Suppose I have a table foo with schema (name, age). So the table looks like:
NAME | AGE
John | 20
Jane | 50
Jason | 30
Now suppose I wanted to select from this table, but I wanted to change all their ages to 20 via the select query. That is, I don't want to update the original table foo, but I want to select, something like:
SELECT name, 20 as age from foo
In other words, I want to manipulate the data on the select. Of course, my real-life example is much more complicated than this but knowing the answer to this will do. Thanks!
You basically just do what you put in your question:
SELECT name, newValue as age
from yourTable
This will give you the name from your table and then the value you specify for the other column.
See SQL Fiddle with Demo
Related
Suppose there is a table which has several identical rows. I can copy the distinct values by
SELECT DISTINCT * INTO DESTINATIONTABLE FROM SOURCETABLE
but if the table has a column named value and for the sake of simplicity its value is 1 for one particular item in that table. Now that row has another 9 duplicates. So the summation of the value column for that particular item is 10. Now I want to remove the 9 duplicates(or copy the distinct value as I mentioned) and for that item now the value should show 10 and not 1. How can this be achieved?
item| value
----+----------------
A | 1
A | 1
A | 1
A | 1
B | 1
B | 1
I want to show this as below
item| value
----+----------------
A | 4
B | 2
Thanks in advance
You can try to use SUM and group by
SELECT item,SUM(value) value
FROM T
GROUP BY item
SQLfiddle:http://sqlfiddle.com/#!18/fac26/1
[Results]:
| item | value |
|------|-------|
| A | 4 |
| B | 2 |
Broadly speaking, you can just us a sum and a GROUP BY clause.
Something like:
SELECT column1, SUM(column2) AS Count
FROM SOURCETABLE
GROUP BY column1
Here it is in action: Sum + Group By
Since your table probably isn't just two columns of data, here is a slightly more complex example showing how to do this to a larger table: SQL Fiddle
Note that I've selected my rows individually so that I can access the necessary data, rather than using
SELECT *
And I have achieved this result without the need for selecting data into another table.
EDIT 2:
Further to your comments, it sounds like you want to alter the actual data in your table rather than just querying it. There may be a more elegant way to do this, but a simple way use the above query to populate a temporary table, delete the contents of the existing table, then move all the data back. To do this in my existing example:
WITH MyQuery AS (
SELECT name, type, colour, price, SUM(number) AS number
FROM MyTable
GROUP BY name, type, colour, price
)
SELECT * INTO MyTable2 FROM MyQuery;
DELETE FROM MyTable;
INSERT INTO MyTable(name, type, colour, price, number)
SELECT * FROM MyTable2;
DROP TABLE MyTable2;
WARNING: If youre going to try this, please use a development environment first (i.e one you don't mind breaking!) to ensure it does exactly what you want it to do. It's imperative that your initial query captures ALL the data you want.
Here is the SQL Fiddle of this example in action: SQL Fiddle
this question was probably asked somewhere but I can't seem to phrase it correctly in the search to find an accurate answer.
I'm doing a query on a Postgres DB, it has quite a few joins, the results are something like this:
WON | name | item
1 Joe A
1 Joe B
2 Smith A
So one row for each entry, I need to somehow get the result back as such:
WON | name | item
1 Joe A, B
2 Smith A
This can be done in the query or with NodeJS, there are hundreds to thousands of results for the query, so getting a distinct row (WON 1) then searching the DB for all entries that match it then repeating for the rest isn't feasible, so this may be better done in Node / Javascript, but I'm somewhat new to that, what would be a (somewhat) efficient way to do this?
If there IS a way to do this in the query itself then that would be my preference though.
Thanks
A sql approach:
SELECT won, name
,STRING_AGG(item, ',' ORDER BY item) AS items
FROM myTable
GROUP BY won, name
ORDER BY won, name
You can use GROUP BY and string_agg to cancat rows, somelike this:
Create table:
CREATE TABLE test
(
won int,
name character varying(255),
item character varying(255)
);
insert into test (won, name, item) values (1,'Joe', 'A'),(1, 'Joe', 'B'),(2, 'Smith', 'A')
And do this in the query:
select won, name, string_agg(item, ',') from test group by won, name order by won
See this example in sqlFiddle
I'm selecting data from multiple tables and I also need to get maximum "timestamp" on those tables. I will need that to create custom cache control.
tbl_name tbl_surname
id | name id | surname
--------- ------------
0 | John 0 | Doe
1 | Jane 1 | Tully
... ...
I have following query:
SELECT name, surname FROM tbl_name, tbl_surname WHERE tbl_name.id = tbl_surname.id
and I need to add following info to result set:
SELECT MAX(ora_rowscn) FROM (SELECT ora_rowscn FROM tbl_name
UNION ALL
SELECT ora_rowscn FROM tbl_surname);
I was trying to use UNION but I get error - mixing group and not single group data - or something like that, I know why I cannot use the union.
I don't want to split this into 2 calls, because I need the timestamp of the current snapshot I took from DB for my cache management. And between select and the call for MAX the DB could change.
Here is result I want:
John | Doe | 123456
Jane | Tully | 123456
where 123456 is approximate time of last change (insert, update, delete) of tables tbl_name and tbl_surname.
I have read only access to DB, so I cannot create triggers, stored procedures, extra tables etc...
Thanks for any suggestions.
EDIT: The value *ora_rowscn* is assigned per block of rows. So in one table this value can differ per row. I need the maximal value from both (all) tables involved in query.
Try:
SELECT name,
surname,
max(greatest(tbl_name.ora_rowscn, tbl_surname.ora_rowscn)) over () as max_rowscn
FROM tbl_name, tbl_surname
WHERE tbl_name.id = tbl_surname.id
There's no need to aggregate here - just include both ora_rowscn values in your query and take the max:
SELECT
n.name,
n.ora_rowscn as n_ora_rowscn,
s.surname,
s.ora_rowscn as s_ora_rowscn,
greatest(n.ora_rowscn, s.ora_rowscn) as last_ora_rowscn
FROM tbl_name n
join tbl_surname s on n.id = s.id
BTW, I've replaced your old-style joins with ANSI style - better readable, IMHO.
This is staight forward I believe:
I have a table with 30,000 rows. When I SELECT DISTINCT 'location' FROM myTable it returns 21,000 rows, about what I'd expect, but it only returns that one column.
What I want is to move those to a new table, but the whole row for each match.
My best guess is something like SELECT * from (SELECT DISTINCT 'location' FROM myTable) or something like that, but it says I have a vague syntax error.
Is there a good way to grab the rest of each DISTINCT row and move it to a new table all in one go?
SELECT * FROM myTable GROUP BY `location`
or if you want to move to another table
CREATE TABLE foo AS SELECT * FROM myTable GROUP BY `location`
Distinct means for the entire row returned. So you can simply use
SELECT DISTINCT * FROM myTable GROUP BY 'location'
Using Distinct on a single column doesn't make a lot of sense. Let's say I have the following simple set
-id- -location-
1 store
2 store
3 home
if there were some sort of query that returned all columns, but just distinct on location, which row would be returned? 1 or 2? Should it just pick one at random? Because of this, DISTINCT works for all columns in the result set returned.
Well, first you need to decide what you really want returned.
The problem is that, presumably, for some of the location values in your table there are different values in the other columns even when the location value is the same:
Location OtherCol StillOtherCol
Place1 1 Fred
Place1 89 Fred
Place1 1 Joe
In that case, which of the three rows do you want to select? When you talk about a DISTINCT Location, you're condensing those three rows of different data into a single row, there's no meaning to moving the original rows from the original table into a new table since those original rows no longer exist in your DISTINCT result set. (If all the other columns are always the same for a given Location, your problem is easier: Just SELECT DISTINCT * FROM YourTable).
If you don't care which values come from the other columns you can use a (bad, IMHO) MySQL extension to SQL and do:
SELECT * FROM YourTable GROUP BY Location
which will give a result set with one row per location and values for the other columns derived from the original data in an undefined fashion.
Multiple rows with identical values in all columns don't have any sense. OK - the question might be a way to correct exactly that situation.
Considering this table, with id being the PK:
kram=# select * from foba;
id | no | name
----+----+---------------
2 | 1 | a
3 | 1 | b
4 | 2 | c
5 | 2 | a,b,c,d,e,f,g
you may extract a sample for every single no (:=location) by grouping over that column, and selecting the row with minimum PK (for example):
SELECT * FROM foba WHERE id IN (SELECT min (id) FROM foba GROUP BY no);
id | no | name
----+----+------
2 | 1 | a
4 | 2 | c
CREATE TABLE `names` ( `name` varchar(20) );
Assume the names table contains all 40 million first names of everyone living in California (for example).
SELECT count(*) as count, name FROM names GROUP BY name ORDER BY name;
How can I optimize this query?
Expected Result:
count | name
9999 | joe
9995 | mike
9990 | kate
.... | ....
2 | kal-el
You have to create an index on the name column of your table. The query is as good as it can be.
Well, what makes you think it's not already optimised? This looks like the sort of query a good database engine should be able to handle relatively easily - particularly if you've got an appropriate index on your table.
Do you actually have a bottleneck here, or are you worrying about something that might happen in the future? If it's the latter, I suggest you try it with your RDBMS (by generating dummy data), and see what happens.