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
Using Oracle 10G
Say for example I have a table with three fields in it, I'd like one query which selects the counts of each column where they are not null. Field name
----------------------------------
| strTest1 | strTest2 | strTest3 |
----------------------------------
I know how to get the count of each one individually:
select count(*) from tablename where strTest1 is not null
but I'd like to know if it's possible to do this within one query for all 3 fields.
Thanks
It sounds like you want:
SELECT COUNT(STRTEST1), COUNT(STRTEST2), COUNT(STRTEST3) FROM YOUR_TABLE
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
I'm using Postgres and cannot manage to get the last record of my table:
my_query = client.query("SELECT timestamp,value,card from my_table");
How can I do that knowning that timestamp is a unique identifier of the record ?
If under "last record" you mean the record which has the latest timestamp value, then try this:
my_query = client.query("
SELECT TIMESTAMP,
value,
card
FROM my_table
ORDER BY TIMESTAMP DESC
LIMIT 1
");
you can use
SELECT timestamp, value, card
FROM my_table
ORDER BY timestamp DESC
LIMIT 1
assuming you want also to sort by timestamp?
Easy way: ORDER BY in conjunction with LIMIT
SELECT timestamp, value, card
FROM my_table
ORDER BY timestamp DESC
LIMIT 1;
However, LIMIT is not standard and as stated by Wikipedia, The SQL standard's core functionality does not explicitly define a default sort order for Nulls.. Finally, only one row is returned when several records share the maximum timestamp.
Relational way:
The typical way of doing this is to check that no row has a higher timestamp than any row we retrieve.
SELECT timestamp, value, card
FROM my_table t1
WHERE NOT EXISTS (
SELECT *
FROM my_table t2
WHERE t2.timestamp > t1.timestamp
);
It is my favorite solution, and the one I tend to use. The drawback is that our intent is not immediately clear when having a glimpse on this query.
Instructive way: MAX
To circumvent this, one can use MAX in the subquery instead of the correlation.
SELECT timestamp, value, card
FROM my_table
WHERE timestamp = (
SELECT MAX(timestamp)
FROM my_table
);
But without an index, two passes on the data will be necessary whereas the previous query can find the solution with only one scan. That said, we should not take performances into consideration when designing queries unless necessary, as we can expect optimizers to improve over time. However this particular kind of query is quite used.
Show off way: Windowing functions
I don't recommend doing this, but maybe you can make a good impression on your boss or something ;-)
SELECT DISTINCT
first_value(timestamp) OVER w,
first_value(value) OVER w,
first_value(card) OVER w
FROM my_table
WINDOW w AS (ORDER BY timestamp DESC);
Actually this has the virtue of showing that a simple query can be expressed in a wide variety of ways (there are several others I can think of), and that picking one or the other form should be done according to several criteria such as:
portability (Relational/Instructive ways)
efficiency (Relational way)
expressiveness (Easy/Instructive way)
If your table has no Id such as integer auto-increment, and no timestamp, you can still get the last row of a table with the following query.
select * from <tablename> offset ((select count(*) from <tablename>)-1)
For example, that could allow you to search through an updated flat file, find/confirm where the previous version ended, and copy the remaining lines to your table.
The last inserted record can be queried using this assuming you have the "id" as the primary key:
SELECT timestamp,value,card FROM my_table WHERE id=(select max(id) from my_table)
Assuming every new row inserted will use the highest integer value for the table's id.
If you accept a tip, create an id in this table like serial. The default of this field will be:
nextval('table_name_field_seq'::regclass).
So, you use a query to call the last register. Using your example:
pg_query($connection, "SELECT currval('table_name_field_seq') AS id;
I hope this tip helps you.
To get the last row,
Get Last row in the sorted order: In case the table has a column specifying time/primary key,
Using LIMIT clause
SELECT * FROM USERS ORDER BY CREATED_TIME DESC LIMIT 1;
Using FETCH clause - Reference
SELECT * FROM USERS ORDER BY CREATED_TIME FETCH FIRST ROW ONLY;
Get Last row in the rows insertion order: In case the table has no columns specifying time/any unique identifiers
Using CTID system column, where ctid represents the physical location of the row in a table - Reference
SELECT * FROM USERS WHERE CTID = (SELECT MAX(CTID) FROM USERS);
Consider the following table,
userid |username | createdtime |
1 | A | 1535012279455 |
2 | B | 1535042279423 | //as per created time, this is the last row
3 | C | 1535012279443 |
4 | D | 1535012212311 |
5 | E | 1535012254634 | //as per insertion order, this is the last row
The query 1 and 2 returns,
userid |username | createdtime |
2 | B | 1535042279423 |
while 3 returns,
userid |username | createdtime |
5 | E | 1535012254634 |
Note : On updating an old row, it removes the old row and updates the data and inserts as a new row in the table. So using the following query returns the tuple on which the data modification is done at the latest.
Now updating a row, using
UPDATE USERS SET USERNAME = 'Z' WHERE USERID='3'
the table becomes as,
userid |username | createdtime |
1 | A | 1535012279455 |
2 | B | 1535042279423 |
4 | D | 1535012212311 |
5 | E | 1535012254634 |
3 | Z | 1535012279443 |
Now the query 3 returns,
userid |username | createdtime |
3 | Z | 1535012279443 |
Use the following
SELECT timestamp, value, card
FROM my_table
ORDER BY timestamp DESC
LIMIT 1
These are all good answers but if you want an aggregate function to do this to grab the last row in the result set generated by an arbitrary query, there's a standard way to do this (taken from the Postgres wiki, but should work in anything conforming reasonably to the SQL standard as of a decade or more ago):
-- Create a function that always returns the last non-NULL item
CREATE OR REPLACE FUNCTION public.last_agg ( anyelement, anyelement )
RETURNS anyelement LANGUAGE SQL IMMUTABLE STRICT AS $$
SELECT $2;
$$;
-- And then wrap an aggregate around it
CREATE AGGREGATE public.LAST (
sfunc = public.last_agg,
basetype = anyelement,
stype = anyelement
);
It's usually preferable to do select ... limit 1 if you have a reasonable ordering, but this is useful if you need to do this within an aggregate and would prefer to avoid a subquery.
See also this question for a case where this is the natural answer.
The column name plays an important role in the descending order:
select <COLUMN_NAME1, COLUMN_NAME2> from >TABLENAME> ORDER BY <COLUMN_NAME THAT MENTIONS TIME> DESC LIMIT 1;
For example: The below-mentioned table(user_details) consists of the column name 'created_at' that has timestamp for the table.
SELECT userid, username FROM user_details ORDER BY created_at DESC LIMIT 1;
In Oracle SQL,
select * from (select row_number() over (order by rowid desc) rn, emp.* from emp) where rn=1;
select * from table_name LIMIT 1;
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