This is too easy, if you have Id column and Value column which has duplicate rows. But in the interview i had been asked how to remove it, if you have only Value column. For example:
table_a input:
Value
A
A
B
A
C
D
D
E
F
F
E
table_a output:
Value
A
B
C
D
E
F
Question: You have table with only one column Value and you have to delete all rows, which have duplicates (as in result upper).
if you are allowed to use CTE:
with cte as (
select
row_number() over(partition by Value order by Value) as row_num,
Value
from Table1
)
delete from cte where row_num > 1
sql fiddle demo
as t-clausen.dk suggested in comments, you don't even need value inside the CTE:
with cte as (
select
row_number() over(partition by Value order by Value) as row_num
from Table1
)
delete from cte where row_num > 1;
Well, gow about using a CTE
A common table expression (CTE) can be thought of as a temporary
result set that is defined within the execution scope of a single
SELECT, INSERT, UPDATE, DELETE, or CREATE VIEW statement. A CTE is
similar to a derived table in that it is not stored as an object and
lasts only for the duration of the query. Unlike a derived table, a
CTE can be self-referencing and can be referenced multiple times in
the same query.
and ROW_NUMBER.
Returns the sequential number of a row within a partition of a result
set, starting at 1 for the first row in each partition.
Something like
;WITH Vals AS (
SELECT [Value],
ROW_NUMBER() OVER(PARTITION BY [Value] ORDER BY [Value]) RowID
FROM MyTable
)
DELETE
FROM Vals
WHERE RowID > 1
select distinct value
from your_table
(not relevant anymore as the question has been updated by the user, see Roman Pekar's answer)
Related
I need to calculate the difference of a column between two lines of a table. Is there any way I can do this directly in SQL? I'm using Microsoft SQL Server 2008.
I'm looking for something like this:
SELECT value - (previous.value) FROM table
Imagining that the "previous" variable reference the latest selected row. Of course with a select like that I will end up with n-1 rows selected in a table with n rows, that's not a probably, actually is exactly what I need.
Is that possible in some way?
Use the lag function:
SELECT value - lag(value) OVER (ORDER BY Id) FROM table
Sequences used for Ids can skip values, so Id-1 does not always work.
SQL has no built in notion of order, so you need to order by some column for this to be meaningful. Something like this:
select t1.value - t2.value from table t1, table t2
where t1.primaryKey = t2.primaryKey - 1
If you know how to order things but not how to get the previous value given the current one (EG, you want to order alphabetically) then I don't know of a way to do that in standard SQL, but most SQL implementations will have extensions to do it.
Here is a way for SQL server that works if you can order rows such that each one is distinct:
select rank() OVER (ORDER BY id) as 'Rank', value into temp1 from t
select t1.value - t2.value from temp1 t1, temp1 t2
where t1.Rank = t2.Rank - 1
drop table temp1
If you need to break ties, you can add as many columns as necessary to the ORDER BY.
WITH CTE AS (
SELECT
rownum = ROW_NUMBER() OVER (ORDER BY columns_to_order_by),
value
FROM table
)
SELECT
curr.value - prev.value
FROM CTE cur
INNER JOIN CTE prev on prev.rownum = cur.rownum - 1
Oracle, PostgreSQL, SQL Server and many more RDBMS engines have analytic functions called LAG and LEAD that do this very thing.
In SQL Server prior to 2012 you'd need to do the following:
SELECT value - (
SELECT TOP 1 value
FROM mytable m2
WHERE m2.col1 < m1.col1 OR (m2.col1 = m1.col1 AND m2.pk < m1.pk)
ORDER BY
col1, pk
)
FROM mytable m1
ORDER BY
col1, pk
, where COL1 is the column you are ordering by.
Having an index on (COL1, PK) will greatly improve this query.
LEFT JOIN the table to itself, with the join condition worked out so the row matched in the joined version of the table is one row previous, for your particular definition of "previous".
Update: At first I was thinking you would want to keep all rows, with NULLs for the condition where there was no previous row. Reading it again you just want that rows culled, so you should an inner join rather than a left join.
Update:
Newer versions of Sql Server also have the LAG and LEAD Windowing functions that can be used for this, too.
select t2.col from (
select col,MAX(ID) id from
(
select ROW_NUMBER() over(PARTITION by col order by col) id ,col from testtab t1) as t1
group by col) as t2
The selected answer will only work if there are no gaps in the sequence. However if you are using an autogenerated id, there are likely to be gaps in the sequence due to inserts that were rolled back.
This method should work if you have gaps
declare #temp (value int, primaryKey int, tempid int identity)
insert value, primarykey from mytable order by primarykey
select t1.value - t2.value from #temp t1
join #temp t2
on t1.tempid = t2.tempid - 1
Another way to refer to the previous row in an SQL query is to use a recursive common table expression (CTE):
CREATE TABLE t (counter INTEGER);
INSERT INTO t VALUES (1),(2),(3),(4),(5);
WITH cte(counter, previous, difference) AS (
-- Anchor query
SELECT MIN(counter), 0, MIN(counter)
FROM t
UNION ALL
-- Recursive query
SELECT t.counter, cte.counter, t.counter - cte.counter
FROM t JOIN cte ON cte.counter = t.counter - 1
)
SELECT counter, previous, difference
FROM cte
ORDER BY counter;
Result:
counter
previous
difference
1
0
1
2
1
1
3
2
1
4
3
1
5
4
1
The anchor query generates the first row of the common table expression cte where it sets cte.counter to column t.counter in the first row of table t, cte.previous to 0, and cte.difference to the first row of t.counter.
The recursive query joins each row of common table expression cte to the previous row of table t. In the recursive query, cte.counter refers to t.counter in each row of table t, cte.previous refers to cte.counter in the previous row of cte, and t.counter - cte.counter refers to the difference between these two columns.
Note that a recursive CTE is more flexible than the LAG and LEAD functions because a row can refer to any arbitrary result of a previous row. (A recursive function or process is one where the input of the process is the output of the previous iteration of that process, except the first input which is a constant.)
I tested this query at SQLite Online.
You can use the following funtion to get current row value and previous row value:
SELECT value,
min(value) over (order by id rows between 1 preceding and 1
preceding) as value_prev
FROM table
Then you can just select value - value_prev from that select and get your answer
There is a table with tow columns(ID, Data) and there are 3 rows with same value.
ID Data
4 192.168.0.22
4 192.168.0.22
4 192.168.0.22
Now I want to change third row DATA column. In update SQL Server Generate an error that I ca not change the value.
I can delete all 3 rows. But I can not delete third row separately.
This table is for a software that I bought and I changed the third Server IP.
You can try the following query
create table #tblSimilarValues(id int, ipaddress varchar(20))
insert into #tblSimilarValues values (4, '192.168.0.22'),
(4, '192.168.0.22'),(4, '192.168.0.22')
Use Below query if you want to change all rows
with oldData as (
select *,
count(*) over (partition by id, ipaddress) as cnt
from #tblSimilarValues
)
update oldData
set ipaddress = '192.168.0.22_1'
where cnt > 1;
select * from #tblSimilarValues
Use Below query if you want to skip firs row
;with oldData as (
select *,
ROW_NUMBER () over (partition by id, ipaddress order by id, ipaddress) as cnt
from #tblSimilarValues
)
update oldData
set ipaddress = '192.168.0.22_2'
where cnt > 1;
select * from #tblSimilarValues
drop table #tblSimilarValues
You can find the live demo live demo here
Since there is no column that allows us to distinguish these rows from each other, there's no "third row" (nor a first or second one for that matter).
We can use a ROW_NUMBER function to apply arbitrary row numbers to these rows, however, and if we place that in a CTE, we can apply DELETE/UPDATE actions via the CTE and use the arbitrary row numbers:
declare #t table (ID int not null, Data varchar(15))
insert into #t(ID,Data) values
(4,'192.168.0.22'),
(4,'192.168.0.22'),
(4,'192.168.0.22')
;With ArbitraryAssignments as (
select *,ROW_NUMBER() OVER (PARTITION BY ID, Data ORDER BY Data) as rn
from #t
)
delete from ArbitraryAssignments where rn > 2
select * from #t
This produces two rows of output - one row was deleted.
Note that I say that the ROW_NUMBER is arbitrary. One of the expressions in both the PARTITION BY and ORDER BY clauses is the same. By definition, then, we know that no real ORDER is defined by this (because all rows within the same partition, by definition, have the same value for that expression).
In this case ID columns allows duplicate value which is wrong, ID should be unique.
Now what you can do is create a new column make that unique or Primary Key or change the duplicate values of ID column and make it Unique/Primary key.
Now as per your Unique key/Primary key you can update DATA column value by query as below:
UPDATE <Table Name>
SET DATA = 'new data'
WHERE ID = 3;
I was wondering is there a way to retrieve, for example, 2nd and 5th row from SQL table that contains 100 rows?
I saw some solutions with WHERE clause but they all assume that the column on which WHERE clause is applied is linear, starting at 1.
Is there other way to query a SQL Server table for a specific rows in case table doesn't have a column whose values start at 1?
P.S. - I know for a solution with temporary tables, where you copy your select statement output and add a linear column to the table. I am using T-SQL
Try this,
SELECT * FROM (
SELECT
ROW_NUMBER() OVER (ORDER BY ColumnName ASC) AS rownumber
FROM TableName
) as temptablename
WHERE rownumber IN (2,5)
With SQL Server:
; WITH Base AS (
SELECT *, ROW_NUMBER() OVER (ORDER BY id) RN FROM YourTable
)
SELECT *
FROM Base WHERE RN IN (2, 5)
The id that you'll have to replace with your primary key or your ordering, YourTable that is your table.
It's a CTE (Common Table Expression) so it isn't a temporary table. It's something that will be expanded together with your query.
There is no 2nd or 5th row in the table.
There is only the 2nd or 5th result in a resultset that you return, as determined by the order you specify in that query.
If you are on SQL Server 2005 or above, you could use Row_Number() function. Ex:
;With CTE as (
select col1, ..., row_number() over (order by yourOrderingCol) rn
from yourTable
)
select col1,...
from cte
where rn in (2,5)
Please note that yourOrderingCol will decide the value of row number (i.e. rn).
I need to calculate the difference of a column between two lines of a table. Is there any way I can do this directly in SQL? I'm using Microsoft SQL Server 2008.
I'm looking for something like this:
SELECT value - (previous.value) FROM table
Imagining that the "previous" variable reference the latest selected row. Of course with a select like that I will end up with n-1 rows selected in a table with n rows, that's not a probably, actually is exactly what I need.
Is that possible in some way?
Use the lag function:
SELECT value - lag(value) OVER (ORDER BY Id) FROM table
Sequences used for Ids can skip values, so Id-1 does not always work.
SQL has no built in notion of order, so you need to order by some column for this to be meaningful. Something like this:
select t1.value - t2.value from table t1, table t2
where t1.primaryKey = t2.primaryKey - 1
If you know how to order things but not how to get the previous value given the current one (EG, you want to order alphabetically) then I don't know of a way to do that in standard SQL, but most SQL implementations will have extensions to do it.
Here is a way for SQL server that works if you can order rows such that each one is distinct:
select rank() OVER (ORDER BY id) as 'Rank', value into temp1 from t
select t1.value - t2.value from temp1 t1, temp1 t2
where t1.Rank = t2.Rank - 1
drop table temp1
If you need to break ties, you can add as many columns as necessary to the ORDER BY.
WITH CTE AS (
SELECT
rownum = ROW_NUMBER() OVER (ORDER BY columns_to_order_by),
value
FROM table
)
SELECT
curr.value - prev.value
FROM CTE cur
INNER JOIN CTE prev on prev.rownum = cur.rownum - 1
Oracle, PostgreSQL, SQL Server and many more RDBMS engines have analytic functions called LAG and LEAD that do this very thing.
In SQL Server prior to 2012 you'd need to do the following:
SELECT value - (
SELECT TOP 1 value
FROM mytable m2
WHERE m2.col1 < m1.col1 OR (m2.col1 = m1.col1 AND m2.pk < m1.pk)
ORDER BY
col1, pk
)
FROM mytable m1
ORDER BY
col1, pk
, where COL1 is the column you are ordering by.
Having an index on (COL1, PK) will greatly improve this query.
LEFT JOIN the table to itself, with the join condition worked out so the row matched in the joined version of the table is one row previous, for your particular definition of "previous".
Update: At first I was thinking you would want to keep all rows, with NULLs for the condition where there was no previous row. Reading it again you just want that rows culled, so you should an inner join rather than a left join.
Update:
Newer versions of Sql Server also have the LAG and LEAD Windowing functions that can be used for this, too.
select t2.col from (
select col,MAX(ID) id from
(
select ROW_NUMBER() over(PARTITION by col order by col) id ,col from testtab t1) as t1
group by col) as t2
The selected answer will only work if there are no gaps in the sequence. However if you are using an autogenerated id, there are likely to be gaps in the sequence due to inserts that were rolled back.
This method should work if you have gaps
declare #temp (value int, primaryKey int, tempid int identity)
insert value, primarykey from mytable order by primarykey
select t1.value - t2.value from #temp t1
join #temp t2
on t1.tempid = t2.tempid - 1
Another way to refer to the previous row in an SQL query is to use a recursive common table expression (CTE):
CREATE TABLE t (counter INTEGER);
INSERT INTO t VALUES (1),(2),(3),(4),(5);
WITH cte(counter, previous, difference) AS (
-- Anchor query
SELECT MIN(counter), 0, MIN(counter)
FROM t
UNION ALL
-- Recursive query
SELECT t.counter, cte.counter, t.counter - cte.counter
FROM t JOIN cte ON cte.counter = t.counter - 1
)
SELECT counter, previous, difference
FROM cte
ORDER BY counter;
Result:
counter
previous
difference
1
0
1
2
1
1
3
2
1
4
3
1
5
4
1
The anchor query generates the first row of the common table expression cte where it sets cte.counter to column t.counter in the first row of table t, cte.previous to 0, and cte.difference to the first row of t.counter.
The recursive query joins each row of common table expression cte to the previous row of table t. In the recursive query, cte.counter refers to t.counter in each row of table t, cte.previous refers to cte.counter in the previous row of cte, and t.counter - cte.counter refers to the difference between these two columns.
Note that a recursive CTE is more flexible than the LAG and LEAD functions because a row can refer to any arbitrary result of a previous row. (A recursive function or process is one where the input of the process is the output of the previous iteration of that process, except the first input which is a constant.)
I tested this query at SQLite Online.
You can use the following funtion to get current row value and previous row value:
SELECT value,
min(value) over (order by id rows between 1 preceding and 1
preceding) as value_prev
FROM table
Then you can just select value - value_prev from that select and get your answer
I need, if possible, a t-sql query that, returning the values from an arbitrary table, also returns a incremental integer column with value = 1 for the first row, 2 for the second, and so on.
This column does not actually resides in any table, and must be strictly incremental, because the ORDER BY clause could sort the rows of the table and I want the incremental row in perfect shape always.
The solution must run on SQL Server 2000
For SQL 2005 and up
SELECT ROW_NUMBER() OVER( ORDER BY SomeColumn ) AS 'rownumber',*
FROM YourTable
for 2000 you need to do something like this
SELECT IDENTITY(INT, 1,1) AS Rank ,VALUE
INTO #Ranks FROM YourTable WHERE 1=0
INSERT INTO #Ranks
SELECT SomeColumn FROM YourTable
ORDER BY SomeColumn
SELECT * FROM #Ranks
Order By Ranks
see also here Row Number
You can start with a custom number and increment from there, for example you want to add a cheque number for each payment you can do:
select #StartChequeNumber = 3446;
SELECT
((ROW_NUMBER() OVER(ORDER BY AnyColumn)) + #StartChequeNumber ) AS 'ChequeNumber'
,* FROM YourTable
will give the correct cheque number for each row.
Try ROW_NUMBER()
http://msdn.microsoft.com/en-us/library/ms186734.aspx
Example:
SELECT
col1,
col2,
ROW_NUMBER() OVER (ORDER BY col1) AS rownum
FROM tbl
It is ugly and performs badly, but technically this works on any table with at least one unique field AND works in SQL 2000.
SELECT (SELECT COUNT(*) FROM myTable T1 WHERE T1.UniqueField<=T2.UniqueField) as RowNum, T2.OtherField
FROM myTable T2
ORDER By T2.UniqueField
Note: If you use this approach and add a WHERE clause to the outer SELECT, you have to added it to the inner SELECT also if you want the numbers to be continuous.