Let's say I have the following table in Postgres:
fruit
fruit_id owner_id fruit_price notes
-------------------------------------------
1 5 15
2 5 30
3 5 20
4 8 10
5 8 80
I am looking for a way to update the cheapest fruit owned by someone.
That is, I am looking for an operation that would allow me to set the notes column for the cheapest fruit owned by an individual. So this should only ever update one row (updating multiple rows is fine if there are several ties for the smallest value).
For example (psuedocode):
UPDATE fruit SET notes = 'wow cheap' WHERE owner_id = 5 AND fruit_price IS cheapest;
And this would update the first row in the above example data, because fruit_id of 1 is the cheapest fruit owned by user 5.
One possible way is simply to use a correlated subquery:
update fruit set
notes = 'some notes'
where owner_id = 5
and fruit_price = (
select min(fruit_price) from fruit f2
where f2.owner_id = fruit.owner_id
);
Related
I have a table in qlik Sense loaded from the database.
Example:
ID
FRUIT
VEG
COUNT
1
Apple
5
2
Figs
10
3
Carrots
20
4
Oranges
12
5
Corn
10
From this I need to make a filter that will display all the Fruit/Veg records along with records from other joined tables, when selected.
The filter needs to be something like this :
|FRUIT_XXX|
|VEG_XXX |
Any help will be appreciated.
I do not know how to do it in qlicksense, but in SQL it's like this:
SELECT
ID
CASE WHEN FRUIT IS NULL THEN VEG ELSE FRUIT END as FruitOrVeg,
COUNT
FROM tablename
Not sure if its possible to be dynamic. Usually I solve these by creating a new field that combines the values from both fields into one field
RawData:
Load * Inline [
ID , FRUIT ,VEG , COUNT
1 , Apple , , 5
2 , Figs , , 10
3 , ,Carrots , 20
4 , Oranges , , 12
5 , ,Corn , 10
];
Combined:
Load
ID,
'FRUIT_' & FRUIT as Combined
Resident
RawData
Where
FRUIT <> ''
;
Concatenate
Load
ID,
'VEG_' & VEG as Combined
Resident
RawData
Where
VEG <> ''
;
This will create new table (Combined) which will be linked to the main table by ID field:
The new Combined field will have the values like this:
And the UI:
P.S. If further processing is needed you can join the Combined table to the RawData table. This way the Combined field will become part of the RawData table. To achieve this just extend the script a bit:
join (RawData)
Load * Resident Combined;
Drop Table Combined;
Say I have a table called list, where there are items like these (the ids are random uuids):
id rank text
--- ----- -----
x 0 Hello
x 1 World
x 2 Foo
x 3 Bar
x 4 Baz
I want to maintain the property that rank column always goes from 0 to n-1 (n being the number of rows)---if a client asks to insert an item with rank = 3, then the pg server should push the current 3 and 4 to 4 and 5, respectively:
id rank text
--- ----- -----
x 0 Hello
x 1 World
x 2 Foo
x 3 New Item!
x 4 Bar
x 5 Baz
My current strategy is to have a dedicated insertion function add_item(item) that scans through the table, filter out items with rank equal or greater than that of the item being inserted, and increment those ranks by one. However, I think this approach will run into all sorts of problems---like race conditions.
Is there a more standard practice or more robust approach?
Note: The rank column is completely independent of rest of the columns, and insertion is not the only operation I need to support. Think of it as the back-end of a sortable to-do list, and the user can add/delete/reorder the items on the fly.
Doing verbatim what you suggest might be difficult or not possible at all, but I can suggest a workaround. Maintain a new column ts which stores the time a record is inserted. Then, insert the current time along with rest of the record, i.e.
id rank text ts
--- ----- ----- --------------------
x 0 Hello 2017-12-01 12:34:23
x 1 World 2017-12-03 04:20:01
x 2 Foo ...
x 3 New Item! 2017-12-12 11:26:32
x 3 Bar 2017-12-10 14:05:43
x 4 Baz ...
Now we can easily generate the ordering you want via a query:
SELECT id, rank, text,
ROW_NUMBER() OVER (ORDER BY rank, ts DESC) new_rank
FROM yourTable;
This would generate 0 to 5 ranks in the above sample table. The basic idea is to just use the already existing rank column, but to let the timestamp break the tie in ordering should the same rank appear more than once.
you can wrap it up to function if you think its worth of:
t=# with u as (
update r set rank = rank + 1 where rank >= 3
)
insert into r values('x',3,'New val!')
;
INSERT 0 1
the result:
t=# select * from r;
id | rank | text
----+------+----------
x | 0 | Hello
x | 1 | World
x | 2 | Foo
x | 3 | New val!
x | 4 | Bar
x | 5 | Baz
(6 rows)
also worth of mention you might have concurrency "chasing condition" problem on highly loaded systems. the code above is just a sample
You can have a “computed rank” which is a double precision and a “displayed rank” which is an integer that is computed using the row_number window function on output.
When a row is inserted that should rank between two rows, compute the new rank as the arithmetic mean of the two ranks.
The advantage is that you don't have to update existing rows.
The down side is that you have to calculate the displayed ranks before you can insert a new row so that you know where to insert it.
This solution (like all others) are subject to race conditions.
To deal with these, you can either use table locks or serializable transactions.
The only way to prevent a race condition would be to lock the table
https://www.postgresql.org/docs/current/sql-lock.html
Of course this would slow you down if there are lots of updates and inserts.
If can somehow limit the scope of your updates then you can do a SELECT .... FOR UPDATE on that scope. For example if the records have a parent_id you can do a select for update on the parent record first and any other insert who does the same select for update would have to wait till your transaction is done.
https://www.postgresql.org/docs/current/explicit-locking.html#:~:text=5.-,Advisory%20Locks,application%20to%20use%20them%20correctly.
Read the section on advisory locks to see if you can use those in your application. They are not enforced by the system so you'll need to be careful of how you write your application.
I have a table that needs to pull the latest date from different categories and the date might not always be filled out. I have tried to use MAX, MIN etc. it has not worked.
e.g. ID 1st Game Date 2nd Game Date 3rd Game Date
Joe 6/1/16 missing missing
Anna missing 7/2/16 7/6/16
Rita missing 7/31/16 missing
Needs to Return:
ID Date
Joe 6/1/16
Anna 7/6/16
Rita 7/31/16
I do have this sql that works well but it requires that all the dates get filled in other wise it doesn't return the latest date:
ApptDate: Switch([Pt1stApptDate]>=[2ndApptDate] And [Pt1stApptDate]>=
[3rdApptDate],[Pt1stApptDate],[2ndApptDate]>=[Pt1stApptDate] And [2ndApptDate]>=
[3rdApptDate],[2ndApptDate],[3rdApptDate]>=[Pt1stApptDate] And [3rdApptDate]>=
[2ndApptDate],[3rdApptDate])
Much appreciation in advance for all your help
Use the Nz function:
ApptDate: Switch(Nz([Pt1stApptDate],0)>=Nz([2ndApptDate],0) And
Nz([Pt1stApptDate],0)>= Nz([3rdApptDate],0), Nz([Pt1stApptDate],0),
Nz([2ndApptDate],0)>=Nz([Pt1stApptDate],0) And Nz([2ndApptDate],0)>=
Nz([3rdApptDate],0),Nz([2ndApptDate],0),
Nz([3rdApptDate],0)>=Nz([Pt1stApptDate],0) And Nz([3rdApptDate],0)>=
Nz([2ndApptDate],0),Nz([3rdApptDate],0))
Having said that, your table design is incorrect.
You should be storing each ApptDate per ID in a separate row:
ApptID ID ApptDate ApptNr
1 Joe 6/1/2016 1
2 Anna 7/2/2016 2
3 Anna 7/6/2016 3
4 Rita 7/31/2016 2
whereas ApptID is an autonumber and ApptNr is a sequence per ID (what you seem to call a category).
When you are having problems writing what should be simple queries (SQL DML) then you should consider you may have design flaws (in your SQL DDL).
The missing values are causing you to avoid the MAX set function and compels you to handle nulls in queries (note the NZ() function will cause errors outside of the Access UI). Better to model missing data by simply not adding a row to a table. Think about it: you want the smallest amount of data possible in your database, you can infer the remainder e.g. if Joe was not gaming on 1 Jan and 2 Jan and 3 Jan and 4 Jan etc then simply don't add anything to your database for all these dates.
The following SQL DDL requires ANSI-92 Query Mode (but you can create the same tables/views using the Access GUI tools):
CREATE TABLE Attendance
( gamer_name VARCHAR( 35 ) NOT NULL REFERENCES Gamers ( gamer_name ),
game_sequence NOT NULL CHECK ( game_sequence BETWEEN 1 AND 3 )
game_date DATETIME NOT NULL,
UNIQUE ( game_date, game_sequence ) );
INSERT INTO Attendance VALUES ( 'Joe', 1, '2016-06-01' );
INSERT INTO Attendance VALUES ( 'Anna', 2, '2016-07-02' );
INSERT INTO Attendance VALUES ( 'Anna', 3, '2016-07-06' );
INSERT INTO Attendance VALUES ( 'Rita', 1, '2016-07-31' );
CREATE VIEW MostRecentAttendance
AS
SELECT gamer_name, MAX ( game_date ) AS game_date
FROM Attendance
GROUP
BY gamer_name;
SELECT *
FROM Attendance a
WHERE EXISTS ( SELECT *
FROM MostRecentAttendance r
WHERE r.gamer_name = a.gamer_name
AND r.game_date = a.game_date );
To find the missing sequence values for players, create a table of all possible sequence numbers { 1, 2, 3 } to which you can 'anti-join' (e.g. NOT EXISTS).
I have 3 tables
items,
item_units,
order_items
First table items has the list of items that can be ordered.
Second table item_units has units for the items as well as the amount of those items in this unit
Third table has items that were ordered... ie... item_code , unit, qty
Here are the columns for items
[item_code]
,[desc]
Here are the columns for item_units
,[item_code]
,[unit]
,[amount]
,[price]
,[default_sprd_sht]
Here are the columns for order_items
,[order_id]
,[item_code]
,[item_desc]
,[unit]
,[qty]
,[price]
Note the [default_sprd_sht]. This field is a boolean. If it's set to true this unit is never put into order_items table. This field will be used as calculation field.
For example:
If 1 customer orders 2 6 packs of bread and another orders 3 dozens of bread, the baker needs to know how many pans of bread to make.
Now a 6 pack unit has 6 breads as amount, meaning 2 * 6 = 12. And a dozen unit has 12 breads.. 12 * 3 = 36. A pan bread unit has 20 breads. So i need to add up all the bread units amounts and divide them by the pan amount like so
((2*6) + (12 * 3)) / 20 = 2.4
So the first thing I did to create a report for the baker was
Create a group for order_items.item_code and then order_item.unit.
This needs to be done since the same item and unit cobination will be repeated in different orders. The baker needs to see how many bagels or breads he needs to bake in total.
in the order_item.unit group header I created a formula field that multiplies the order_item.unit by item.amount
Sum ({order_items.qty}, {order_items.unit}) * {items_units.amount}
That was easy.
But I aslo need to group all order items if there exists a record in the items_units with the same item_code and with [default_sprd_sht] set to true
This would look like so
(Sum ({order_items.qty}, {order_items.unit}) * {items_units.amount}) / (get amount for unit with the same item_code and [default_sprd_sht] = 1)
I have two problems accomplishing this.
How to check if this order item has a unit with same item_code and
[default_sprd_sht] = 1?
How to further group order items only if there is a unit with same
item_code and [default_sprd_sht] = 1?
How can I write a calculation field in a table that outputs '1' if there are other (related) records in the same table that meet a given set of criteria and '0' otherwise?
Here's my problem explained in more detail:
I have a table containing 'students' and another containing 'exam results'. The 'exam results' table looks like this:
StudentID SubjectID Level Result
3234 1 2 A-
3234 2 4 B+
4739 1 4 C+
A student can only pass a Level 4 exam in subject 2 if they have also passed a Level 2 exam in subject 1 with a B+ or higher. I want to define a field in the 'students' table that contains a '1' if there exists an exam result belonging to the right student that meets these criteria and a '0' otherwise.
What would be the best way to do this?
Let us take an example of a Results table where the results are also calculated as a numeric value, e.g.
StudentID SubjectID Level Result cResultNum
3234 1 2 A- 95
3234 2 4 B+ 85
4739 1 4 C+ 75
and an Exams table with the following fields (among others):
RequiredSubjectID
RequiredLevel
RequiredResultNum
Given these, you can construct a relationship between Exams and (another occurrence of) Results as:
Exams::RequiredSubjectID = Results 2::SubjectID
AND
Exams::RequiredLevel = Results 2::Level
AND
Exams::RequiredResultNum ≤ Results 2::cResultNum
This allows each exam record to calculate a list of students that are eligible to take that exam as =
List ( Results 2::StudentID )
I want to define a field in the 'students' table that contains a '1'
if there exists an exam result belonging to the right student that
meets these criteria and a '0' otherwise.
This request is unclear, because there are many exams a student may want to take, and a field in the Students table can calculate only one result.
You need to do a self-join in the table for the field you want to check, for example:
Exam::Level = Exam2::Level
Exam::Student = Exam2::Student
And for the "was passed" criteria I think you could do an "If" on the calculation like this:
If ( Last(Exam2::Result) = "D" and ...(all the pass values) ; 1 ; 0 )
Edit:
It could be just with the not pass value hehe I miss that it will be like this:
If ( Last(Exam2::Result) = "F" ; 0 ; 1 )
I hope this helps you.