Maybe i should do this in C# but i have more then one row with linkId X. I would like to remove it but i am unsure how. In code i could just use a foreach from 0 to n and remove any found rows with a greater (or !=) id but thats in code. Is there a less difficult way of doing it using sqlite?
Assuming the table's name is tableName and there is a primary key field named id, the following sql would do it. I think the following SQL query is general enough and should be able to be executed under any database engine.
delete from tableName
where id not in (
select min(id) from tableName
group by linkId
)
Related
Similar to this question and this solution for PostgreSQL (in particular "INSERT missing FK rows at the same time"):
Suppose I am making an address book with a "Groups" table and a "Contact" table. When I create a new Contact, I may want to place them into a Group at the same time. So I could do:
INSERT INTO Contact VALUES (
"Bob",
(SELECT group_id FROM Groups WHERE name = "Friends")
)
But what if the "Friends" Group doesn't exist yet? Can we insert this new Group efficiently?
The obvious thing is to do a SELECT to test if the Group exists already; if not do an INSERT. Then do an INSERT into Contacts with the sub-SELECT above.
Or I can constrain Group.name to be UNIQUE, do an INSERT OR IGNORE, then INSERT into Contacts with the sub-SELECT.
I can also keep my own cache of which Groups exist, but that seems like I'm duplicating functionality of the database in the first place.
My guess is that there is no way to do this in one query, since INSERT does not return anything and cannot be used in a subquery. Is that intuition correct? What is the best practice here?
My guess is that there is no way to do this in one query, since INSERT
does not return anything and cannot be used in a subquery. Is that
intuition correct?
You could use a Trigger and a little modification of the tables and then you could do it with a single query.
For example consider the folowing
Purely for convenience of producing the demo:-
DROP TRIGGER IF EXISTS add_group_if_not_exists;
DROP TABLE IF EXISTS contact;
DROP TABLE IF EXISTS groups;
One-time setup SQL :-
CREATE TABLE IF NOT EXISTS groups (id INTEGER PRIMARY KEY, group_name TEXT UNIQUE);
INSERT INTO groups VALUES(-1,'NOTASSIGNED');
CREATE TABLE IF NOT EXISTS contact (id INTEGER PRIMARY KEY, contact TEXT, group_to_use TEXT, group_reference TEXT DEFAULT -1 REFERENCES groups(id));
CREATE TRIGGER IF NOT EXISTS add_group_if_not_exists
AFTER INSERT ON contact
BEGIN
INSERT OR IGNORE INTO groups (group_name) VALUES(new.group_to_use);
UPDATE contact SET group_reference = (SELECT id FROM groups WHERE group_name = new.group_to_use), group_to_use = NULL WHERE id = new.id;
END;
SQL that would be used on an ongoing basis :-
INSERT INTO contact (contact,group_to_use) VALUES
('Fred','Friends'),
('Mary','Family'),
('Ivan','Enemies'),
('Sue','Work colleagues'),
('Arthur','Fellow Rulers'),
('Amy','Work colleagues'),
('Henry','Fellow Rulers'),
('Canute','Fellow Ruler')
;
The number of values and the actual values would vary.
SQL Just for demonstration of the result
SELECT * FROM groups;
SELECT contact,group_name FROM contact JOIN groups ON group_reference = groups.id;
Results
This results in :-
1) The groups (noting that the group "NOTASSIGNED", is intrinsic to the working of the above and hence added initially) :-
have to be careful regard mistakes like (Fellow Ruler instead of Fellow Rulers)
-1 used because it would not be a normal value automatically generated.
2) The contacts with the respective group :-
Efficient insertion
That could likely be debated from here to eternity so I leave it for the fence sitters/destroyers to decide :). However, some considerations:-
It works and appears to do what is wanted.
It's a little wasteful due to the additional wasted column.
It tries to minimise the waste by changing the column to an empty string (NULL may be even more efficient, but for some can be confusing)
There will obviously be an overhead BUT in comparison to the alternatives probably negligible (perhaps important if you were extracting every Facebook user) but if it's user input driven likely irrelevant.
What is the best practice here?
Fences again. :)
Note Hopefully obvious, but the DROP statements are purely for convenience and that all other SQL up until the INSERT is run once
to setup the tables and triggers in preparation for the single INSERT
that adds a group if necessary.
I'm trying to add an auto-calculated field in SQL Server 2012 Express, that stores the % of project completion, by calculating the date difference by using:
ALTER TABLE dbo.projects
ADD PercentageCompleted AS (select COUNT(*) FROM projects WHERE project_finish > project_start) * 100 / COUNT(*)
But I am getting this error:
Msg 1046, Level 15, State 1, Line 2
Subqueries are not allowed in this context. Only scalar expressions are allowed.
What am I doing wrong?
Even if it would be possible (it isn't), it is anyway not something you would want to have as a caculated column:
it will be the same value in each row
the entire table would need to be updated after every insert/update
You should consider doing this in a stored procedure or a user defined function instead.Or even better in the business logic of your application,
I don't think you can do that. You could write a trigger to figure it out or do it as part of an update statement.
Are you storing "percentageCompleted" as a duplicated column value in the same table as your project data?
If this is the case, I would not recommend this, because it would duplicate the data.
If you don't care about duplicate data, try something separating the steps out like this:
ALTER TABLE dbo.projects
ADD PercentageCompleted decimal(2,2) --You could also store it as a varchar or char
declare #percentageVariable decimal(2,2)
select #percentageVariable = (select count(*) from projects where Project_finish > project_start) / (select count(*) from projects) -- need to get ratio by completed/total
update projects
set PercentageCompleted = #percentageVariable
this will give you a decimal value in that table, then you can format it on select if you desire to % + PercentageCompleted * 100
I have a column in SQLite of GUID type, I have tried a query like this, and it returns no error, but the row is not deleted
DELETE FROM MyTable WHERE Id='4ffbd580-b17d-4731-b162-ede8d698e026';
In SQLite Browser the Id values look like binary values, they have strange characters.
I also have tried this, but still does not work
DELETE FROM MyTable WHERE Id='{4ffbd580-b17d-4731-b162-ede8d698e026}';
I know I'm late for this, but it might just be useful for someone with the same problem.
I have a uniqueidentifier type of column in one of my tables and when I execute a select query without any conditions, it returns the result guid column values in this format -
{000B6A69-04D6-C557-7EA3-08CF8C8AD84B}
(Yes, with the braces)
I found out using typeof() function that my guid column values had been stored as text. So, I just tried out four different statements and luckily, the 4th one worked -
1. select myGuidColumn, typeof(myGuidColumn) from MyTable WHERE [myGuidColumn] = '000B6A69-04D6-C557-7EA3-08CF8C8AD84B' --didn't work
2. select myGuidColumn, typeof(myGuidColumn) from MyTable WHERE [myGuidColumn] = '{000B6A69-04D6-C557-7EA3-08CF8C8AD84B}' --didn't work
3. select myGuidColumn, typeof(myGuidColumn) from MyTable WHERE [myGuidColumn] LIKE '{000B6A69-04D6-C557-7EA3-08CF8C8AD84B}' --didn't work
4. select myGuidColumn, typeof(myGuidColumn) from MyTable WHERE [myGuidColumn] LIKE '000B6A69-04D6-C557-7EA3-08CF8C8AD84B' --it works!
Try this command. Id is a probably a binary blob field
DELETE FROM MyTable WHERE Id= X'4ffbd580b17d4731b162ede8d698e026';
I want to insert a record into a sqlite table if its actually not inserted.
Let's say it has three fields pk, name, address
I want to INSERT new record with name if that name not added preveously.
Can we do with this in a single Query. Seems like its slightly different from SQL Queries sometimes.
Yes, you can do that with a single query.
INSERT ON CONFLICT IGNORE should help you: http://www.sqlite.org/lang_conflict.html
Put a unique key on the name, this will create a conflict when you try inserting a record if the name already exists.
The default is ABORT, so without the IGNORE, the statement will return an error. If you don't want that, use IGNORE.
If you can't make use of a UNIQUE INDEX in combination with INSERT INTO or INSERT OR IGNORE INTO, you could write a query like this;
INSERT INTO table (column)
SELECT value
WHERE NOT EXISTS (SELECT 1
FROM table
WHERE column = value)
I want to get the number of selected rows as well as the selected data. At the present I have to use two sql statements:
one is
select * from XXX where XXX;
the other is
select count(*) from XXX where XXX;
Can it be realised with a single sql string?
I've checked the source code of sqlite3, and I found the function of sqlite3_changes(). But the function is only useful when the database is changed (after insert, delete or update).
Can anyone help me with this problem? Thank you very much!
SQL can't mix single-row (counting) and multi-row results (selecting data from your tables). This is a common problem with returning huge amounts of data. Here are some tips how to handle this:
Read the first N rows and tell the user "more than N rows available". Not very precise but often good enough. If you keep the cursor open, you can fetch more data when the user hits the bottom of the view (Google Reader does this)
Instead of selecting the data directly, first copy it into a temporary table. The INSERT statement will return the number of rows copied. Later, you can use the data in the temporary table to display the data. You can add a "row number" to this temporary table to make paging more simple.
Fetch the data in a background thread. This allows the user to use your application while the data grid or table fills with more data.
try this way
select (select count() from XXX) as count, *
from XXX;
select (select COUNT(0)
from xxx t1
where t1.b <= t2.b
) as 'Row Number', b from xxx t2 ORDER BY b;
just try this.
You could combine them into a single statement:
select count(*), * from XXX where XXX
or
select count(*) as MYCOUNT, * from XXX where XXX
To get the number of unique titles, you need to pass the DISTINCT clause to the COUNT function as the following statement:
SELECT
COUNT(DISTINCT column_name)
FROM
'table_name';
Source: http://www.sqlitetutorial.net/sqlite-count-function/
For those who are still looking for another method, the more elegant one I found to get the total of row was to use a CTE.
this ensure that the count is only calculated once :
WITH cnt(total) as (SELECT COUNT(*) from xxx) select * from xxx,cnt
the only drawback is if a WHERE clause is needed, it should be applied in both main query and CTE query.
In the first comment, Alttag said that there is no issue to run 2 queries. I don't agree with that unless both are part of a unique transaction. If not, the source table can be altered between the 2 queries by any INSERT or DELETE from another thread/process. In such case, the count value might be wrong.
Once you already have the select * from XXX results, you can just find the array length in your program right?
If you use sqlite3_get_table instead of prepare/step/finalize you will get all the results at once in an array ("result table"), including the numbers and names of columns, and the number of rows. Then you should free the result with sqlite3_free_table
int rows_count = 0;
while (sqlite3_step(stmt) == SQLITE_ROW)
{
rows_count++;
}
// The rows_count is available for use
sqlite3_reset(stmt); // reset the stmt for use it again
while (sqlite3_step(stmt) == SQLITE_ROW)
{
// your code in the query result
}