I have a table contains columns 'employeename' and 'id', how can I sort the 'employeename' column following alphabetical order of the names initial?
Say the table is like this now:
employeename rid eid
Dave 1 1
Ben 4 2
Chloe 6 6
I tried the command ORDER BY, it shows what I want but when I query the data again by SELECT, the showed table data is the same as original, indicting ORDER BY does not modify the data, is this correct?
SELECT *
FROM employee
ORDER BY employeename ASC;
I expect the table data to be modified (sorted by names alphabetical order) like this:
employeename rid eid
Ben 4 2
Chloe 6 6
Dave 1 1
the showed table data is the same as original, indicting ORDER BY does not modify the data, is this correct?
Yes, this is correct. A SELECT statement does not change the data in a table. Only UPDATE, DELETE, INSERT or TRUNCATE statements will change the data.
However, your question shows a misconception on how a relational database works.
Rows in a table (of a relational database) are not sorted in any way. You can picture them as balls in a basket.
If you want to display data in a specific sort order, the only (really: the only) way to do that is to use an ORDER BY in your SELECT statement. There is no alternative to that.
Postgres allows to define a VIEW that includes an ORDER BY which might be an acceptable workaround for you:
CREATE VIEW sorted_employee;
AS
SELECT *
FROM employee
ORDER BY employeename ASC;
Then you can simply use
select *
from sorted_employees;
But be aware of the drawbacks. If you run select * from sorted_employees order by id then the data will be sorted twice. Postgres is not smart enough to remove the (useless) order by from the view's definition.
Some related questions:
Default row order in SELECT query - SQL Server 2008 vs SQL 2012
What is the default SQL result sort order with 'select *'?
Is PostgreSQL order fully guaranteed if sorting on a non-unique attribute?
Why do results from a SQL query not come back in the order I expect?
Related
I would like to organize my postgresql table in ascending order from the date it was created on.
So I tried:
SELECT *
FROM price
ORDER BY created_on;
And it did show me the database in that order, however it did not save it.
Is there a way I can make it so it gets saved?
Tables in a relational database represent unordered sets. There is no such thing as the "order of rows" in a table.
If you need a specific sort order, the only way is to use an order by in a select statement as you did.
If you don't want to type the order by each time, you can create a view that does that:
create view sorted_price
as
select *
from price
order by created_on;
But be warned: if you sort the rows from the view in a different way, e.g. select * from sorted_price order by created_on desc Postgres will actually apply two sorts. The query optimizer is unfortunately not smart enough to remove the one store in the view's definition.
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 am using a PostgreSQL database. I have one select query:
select userid, name, age from tbluser;
Now there is another table, tblcalculatedtax, which is generated on the fly and their column names are not predefined, the only mapping between that table and this table is userid. I want to get records after joining two tables. How can I get that?
Simpler:
SELECT *
FROM tbluser
JOIN tblcalculatedtax USING (userid)
Details in the fine manual about SELECT.
Your need SQL Joins. Here's a W3Schools tutorial: http://www.w3schools.com/sql/sql_join.asp
To quickly answer your question, though:
SELECT * FROM tbluser
INNER JOIN tblcalculatedtax
ON tbluser.userid=tblcalculatedtext.userid
The * selects all the columns, so you don't need to know their names. Of course, I'm not sure what use a column is to you if you don't know it's name: do you know what data it contains?
I was wondering if there is a way using TSQL join statement (or any other available option) to only display certain values. I will try and explain exactly what I mean.
My database has tables called Job, consign, dechead, decitem. Job, consign, and dechead will only ever have one line per record but decitem can have multiple records all tied to the dechead with a foreign key. I am writing a query that pulls various values from each table. This is fine with all the tables except decitem. From dechead I need to pull an invoice value and from decitem I need to grab the net wieghts. When the results are returned if dechead has multiple child decitem tables it displays all values from both tables. What I need it to do is only display the dechad values once and then all the decitems values.
e.g.
1 ¦123¦£2000¦15.00¦1
2 ¦--¦------¦20.00¦2
3 ¦--¦------¦25.00¦3
Line 1 displays values from dechead and the first line/Join from decitems. Lines 2 and 3 just display values from decitem. If I then export the query to say excel I do not have duplicate values in the first two fileds of lines 2 and 3
e.g.
1 ¦123¦£2000¦15.00¦1
2 ¦123¦£2000¦20.00¦2
3 ¦123¦£2000¦25.00¦3
Thanks in advance.
Check out 'group by' for your RDBMS http://msdn.microsoft.com/en-US/library/ms177673%28v=SQL.90%29.aspx
this is a task best left for the application, but if you must do it in sql, try this:
SELECT
CASE
WHEN RowVal=1 THEN dt.col1
ELSE NULL
END as Col1
,CASE
WHEN RowVal=1 THEN dt.col2
ELSE NULL
END as Col2
,dt.Col3
,dt.Col4
FROM (SELECT
col1, col2, col3
,ROW_NUMBER OVER(PARTITION BY Col1 ORDER BY Col1,Col4) AS RowVal
FROM ...rest of your big query here...
) dt
ORDER BY dt.col1,dt.Col4
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