressions in the select list is the level of detail of the value they produce. The select list of the SELECT statement can include only expressions that produce values that are on the same level of detail as others in that select list.
Example 1: incorrect
SELECT avg(col1) --> level of detail of the value is aggregated
,col2 --> level of detail of the value is only for one row
FROM table_a;
Example 2: correct
SELECT avg(col1) --> level of detail of the value is aggregated
,col2 --> level of detail of the value is aggregated
FROM table_a
GROUP BY col2;
By including a column in the GROUP BY clause you aggregate the specified column and change its level of detail from single row to aggregate.
So I know enough SQL just to be really dangerous (I don't normally work the back-end) but cannot get the following view to be created successfully ;) The result set I'm after is a data set that has rows assigned as a column alias from multiple tables (instead of a 1xN flat of all columns). There is a many-to-one relationship when looking at the main table, based on foreign keys associated to the row id of the appropriate related table.
Ideally I'd like a data set that looks like this in the return:
dataset.transaction_row[n]: col1, col2, col3, coln... (columns from the transaction table)
dataset.category_row[n]: col1, co2, col3, coln... (columns from the category table)
and so on...
I get the following error:
Query Error: near "AS": syntax error Unable to execute statement
From:
CREATE VIEW view_unreconciled_transactions
AS SELECT account_transaction.* AS transaction_row,
category.* AS category_row,
memorized.name_rule_replace OR account_transaction.name AS payee
FROM account_transaction
LEFT JOIN memorized ON account_transaction.memorized_key = memorized.id
LEFT JOIN category ON account_transaction.category_key = category.id
WHERE status != 2
ORDER BY account_transaction.dt_posted DESC
It seems easy enough since the result-column selector is repeatable which includes expressions (referencing sqlite's syntax diagrams). In reference to the error, I'm assuming it's complaining about the 2nd 'AS' where I'm trying to get table.* assigned as an alias. Any help in the right direction is appreciated. If I had to, I suppose I could explicitly state all columns but that feels like a kludge.
The AS modifier can only be applied to a single column, not to a collection such as the * you used. You will have to break them out into specific names, (which is best practice IMHO anyway)
It looks like you want to make a "pivot table". They can be tricky to make in a database. I can say that if you a data result, where each row comes from a different table source, and the columns form each table are IDENTICAL, then you could try using a UNION statement to join the different results together like they are just one dataset.
NOTE that the columns all take their naming cue from the first dataset in a UNION and the datatype all need to be the same.
I have two tables, one contains a list of items which is called watch_list with some important attributes and the other is just a list of prices which is called price_history. What I would like to do is group together 10 of the lowest prices into a single column with a group_concat operation and then create a row with item attributes from watch_list along with the 10 lowest prices for each item in watch_list. First I tried joins but then I realized that the operations where happening in the wrong order so there was no way I could get the desired result with a join operation. Then I tried the obvious thing and just queried the price_history for every row in the watch_list and just glued everything together in the host environment which worked but seemed very inefficient. Now I have the following query which looks like it should work but it's not giving me the results that I want. I would like to know what is wrong with the following statement:
select w.asin,w.title,
(select group_concat(lowest_used_price) from price_history as p
where p.asin=w.asin limit 10)
as lowest_used
from watch_list as w
Basically I want the limit operation to happen before group_concat does anything but I can't think of a sql statement that will do that.
Figured it out, as somebody once said "All problems in computer science can be solved by another level of indirection." and in this case an extra select subquery did the trick:
select w.asin,w.title,
(select group_concat(lowest_used_price)
from (select lowest_used_price from price_history as p
where p.asin=w.asin limit 10)) as lowest_used
from watch_list as w