Scenario is like
Before inserting into sqlite db I have to check whether is it reached a particular number, say 10. I know it can be done by using 2 queries for get and insert.
Can it be done in 1 query in android and sqlite
INSERT INTO Customers (name, age)
SELECT 'MM', 20 WHERE (SELECT count(*) from Customers) < 10;
By use this query, we only insert new customer to database when total customers in database < 10
Considering the documentation for INSERT you can also insert a result of a select statement. So instead of inserting the values directly, you could assemble a select statement to only return your default values with coalesce if and only if a condition for the count yields true.
This is mostly an idea and theoretical approach, but worth trying.
To check how many rows in database, you need to make
select * from
query and get the cursor object. Cursor object have getCount method which return you size of cursor, its simply means to show all your records in table.
Cheers!!!
Related
I am using this query
"select * from SomeTable group by SomeColumn"
It is returns list with accenting order, but i need to same order like in database.
For example the order in database is:
p
a
s
But result is:
a
i
p
Sample
The result need to be like distinct by CityEN but with all columns and order like 1.Paris 2.Amsterdam 3.Istanbul
In Sqlite, each row of a table has a unique rowid, which you can use for sorting.
select * from SomeTable group by SomeColumn order by rowid;
In your statement, add this line to sort the results:
order by min(rowid)
Your query does not enforce any order with ORDER BY clause so no assumption about row order should be made. If you want specific order add i.e. ORDER BY SomeColumn. See docs about all available order options: https://www.sqlite.org/lang_select.html#orderby
By the rules of SQL, you can't count on getting records back in any specific order without specifying an ORDER BY clause in your SQL query.
In practice servers sometimes return values in the order in which they're inserted, in the order of the first index created, or in the order of the primary key--but you can't count on this behavior, and in fact I've seen the behavior change between database maintenance windows or after the database version is upgraded. You definitely wouldn't want to count on a DB engine to give you back records in any particular order if you write a SELECT statement without an ORDER BY clause.
The only real way to get your records back in the order you inserted them is to create a timestamp column and then sort on it during the SELECT. If you don't want to worry about populating that column on INSERT, have that column auto-populate itself with a timestamp (depending on your DB engine).
I am trying to test if a sqlite database is empty or not. I've read stackoverflow posts and many recommend using rawQuery and getCount() methods. Others recommend using DatabaseUtils.longForQuery and DatabaseUtils.queryNumEntries.
I am looking for speed since the number of rows in the database is large. I am looking for quick test to see if the number of rows is zero. If not, it is because there are 1 or more rows in the database. Would there be a method that aborts the count after 1 row is identified rather than counting all of the rows? If not, any thoughts on getCount vs. DatabaseUtils?
queryNumEntries as Google Doc where Added in API level 11
long count=0;
Cursor cursor = Database.DB.rawQuery("select count(*) from table;",null);
count=cursor.getLong(0);
The sql query SELECT COUNT(*) FROM table_name; runs pretty fast.
DatabaseUtils.queryNumEntries() uses this method. I would go with that.
You could run your own SQLiteDatabase.rawQuery() that does the above query. That should be good too.
Anyone know of a way to limit the number of rows deleted when using an sql DELETE statement?I just need to delete a row that holds a certain value one time instead of deleting every instance of the value. It's my understanding that the LIMIT clause cannot be added to DELETE statements in SQLITE. Now, I can't see a way to limit the number of rows deleted just using _id because I don't know what row _id will be deleted ahead of time; the rows are being deleted based on a value held in a variable and they could be anywhere in the DB. I hope this makes sense. Here's the delete statement:
String sql = "DELETE FROM strategyTotal WHERE strategy_prices = (?)" ;
db.execSQL(sql, new Double[] {subtractedStrategyPrice });
Use a subquery:
String sql = "DELETE FROM strategyTotal WHERE _id IN (SELECT _id FROM strategyTotal WHERE strategy_prices = (?) LIMIT 1);" ;
db.execSQL(sql, new Double[] {subtractedStrategyPrice });
delete from tablename where rowid in (
select rowid from tablename condition LIMIT 1)
try above work around or you may need to enable SQLITE ENABLE UPDATE DELETE LIMIT
my query is just an example. replace it with your own query.
Will the order of rows returned by a query will be the same as the order in which the rows were inserted into the table, of SQLite database?
If Yes, Is this behaviour consistent?
If No, Can this be enforced?
I have a requirement of storing approx 500 rows of data, and which requires sorting/ordering from time to time. The data is in proper order, before the insertion.
Given the small number of rows in your table, this is probably what you need:
SELECT * FROM yourtable ORDER BY ROWID
For more information on ROWID, see these two links:
SQLite Autoincrement and ROWIDs and the INTEGER PRIMARY KEY
Even if the order may be consistent in one scenario, there is afaik no guarantee.
That is why SQL has the ORDER BY operator:
SELECT foo,bar FROM Table FOO WHERE frobnitz LIKE 'foo%' ORDER BY baz ASC;
Will the order of rows returned by a
query will be the same as the order in
which the rows were inserted into the
table, of SQLite database?
No, you can't count on that. All query optimizers have a lot of freedom when it comes to speeding up queries. One thing they're free to do is to return rows in whatever order is the fastest. That's true even if a particular dbms supports clustered indexes. (A clustered index imposes a physical ordering on the rows.)
There's only one way to guarantee the order of returned rows in a SQL database: use an ORDER BY clause.
I want to get the number of NOT NULL records from my SQLite database. Since I'm using autoincrement, the last inserted row won't give me the real number of records in the table as even if I delete any middle record, it'll insert at the position higher than the last inserted record.
The insert statement returns me the number of last inserted row, but I want this value on the fly.
Doing a count before on the table should work. Simply query for the id column with the where check of NOT NULL and on the returned cursor just call the getCount()
Just to be sure: You should never ever, really never ever, manipulate the auto increment in a productive database. If you delete a record, than the "gap" should stay there. It has no impact on performance or anything else. If you insert a new record in the gap, you can create a lot of trouble...
So you just want to find the number of rows? (There no such thing as a "null record" as far as I'm aware.)
Can you not just do
select count(1) from YourTableName
or
select count(*) from YourTableName
? (Some databases are faster using one form or other... I don't know about sqlite.)