I have an SQL table which contains flashcard objects. The table has a column indicated whether a flashcard is free or paid. Flashcards are divided into categories. In my android app I need to display the number of flashcards which are free, and also the number of flashcards which are paid, for each category. If a flashcard isn't free, it is paid.
My SQL isn't great, so far I have a query which returns the number of flashcards which are free:
SELECT _id, category_primary, count(category_primary) FROM Flashcards WHERE available = '1' GROUP BY category_primary;
I want to try to get the count of both free and paid flashcards in a single query/cursor as I display the result in a ListView using an adapter.
You can add the available column to the GROUP BY:
SELECT
_id,
category_primary,
available,
count(category_primary)
FROM
Flashcards
GROUP BY
available,
category_primary;
As an aside, I would have expected you to need the _id column in both your original query and this updated version - I have left it out because I'm assuming your original query works fine.
select f.category_primary, count(f1._id)as available_count, count(f0._id)as disable_count
from (select _id, category_primary from Flashcards) f
left join (select _id from Flashcards where available='1') f1 on f1._id=f._id
left join (select _id from Flashcards where available='0') f0 on f0._id=f._id
group by f.category_primary
Steven Fenton's answer came close, but wasn't quite what I wanted. I found my answer here: How to get multiple counts with one SQL query?.
The query that worked for me is:
SELECT
_id,
category_primary,
sum(CASE WHEN Flashcards.available = '1' THEN 1 ELSE 0 END) category_free,
sum(CASE WHEN Flashcards.available = '0' THEN 1 ELSE 0 END) category_paid
FROM
Flashcards
GROUP BY
category_primary;
The naming of the column available is pretty poor on my part, and I didn't phrase my question very clearly. A better name for the column would be is_free Thanks for those who helped answer my question!
Related
I have ids in my table, ids start from 1 to 20, I want a query, to find the first and last records in a given table but I want the result by some condition.
For example: if I have the record
1,2,3,4,5,9,10,11,12,13, 19,20
I need a result like 1-5, 9-13, 19-20 like this I need results
This is the island part of the classic gaps and islands problem (With the gaps part being finding the missing values in between each island). If you search for that term, you'll find a ton of material about how to calculate them.
One approach (Requires Sqlite 3.25 or newer for window function support):
sqlite> CREATE TABLE ex(id INTEGER PRIMARY KEY);
sqlite> INSERT INTO ex VALUES (1),(2),(3),(4),(5),(9),(10),(11),(12),(13),(19),(20);
sqlite> WITH cte AS (SELECT id, id - row_number() OVER (ORDER BY id) AS grp FROM ex)
...> SELECT min(id) AS rangestart, max(id) AS rangeend FROM cte GROUP BY grp;
rangestart rangeend
---------- ----------
1 5
9 13
19 20
SQL Query to find first record in your table:
SELECT * FROM <table_name> ORDER BY <column_name> ASC LIMIT 1
SQL Query to find last record in your table:
SELECT * FROM <table_name> ORDER BY <column_name> DESC LIMIT 1
For example: if I have the record 1,2,3,4,5,9,10,11,12,13, 19,20
I need a result like 1-5, 9-13, 19-20 like this I need results
If you need result like you have mentioned, then you can set LIMIT in your query to get how many records you can have in that query.
QUERY:
SELECT * FROM <table_name> LIMIT <any_number>
I have added a new columnt (position) to the table. It needs to be ordered from 0 to n. And should reflect the order of rows by _id. But the thing is that _id has gaps, because some rows were deleted, and position shouldn't have gaps.
Like this:
_id position[new column]
1 0
4 1
8 2
17 3
How can I achive this during the upgrade of the database? It would be great to have only one sql statement to perform with execSQL, without need to query existing data with cursors.
Other DBMS have ROW_NUMBER() to make this easier.
In SQLite you can use a correlated sub query to count "how many rows have a lower id than this one". The answer to which happens to exactly match the enumerator column you want (position).
UPDATE
yourTable
SET
position = (SELECT COUNT(*)
FROM yourTable lookup
WHERE lookup._id < yourTable._id
)
I need get the total SUM for each rows in my query, but I don't want go twice in the table.
I tried do this:
SELECT id, value, SUM(value) as total FROM product
But my result was this:
id value total
3 30 60
If I do the bellow query I get my wanted result, but I need go twice in the table:
SELECT id, value, (SELECT SUM(value) FROM product) as total FROM product
Or if I use 'WITH' clause, but this is not supported before Android 5:
WITH data AS (SELECT id, value FROM product)
SELECT id, value, (SELECT SUM(value) FROM data) as total FROM data
Wanted result:
id value total
1 10 60
2 20 60
3 30 60
Thank you!
It's not possible using your SQLite version. You'll have to use two selects.
Basically you have to use a subquery.
However, perhaps you may be less concerned about the 2nd table as I believe that the Query Planner will determine that it only needs to calculate the sum once and does away with the need for a variable as it stores the value in cache.
I believe that the results of using EXPLAIN QUERY PLAN your_query shows this. i.e. using
EXPLAIN QUERY PLAN SELECT id, value, (SELECT sum(value) FROM products) AS total FROM products;
results in :-
This being explained as (see bolded statements) :-
1.3. Subqueries
In all the examples above, the first column (column "selectid") is
always set to 0. If a query contains sub-selects, either as part of
the FROM clause or as part of SQL expressions, then the output of
EXPLAIN QUERY PLAN also includes a report for each sub-select. Each
sub-select is assigned a distinct, non-zero "selectid" value. The
top-level SELECT statement is always assigned the selectid value 0.
For example:
sqlite> EXPLAIN QUERY PLAN SELECT (SELECT b FROM t1 WHERE a=0), (SELECT a FROM t1 WHERE b=t2.c) FROM t2;
0|0|0|SCAN TABLE t2
0|0|0|EXECUTE SCALAR SUBQUERY 1
1|0|0|SEARCH TABLE t1 USING COVERING INDEX i2 (a=?)
0|0|0|EXECUTE CORRELATED SCALAR SUBQUERY 2
2|0|0|SEARCH TABLE t1 USING INDEX i3 (b=?)
The example above contains a pair of scalar subqueries assigned
selectid values 1 and 2. As well as a SCAN record, there are also 2
"EXECUTE" records associated with the top level subquery (selectid 0),
indicating that subqueries 1 and 2 are executed by the top level query
in a scalar context. The CORRELATED qualifier present in the EXECUTE
record associated with scalar subquery 2 indicates that the query must
be run separately for each row visited by the top level query. Its
absence in the record associated with subquery 1 means that the
subquery is only run once and the result cached. In other words,
subquery 2 may be more performance critical, as it may be run many
times whereas subquery 1 is only ever run once.
Unless the flattening optimization is applied, if a subquery appears
in the FROM clause of a SELECT statement, SQLite executes the subquery
and stores the results in a temporary table. It then uses the contents
of the temporary table in place of the subquery to execute the parent
query. This is shown in the output of EXPLAIN QUERY PLAN by
substituting a "SCAN SUBQUERY" record for the "SCAN TABLE" record that
normally appears for each element in the FROM clause. For example:
sqlite> EXPLAIN QUERY PLAN SELECT count(*) FROM (SELECT max(b) AS x FROM t1 GROUP BY a) GROUP BY x;
1|0|0|SCAN TABLE t1 USING COVERING INDEX i2
0|0|0|SCAN SUBQUERY 1
0|0|0|USE TEMP B-TREE FOR GROUP BY
If the flattening optimization is used on a subquery in the FROM
clause of a SELECT statement, then the output of EXPLAIN QUERY PLAN
reflects this. For example, in the following there is no "SCAN
SUBQUERY" record even though there is a subquery in the FROM clause of
the top level SELECT. Instead, since the flattening optimization does
apply in this case, the EXPLAIN QUERY PLAN report shows that the top
level query is implemented using a nested loop join of tables t1 and
t2.
sqlite> EXPLAIN QUERY PLAN SELECT * FROM (SELECT * FROM t2 WHERE c=1), t1;
0|0|0|SEARCH TABLE t2 USING INDEX i4 (c=?)
0|1|1|SCAN TABLE t1
EXPLAIN QUERY PLAN
End Note
Perhaps of relevance is this statement :-
The best feature of SQL (in all its implementations, not just SQLite)
is that it is a declarative language, not a procedural language. When
programming in SQL you tell the system what you want to compute, not
how to compute it. The task of figuring out the how is delegated to
the query planner subsystem within the SQL database engine.
Query Planning
You may also find this of interest he SQLite Query Optimizer Overview noting that as of release 3.8.0 The Next-Generation Query Planner is utilised.
I had following Table
CREATE TABLE Customer
( `Name` varchar(7), `Address` varchar(55), `City` varchar(15),`Contact` int,`timestamp` int)
;
INSERT INTO Customer
(`Name`,`Address`, `City`, `Contact`,`timestamp`)
VALUES
('Jack','New City','LA',79878458,456125),
('Joseph','New Lane23','LA',87458458,794865),
('Rosy','Old City','Paris',79878458,215125),
('Maria','New City','LA',79878458,699125),
('Jack','New City','LA',79878458,456125),
('Rosy','Old City','Paris',79878458,845125),
('Jack','New Main Street','New York',79878458,555525),
('Joseph','Near Bank','SAn Francisco',79878458,984521)
;
I want to get all customer record with highest timestamp without duplication.
Try the following.
select name,max(timestamp),Address,City,Contact from Customer group by name
I want to get all customer record with highest timestamp without
duplication.
Use DISTINCT operator and ORDER BY clause like
select distinct `Name`,`Address`, `City`, `Contact`,`timestamp`
from customer
order by `timestamp` desc;
In that case you can use JOIN query like
select t1.*
from customer t1 join
(select Name, max(`timestamp`) as maxstamp
from customer
group by Name) xx
on t1.Name = xx.Name
and t1.`timestamp` = xx.maxstamp;
Try this:
SELECT * FROM `customer`
group by name,Address,City,Contact,timestamp
order by timestamp desc
I'm joining the Customer table with itself, the condition c1.timestamp<c2.timestamp on the join clause combined with c2.timestamp IS NULL
will make sure that only the latest record for each person is returned. I put DISTINCT because on your sample data there are two records for Jack with the same timestamp:
SELECT DISTINCT
c1.*
FROM
Customer c1 LEFT JOIN Customer c2
ON c1.Name=c2.Name
AND c1.Contact=c2.Contact -- you might want to remove this
AND c1.timestamp<c2.timestamp
WHERE
c2.timestamp IS NULL
ORDER BY
Name, Address
Please see a fiddle here.
I have a news database table that has two kinds of articles in it: social and official (mutually exclusive).
What I would like to do is reduce the number of social rows in the table to a specific number, while leaving the official news articles alone.
Here is what I've come up with so far:
DELETE
FROM News
WHERE _id NOT IN
(SELECT _id
FROM News
WHERE IsOfficialNews=0
ORDER BY Date DESC LIMIT 20
UNION SELECT _id
FROM News
WHERE IsOfficialNews=1)
However, I get an error that states ORDER BY clause should come after UNION not before. Moving the ORDER BY to the end of the inner SELECT results in LIMIT clause should come after UNION not before.
I understand the error message but I wonder if there is some other way to accomplish what I'm trying to do. If I move the LIMIT clause to the end of the inner SELECT then I will still have more than 20 social rows left in the table (because it will also count official rows).
Maybe like this (see comment above):
DELETE FROM News
WHERE IsOfficialNews=0
AND _id NOT IN
(SELECT _id
FROM News
WHERE IsOfficialNews=0
ORDER BY Date DESC LIMIT 20)