I need to create a view that automatically adds virtual row number in the result. the graph here is totally random all that I want to achieve is the last column to be created dynamically.
> +--------+------------+-----+
> | id | variety | num |
> +--------+------------+-----+
> | 234 | fuji | 1 |
> | 4356 | gala | 2 |
> | 343245 | limbertwig | 3 |
> | 224 | bing | 4 |
> | 4545 | chelan | 5 |
> | 3455 | navel | 6 |
> | 4534345| valencia | 7 |
> | 3451 | bartlett | 8 |
> | 3452 | bradford | 9 |
> +--------+------------+-----+
Query:
SELECT id,
variety,
SOMEFUNCTIONTHATWOULDGENERATETHIS() AS num
FROM mytable
Use:
SELECT t.id,
t.variety,
(SELECT COUNT(*) FROM TABLE WHERE id < t.id) +1 AS NUM
FROM TABLE t
It's not an ideal manner of doing this, because the query for the num value will execute for every row returned. A better idea would be to create a NUMBERS table, with a single column containing a number starting at one that increments to an outrageously large number, and then join & reference the NUMBERS table in a manner similar to the variable example that follows.
MySQL Ranking, or Lack Thereof
You can define a variable in order to get psuedo row number functionality, because MySQL doesn't have any ranking functions:
SELECT t.id,
t.variety,
#rownum := #rownum + 1 AS num
FROM TABLE t,
(SELECT #rownum := 0) r
The SELECT #rownum := 0 defines the variable, and sets it to zero.
The r is a subquery/table alias, because you'll get an error in MySQL if you don't define an alias for a subquery, even if you don't use it.
Can't Use A Variable in a MySQL View
If you do, you'll get the 1351 error, because you can't use a variable in a view due to design. The bug/feature behavior is documented here.
Oracle has a rowid pseudo-column. In MySQL, you might have to go ugly:
SELECT id,
variety,
1 + (SELECT COUNT(*) FROM tbl WHERE t.id < id) as num
FROM tbl
This query is off the top of my head and untested, so take it with a grain of salt. Also, it assumes that you want to number the rows according to some sort criteria (id in this case), rather than the arbitrary numbering shown in the question.
Related
+----+------+-------+---------+---------+
| id | order| value | type | account |
+----+------+-------+---------+---------+
| 1 | 1 | a | 2 | 1 |
| 1 | 2 | b | 1 | 1 |
| 1 | 3 | c | 4 | 1 |
| 1 | 4 | d | 2 | 1 |
| 1 | 5 | e | 1 | 1 |
| 1 | 5 | f | 6 | 1 |
| 2 | 6 | g | 1 | 1 |
+----+------+-------+---------+---------+
I need get a select of all fields of this table but only getting 1 row for each combination of id+type (I don't care the value of the type). But I tried some approach without result.
At the moment that I make an DISTINCT I cant include rest of the fields to make it available in a subquery. If I add ROWNUM in the subquery all rows will be different making this not working.
Some ideas?
My better query at the moment is this:
SELECT ID, TYPE, VALUE, ACCOUNT
FROM MYTABLE
WHERE ROWID IN (SELECT DISTINCT MAX(ROWID)
FROM MYTABLE
GROUP BY ID, TYPE);
It seems you need to select one (random) row for each distinct combination of id and type. If so, you could do that efficiently using the row_number analytic function. Something like this:
select id, type, value, account
from (
select id, type, value, account,
row_number() over (partition by id, type order by null) as rn
from your_table
)
where rn = 1
;
order by null means random ordering of rows within each group (partition) by (id, type); this means that the ordering step, which is usually time-consuming, will be trivial in this case. Also, Oracle optimizes such queries (for the filter rn = 1).
Or, in versions 12.1 and higher, you can get the same with the match_recognize clause:
select id, type, value, account
from my_table
match_recognize (
partition by id, type
all rows per match
pattern (^r)
define r as null is null
);
This partitions the rows by id and type, it doesn't order them (which means random ordering), and selects just the "first" row from each partition. Note that some analytic functions, including row_number(), require an order by clause (even when we don't care about the ordering) - order by null is customary, but it can't be left out completely. By contrast, in match_recognize you can leave out the order by clause (the default is "random order"). On the other hand, you can't leave out the define clause, even if it imposes no conditions whatsoever. Why Oracle doesn't use a default for that clause too, only Oracle knows.
I've got a table that looks like this:
player_id | violation
---------------------
1 | A
1 | A
1 | B
2 | C
3 | D
3 | A
And I want to turn it into this, with a bunch of new columns that refer to the types of violations, and then the sum of the number of each individual type of violation that each player got (not that concerned with what the columns are called; a/b/c/d would work great as well):
player_id | violation_a | violation_b | violation_c | violation_d
-----------------------------------------------------------------
1 | 2 | 1 | 0 | 0
2 | 0 | 0 | 1 | 0
3 | 1 | 0 | 0 | 1
I know how I could do this, but it would take a ton of lines of code, since there are in reality 100+ types of violations. Is there any way (perhaps with a tablefunc()?) that I could do this more concisely than spelling out each of the new 100+ columns that I want and the logic for them each individually?
In pure SQL I don't see how you could avoid declaring the columns yourself. You either have to create subselects or filters in every column ..
SELECT DISTINCT ON (t.player_id)
t.player_id,
count(*) FILTER (WHERE violation = 'A') AS violation_a,
count(*) FILTER (WHERE violation = 'B') AS violation_b,
count(*) FILTER (WHERE violation = 'C') AS violation_c,
count(*) FILTER (WHERE violation = 'D') AS violation_d
FROM t
GROUP BY t.player_id;
.. or create a pivot table:
SELECT *
FROM crosstab(
'SELECT player_id, t2.violation, count(*) FILTER (WHERE t.violation = t2.violation)::INT
FROM t,(SELECT DISTINCT violation FROM t) t2
GROUP BY player_id, t2.violation'
) AS ct(player_id INT,violation_a int,violation_b int,violation_c int,violation_d int);
Demo: db<>fiddle
For example, I have a table of:
id | code | name | type | deviceType
---+------+------+------+-----------
1 | 23 | xyz | 0 | web
2 | 23 | xyz | 0 | mobile
3 | 24 | xyzc | 0 | web
4 | 25 | xyzc | 0 | web
I want the result to be:
id | code | name | type | deviceType
---+------+------+------+-----------
1 | 23 | xyz | 0 | web&mobile
2 | 24 | xyzc | 0 | web
3 | 25 | xyzc | 0 | web
How do I do this in SQL Server using UPDATE and DELETE statements?
Any help is greatly appreciated!
I might actually suggest just leaving the original data intact, and instead creating a view here:
CREATE VIEW yourView AS
SELECT ROW_NUMBER() OVER (ORDER BY MIN(id)) AS id,
code, name, type,
STRING_AGG(deviceType, '&') WITHIN GROUP (ORDER BY id) AS deviceType
FROM yourTable
GROUP BY code, name, type;
Demo
One main reason for not actually doing the update is that every time new data comes in, you might possibly have to run that update, over and over. Instead, just keeping the original data and running the view occasionally might perform better here.
Note that I assume that you are using SQL Server 2017 or later. If not, then STRING_AGG would have to be replaced with an uglier approach, but you should consider upgrading in this case.
To do what you want, you would need two separate statements.
This updates the "first" row of each group with all the device types in the group:
update t
set t.devicetype = t1.devicetype
from mytable t
inner join (
select min(id) as id, string_agg(devicetype, '&') within group(order by id) as devicetype
from mytable
group by code, name, type
having count(*) > 1
) t1 on t1.id = t.id
This deletes everything but the first row per group:
with t as (
select row_number() over(partition by code, name, type order by id) rn
from mytable
)
delete from t where rn > 1
Demo on DB Fiddle
I have event data that looks like this:
id | instance_id | value
1 | 1 | a
2 | 1 | ap
3 | 1 | app
4 | 1 | appl
5 | 2 | b
6 | 2 | bo
7 | 1 | apple
8 | 2 | boa
9 | 2 | boat
10 | 2 | boa
11 | 1 | appl
12 | 1 | apply
Basically, each row is a user typing a new letter. They can also delete letters.
I'd like to create a dataset that looks like this, let's call it data
id | instance_id | value
7 | 1 | apple
9 | 2 | boat
12 | 1 | apply
My goal is to extract all the complete words in each instance, accounting for deletion as well - so it's not sufficient to just get the longest word or the most recently typed.
To do so, I was planning to do a regex operation like so:
select * from data
where not exists (select * from data d2 where d2.value ~ (d.value || '.'))
Effectively I'm trying to build a dynamic regex that adds matches one character more than is present, and is specific to the row it's matching against.
The code above doesn't seem to work. In Python, I can "compile" a regex pattern before I use it. What is the equivalent in PostgreSQL to dynamically build a pattern?
Try simple LIKE operator instead of regex patterns:
SELECT * FROM data d1
WHERE NOT EXISTS (
SELECT * FROM data d2
WHERE d2.value LIKE d1.value ||'_%'
)
Demo: https://dbfiddle.uk/?rdbms=postgres_9.6&fiddle=cd064c92565639576ff456dbe0cd5f39
Create an index on value column, this should speed up the query a bit.
To find peaks in the sequential data window functions is a good choice. You just need to compare each value with previous and next ones using lag() and lead() functions:
with cte as (
select
*,
length(value) > coalesce(length(lead(value) over (partition by instance_id order by id)),0) and
length(value) > coalesce(length(lag(value) over (partition by instance_id order by id)),length(value)) as is_peak
from data)
select * from cte where is_peak order by id;
Demo
I have performing some queries using PostgreSQL SELECT DISTINCT ON syntax. I would like to have the query return the total number of rows alongside with every result row.
Assume I have a table my_table like the following:
CREATE TABLE my_table(
id int,
my_field text,
id_reference bigint
);
I then have a couple of values:
id | my_field | id_reference
----+----------+--------------
1 | a | 1
1 | b | 2
2 | a | 3
2 | c | 4
3 | x | 5
Basically my_table contains some versioned data. The id_reference is a reference to a global version of the database. Every change to the database will increase the global version number and changes will always add new rows to the tables (instead of updating/deleting values) and they will insert the new version number.
My goal is to perform a query that will only retrieve the latest values in the table, alongside with the total number of rows.
For example, in the above case I would like to retrieve the following output:
| total | id | my_field | id_reference |
+-------+----+----------+--------------+
| 3 | 1 | b | 2 |
+-------+----+----------+--------------+
| 3 | 2 | c | 4 |
+-------+----+----------+--------------+
| 3 | 3 | x | 5 |
+-------+----+----------+--------------+
My attemp is the following:
select distinct on (id)
count(*) over () as total,
*
from my_table
order by id, id_reference desc
This returns almost the correct output, except that total is the number of rows in my_table instead of being the number of rows of the resulting query:
total | id | my_field | id_reference
-------+----+----------+--------------
5 | 1 | b | 2
5 | 2 | c | 4
5 | 3 | x | 5
(3 rows)
As you can see it has 5 instead of the expected 3.
I can fix this by using a subquery and count as an aggregate function:
with my_values as (
select distinct on (id)
*
from my_table
order by id, id_reference desc
)
select count(*) over (), * from my_values
Which produces my expected output.
My question: is there a way to avoid using this subquery and have something similar to count(*) over () return the result I want?
You are looking at my_table 3 ways:
to find the latest id_reference for each id
to find my_field for the latest id_reference for each id
to count the distinct number of ids in the table
I therefore prefer this solution:
select
c.id_count as total,
a.id,
a.my_field,
b.max_id_reference
from
my_table a
join
(
select
id,
max(id_reference) as max_id_reference
from
my_table
group by
id
) b
on
a.id = b.id and
a.id_reference = b.max_id_reference
join
(
select
count(distinct id) as id_count
from
my_table
) c
on true;
This is a bit longer (especially the long thin way I write SQL) but it makes it clear what is happening. If you come back to it in a few months time (somebody usually does) then it will take less time to understand what is going on.
The "on true" at the end is a deliberate cartesian product because there can only ever be exactly one result from the subquery "c" and you do want a cartesian product with that.
There is nothing necessarily wrong with subqueries.