So I have 2 tables
1 table User with columns Username(nvarchar) and Active(boolean)
and a second table UserCompany with column a Username(nvarchar) column.
I want to make all the Users from table one that dont exist in table 2 inactive.
Is there a way to do some kind of foreach from all the users and if the result comes out as NULL the user can be set as inactive? Searched online but coudn't find it.
You can use a condition in the update. It looks something like this:
update users u
set active = 0
where not exists (select 1 from usercompany uc where uc.username = u.username);
If you want to maintain this over time, you will need insert, update, and delete triggers on usercompany.
Related
I am creating 2 tables, one called quizes with:
id, quiz_name, plays
and the second one called quizes taken with:
session_id, quiz_id
and I would like to store count of rows, inside of plays column in table quizes, but I have no clue how to do it.
In more detail, what I am trying to achieve, is that on every update/insert into quizes_taken, in table quizes, the play" column updates with the row count of that quiz's ID from table quizes_taken.
If someone could explain how to achieve it, I'd be grateful!
Thanks in advance!
you could make a view or materialized view for the expected result output,
CREATE OR REPLACE VIEW AS quizes_live_update AS
SELECT a.id , a.quiz_name, COUNT(b.quiz_id) as plays
FROM quizes a
JOIN quizes_taken b ON a.id = b.quiz_id
GROUP BY a.id, a.quiz_name
Postgres: Create view
Although, if you need to proceed with tables approach, you can set up some upsert triggers
on quizes_taken table to perform update and insert on quizes table, plays column.
I have a situation where I have multiple (potentially hundreds) threads repeating the same task (using a java scheduled executor, if you are curious). This task entails selecting rows of changes (from a table called change) that have not yet been processed (processed changes are kept track in a m:n join table called process_change_rel that keeps track of the process id, record id and status) processing them, then updating back the status.
My question is, how is the best way to prevent two threads from the same process from selecting the same row? Will the below solution (using for update to lock rows ) work? If not, please suggest a working solution
Create table change(
—id , autogenerated pk
—other fields
)
Create table change_process_rel(
—change id (pk of change table)
—process id (pk of process table)
—status)
Query I would use is listed below
Select * from
change c
where c.id not in(select changeid from change_process_rel with cs) for update
Please let me know if this would work
You have to "lock" a row which you are going to process somehow. Such a "locking" should be concurrent of course with minimum conflicts / errors.
One way is as follows:
Create table change
(
id int not null generated always as identity
, v varchar(10)
) in userspace1;
insert into change (v) values '1', '2', '3';
Create table change_process_rel
(
id int not null
, pid int not null
, status int not null
) in userspace1;
create unique index change_process_rel1 on change_process_rel(id);
Now you should be able to run the same statement from multiple concurrent sessions:
SELECT ID
FROM NEW TABLE
(
insert into change_process_rel (id, pid, status)
select c.id, mon_get_application_handle(), 1
from change c
where not exists (select 1 from change_process_rel r where r.id = c.id)
fetch first 1 row only
with ur
);
Every such a statement inserts 1 or 0 rows into the change_process_rel table, which is used here as a "lock" table. The corresponding ID from change is returned, and you may proceed with processing of the corresponding event in the same transaction.
If the transaction completes successfully, then the row inserted into the change_process_rel table is saved, so, the corresponding id from change may be considered as processed. If the transaction fails, the corresponding "lock" row from change_process_rel disappears, and this row may be processed later by this or another application.
The problem of this method is, that when both tables become large enough, such a sub-select may not work as quick as previously.
Another method is to use Evaluate uncommitted data through lock deferral.
It requires to place the status column into the change table.
Unfortunately, Db2 for LUW doesn't have SKIP LOCKED functionality, which might help with such a sort of algorithms.
If, let's say, status=0 is "not processed", and status<>0 is some processing / processed status, then after setting these DB2_EVALUNCOMMITTED and DB2_SKIP* registry variables and restart the instance, you may "catch" the next ID for processing with the following statement.
SELECT ID
FROM NEW TABLE
(
update
(
select id, status
from change
where status=0
fetch first 1 row only
)
set status=1
);
Once you get it, you may do further processing of this ID in the same transaction as previously.
It's good to create an index for performance:
create index change1 on change(status);
and may be set this table as volatile or collect distribution statistics on this column in addition to regular statistics on table and its indexes periodically.
Note that such a registry variables setting has global effect, and you should keep it in mind...
I need to create a set of views in which a database user with select permissions will query data, but I want to create a generic view, so if other users want to query the same data, I wouldn't need to create more views based on theirs "user_id". An example would be:
CREATE OR REPLACE VIEW user_data AS SELECT * FROM users JOIN something WHERE user_id = ?
Is there any way to set that if the database user querying data is "db_user1", then the view with the above select would query using 1 as the user_id? And if the database user is db_user2 than the where user_id would be set to 2?
For this specific use case, you could do something as simple as this:
create or replace view user_data AS
select * from users
where user_id = (select substring(current_user, length(current_user))::int);
However, this will return an error if the user's name does not end with something that can be turned into an int.
You may want to look into row level security for something a bit more robust and configurable.
I have PostgreSQL table:
Username1 SomeBytes1
Username2 SomeBytes1
Username1 SomeBytes1
Username1 SomeBytes1
I need to get some rows from with name Username1 but from the end of the table. For example i need last to rows with Username1
select from my_table where user = Username1 LIMIT 2
Gives me first 2 rows, but i need last two.
How can i select it?
Thank you.
first and last in a table is very arbitrary. In order to have a good predictable result you should always have an order by clause. And if you have that, then getting the last two rows will become easy.
For instance, if you have a primary key or something like an ID (which is populated by a sequence), then you can do:
select * from my_table where user = 'Username1' order by ID desc limit 2.
desc tells the database to sort the rows in reverse order, which means that last will be first.
Does your table have a primary key ? / Can your table be sorted?
Because the notion of 'first' and 'last' implies some sorting of the tuples. If this is the case, you could sort the data the other way around, so that your 'last' entries are on top. Then you can access them with the statement you tried.
To view tail of a table you may use ctid. It is a temporary physical identifier of a record in PostgreSQL.
SELECT * from my_table
WHERE user = Username1
ORDER BY ctid DESC
LIMIT 2
I have a table in my database and I want for each row in my table to have an unique id and to have the rows named sequently.
For example: I have 10 rows, each has an id - starting from 0, ending at 9. When I remove a row from a table, lets say - row number 5, there occurs a "hole". And afterwards I add more data, but the "hole" is still there.
It is important for me to know exact number of rows and to have at every row data in order to access my table arbitrarily.
There is a way in sqlite to do it? Or do I have to manually manage removing and adding of data?
Thank you in advance,
Ilya.
It may be worth considering whether you really want to do this. Primary keys usually should not change through the lifetime of the row, and you can always find the total number of rows by running:
SELECT COUNT(*) FROM table_name;
That said, the following trigger should "roll down" every ID number whenever a delete creates a hole:
CREATE TRIGGER sequentialize_ids AFTER DELETE ON table_name FOR EACH ROW
BEGIN
UPDATE table_name SET id=id-1 WHERE id > OLD.id;
END;
I tested this on a sample database and it appears to work as advertised. If you have the following table:
id name
1 First
2 Second
3 Third
4 Fourth
And delete where id=2, afterwards the table will be:
id name
1 First
2 Third
3 Fourth
This trigger can take a long time and has very poor scaling properties (it takes longer for each row you delete and each remaining row in the table). On my computer, deleting 15 rows at the beginning of a 1000 row table took 0.26 seconds, but this will certainly be longer on an iPhone.
I strongly suggest that you re-think your design. In my opinion your asking yourself for troubles in the future (e.g. if you create another table and want to have some relations between the tables).
If you want to know the number of rows just use:
SELECT count(*) FROM table_name;
If you want to access rows in the order of id, just define this field using PRIMARY KEY constraint:
CREATE TABLE test (
id INTEGER PRIMARY KEY,
...
);
and get rows using ORDER BY clause with ASC or DESC:
SELECT * FROM table_name ORDER BY id ASC;
Sqlite creates an index for the primary key field, so this query is fast.
I think that you would be interested in reading about LIMIT and OFFSET clauses.
The best source of information is the SQLite documentation.
If you don't want to take Stephen Jennings's very clever but performance-killing approach, just query a little differently. Instead of:
SELECT * FROM mytable WHERE id = ?
Do:
SELECT * FROM mytable ORDER BY id LIMIT 1 OFFSET ?
Note that OFFSET is zero-based, so you may need to subtract 1 from the variable you're indexing in with.
If you want to reclaim deleted row ids the VACUUM command or pragma may be what you seek,
http://www.sqlite.org/faq.html#q12
http://www.sqlite.org/lang_vacuum.html
http://www.sqlite.org/pragma.html#pragma_auto_vacuum