I want to get the latest 5 records in my table, so far i tried this but, it did not work out very well. So, what is the cleanest and efficient way to get last 5 records in the table ?
"select * from (select * from People order by Date DESC limit 5) order by Date ASC;"
Your query works just fine.
To make it efficient, ensure that there is an index on the Date column; then SQLite will just read the last five entries from the index and the table and does not need to scan the entire table.
If this table has an autoincrementing ID column, and if "latest" means the insertion order, then you can use that ID for sorting; this will be as efficient as your original query with an index on Date:
SELECT * FROM (SELECT * FROM People
ORDER BY _id DESC
LIMIT 5)
ORDER BY Date ASC
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 a database from which I would like to take the last 3 records. For example if I had the lines 1,2,3,4,5,6, ... 10,11,12,13,14, I would like 12,13,14 no matter the order (12,13,14 for me is equal to 14,13,12).
I tried to follow another question at this link Android SQLite Query - Getting latest 10 records
but what I get is just showing the first 3 rows of the database.
This is my query
String query2 ="select * from (select * from USERS order by ID ASC limit 3)";
In any case you can sort by rowid descending and get 3 rows:
select * from USERS order by rowid desc limit 3
If you want to sort by a specific column:
select * from USERS order by columnname desc limit 3
I have some Entity to store my data into ROOM. How to store only 10 last row to my db using room. For now I'n using #Query("SELECT * FROM Entity LIMIT 10") but it's dont looks right
We you want only first 10 or less records in your database then you have to set the id as auto increment and try to delete all those records where the ids doesn't match the first 10 results (after every insertion)
DELETE FROM tableName where id NOT IN (SELECT id from tableName ORDER BY id DESC LIMIT 10)
Here is a link to explore more:-
Limit the amount of rows in a room database
Does it work?
This is the correct way to do it:
SELECT expressions
FROM tables
[WHERE conditions]
[ORDER BY expression [ ASC | DESC ]]
LIMIT number_rows [ OFFSET offset_value ];
And here's a real example:
SELECT contact_id, last_name, first_name
FROM contacts
WHERE website = 'TechOnTheNet.com'
ORDER BY contact_id ASC
LIMIT 5;
I got these from https://www.techonthenet.com/sql/select_limit.php.
There could by slight differences in syntax as I do not know which type of SQL you are using.
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 am developing an android application in which i have a database table and i want to display only last three rows from the database how can i achieve it ?
As a SQL table has no inherent order, you have to specify something you want to order by, like a date or id and limit the result to 3.
SELECT * FROM yourTable ORDER BY date DESC LIMIT 3