This is a dumbed down version of the real table data, so may look bit silly.
Table 1 (users):
id INT
username TEXT
favourite_food TEXT
food_pref_id INT
Table 2 (food_preferences):
id INT
food_type TEXT
The logic is as follows:
Let's say I have this in my food preference table:
1, 'VEGETARIAN'
and this in the users table:
1, 'John', NULL, 1
2, 'Pete', 'Curry', 1
In which case John defaults to be a vegetarian, but Pete should show up as a person who enjoys curry.
Question, is there any way to combine the query into one select statement, so that it would get the default from the preferences table if the favourite_food column is NULL?
I can obviously do this in application logic, but would be nice just to offload this to SQL, if possible.
DB is SQLite3...
You could use COALESCE(X,Y,...) to select the first item that isn't NULL.
If you combine this with an inner join, you should be able to do what you want.
It should go something like this:
SELECT u.id AS id,
u.username AS username,
COALESCE(u.favorite_food, p.food_type) AS favorite_food,
u.food_pref_id AS food_pref_id
FROM users AS u INNER JOIN food_preferences AS p
ON u.food_pref_id = p.id
I don't have a SQLite database handy to test on, however, so the syntax might not be 100% correct, but it's the gist of it.
Related
I'm new to the relational database stuff and Im having a hard time understanding how to write a query to do what I want. I have two tables that have a relationship.
CREATE TABLE DocumentGroups (
id INTEGER PRIMARY KEY AUTOINCREMENT,
comments TEXT,
Username TEXT NOT NULL,
)
CREATE TABLE Documents (
id INTEGER PRIMARY KEY,
documentGroupId INT NOT NULL,
documentTypeId INT NOT NULL,
documentTypeName TEXT NOT NULL,
succesfullyUploaded BIT
)
I would like to query the Documents table and get the record count for each username. Here is the query that I came up with:
SELECT Count(*)
FROM DOCUMENTS
JOIN DocumentGroups ON Documents.documentGroupId=DocumentGroups.id
GROUP BY Username
I currently have 2 entries in the Documents table, 1 from each user. This query prints out:
[{Count(*): 1}, {Count(*): 1}]
This looks correct, but is there anyway for me to get he username associated with each count. Right now there is no way of me knowing which count belongs to each user.
You are almost there. Your query already produces one row per user name (that's your group by clause). All that is left to do is to put that column in the select clause as well:
select dg.username, count(*) cnt
from documents d
join documentgroups dg on d.documentgroupid = dg.id
group by dg.username
Side notes:
table aliases make the queries easier to read and write
in a multi-table query, always qualify all columns with the (alias of) table they belong to
you probably want to alias the result of count(*), so it is easier to consume it from your application
Imagine I have a table like such
UserID Name Hobbies
00001 Jim Baseball, Hockey, Astonomy
00002 Jack Baseball, Football, Video Games
00003 Jill Astronomy, Shopping, Soccer
00004 Jane Hockey, Astronomy, Video Games
00005 Jacob Football, Basketball, Video Games
Now, what I want to do is get a count of hobbies in common. So, let's say I plug in 00001 into a textbox or query string or whatever. I want to see something like:
Name Hobbies
Jack You have (1) hobby in common
Jill You have (1) hobby in common
Jane You have (2) hobbies in common
Jacob You have (0) hobbies in common
How would I write the code for that? I'm stumped. I'm thinking it's got to do with string matching, but I have no idea how to do that.
The first choice is to fix your data structure. Comma-delimited lists are bad, bad, bad. A separate table storing one row per person and per hobby is good, good, good.
If you are stuck with someone else's bad decisions, there is a little recourse. First Google "sql server split" and get your favorite string splitting function.
Then, you can do:
with t as (
select t.*, s.val as hobby
from table t cross apply
dbo.split(t.Hobbies, ', ') as s(val) -- Note, some `split()` implementations also have a `pos` value
)
select t.userName, count(tuser.userId) as NumInCommon
from t left join
t tuser
on t.hobby = tuser.hobby and tuser.userId = '00001'
group by t.userId, t.userName;
It is not worth constructing the full sentence in SQL, unless you really want to. Use SQL primarily to get the data you want. (Formatting in SQL can be useful sometimes, but it is really more for the application code.)
create table #temp_hobbies
(hobby_id int
,hobby varchar(50))
insert into #temp_hobbies values
(1, 'football')
,(2,'baseball')
create table #temp_people
(user_ids int,
name varchar(50),
hobby_ids int)
insert into #temp_people values
(01,'Adam',1)
,(01,'Adam',2)
,(02,'Dave',1)
,(03,'Matt',2)
select count(distinct hobby) , count(distinct name)
from #temp_hobbies a
inner join #temp_people b on a.hobby_id = b.hobby_ids
part of your solution you now need to add query that will give computed column of each user's hobby compared to other.
But per other user's try seperating hobby's into a seperate table and use int to do joins. Sql server is faster to process ints than varchar's esp if you will need to do this for thousand's of records.
First of all please NORMALIZE your data. you can see lot of repeatating hobbies in each row, also it will be tedious to serach and for maintainability.
you can have all your USERS data in one table as below :
CREATE TABLE USERS ( UserID , NAME ); --> USERID being PRIMARY KEY
you can have all your HOBBIES in another table as below :
CREATE TABLE HOBBIES ( HOBBYID, HOBBYNAME); --> HOBBYID being PRIMARY KEY
you can have another table which maps USERS with HOBBIES as below :
CREATE USERS_HOBBIES( USERID , HOBBYID );
once the table is normalized as above, you can get the desired result by querying as below :
SELECT u.NAME , count(*) AS Hobbies FROM USERS u INNER JOIN
USERS_HOBBIES uh ON u.UserID = uh.USERID INNER JOIN HOBBIES h ON
uh.HOBBYID = h.HOBBYID WHERE h.HOBBYID IN (
(SELECT a.HOBBYID as HOBBYID FROM
(SELECT DISTINCT(HOBBYID) as HOBBYID FROM USERS_HOBBIES WHERE
USERID = '00001' ) a INNER JOIN
(SELECT DISTINCT(HOBBYID) as HOBBYID FROM USERS_HOBBIES WHERE
USERID <> '00001' ) b ON a.HOBBYID = b.HOBBYID) )
AND u.USERID = '00001' GROUP BY u.NAME
P.S : The above query syntax is in ORACLE
I have two tables one table tbl_OrderLine with order ID and one tbl_Student with Student ID.
I want to select the OrderLineID from tbl_OrderLine and The StudentID From tbl_Student
This is what I have tried so far:
INSERT INTO tbl_StudentPurchaseFromUnibooks
(OrderLineID, StudentID )
SELECT tbl_OrderLine.OrderLineID,
SELECT tbl_Student.StudentID
WHERE tbl_Student.LoggedIn ="Yes";
tbl_OrderLine:
OrderLineID Price Qty
1 5 2
tbl_Student:
StudentID Name LoggedIn
1 Joe Yes
tbl_StudentPurchaseFromUnibooks:
StudentPurchaseID OrderLineID StudentID PurchaseDate
1 1 1 09/12/2012
Also, does any one know a simple way of mimicking a login in Microsoft Access. As this is only dummy database for a Microsoft Access project security isn't an issue but I just would like to know a way of logging users in. At the moment i update their LoggedIn value to "Yes" if they are logged in but obviously this isn't efficient. If no one is able to help with this I will post as a seperate question later :) Thanks!
You want to define your select as a join instead of two separate SQLs. I am not really sure how MS works, but something like this should do the trick:
INSERT INTO tbl_StudentPurchaseFromUnibooks (OrderLineID, StudentID )
SELECT tbl_OrderLineID.OrderLineID, tbl_Student.StudentID
from tbl_OrderLineId
left join tbl_Student on ???
WHERE tbl_Student.LoggedIn ="Yes";
I am not sure that your table should really be named tbl_OrderLineID and you would need to know your join condition.
Using Postgres. Here's my scenario:
I have three different tables. One is a title table. The second is a genre table. The third table is used to join the two. When I designed the database, I expected that each title would have one top level genre. After filling it with data, I discovered that there were titles that had two, sometimes, three top level genres.
I wrote a query that retrieves titles and their top level genres. This obviously requires that I join the two tables. For those that only have one top level genre, there is one record. For those that have more, there are multiple records.
I realize I'll probably have to write a custom function of some kind that will handle this for me, but I thought I'd ask if it's possible to do this without doing so just to make sure I'm not missing anything.
Is it possible to write a query that will allow me to select all of the distinct titles regardless of the number of genres that it has, but also include the genre? Or even better, a query that would give me a comma delimited string of genres when there are multiples?
Thanks in advance!
Sounds like a job for array_agg to me. With tables like this:
create table t (id int not null, title varchar not null);
create table g (id int not null, name varchar not null);
create table tg (t int not null, g int not null);
You could do something like this:
SELECT t.title, array_agg(g.name)
FROM t, tg, g
WHERE t.id = tg.t
AND tg.g = g.id
GROUP BY t.title, t.id
to get:
title | array_agg
-------+-----------------------
one | {g-one,g-two,g-three}
three | {g-three}
two | {g-two}
Then just unpack the arrays as needed. If for some reason you really want a comma delimited string instead of an array, then string_agg is your friend:
SELECT t.title, string_agg(g.name, ',')
FROM t, tg, g
WHERE t.id = tg.t
AND tg.g = g.id
GROUP BY t.title, t.id
and you'll get something like this:
title | string_agg
-------+---------------------
one | g-one,g-two,g-three
three | g-three
two | g-two
I'd go with the array approach so that you wouldn't have to worry about reserving a character for the delimiter or having to escape (and then unescape) the delimiter while aggregating.
Have a look at this thread which might answer your question.
What the simplest way to sub-query a variable number of rows into fields of the parent query?
PeopleTBL
NameID int - unique
Name varchar
Data: 1,joe
2,frank
3,sam
HobbyTBL
HobbyID int - unique
HobbyName varchar
Data: 1,skiing
2,swimming
HobbiesTBL
NameID int
HobbyID int
Data: 1,1
2,1
2,2
The app defines 0-2 Hobbies per NameID.
What the simplest way to query the Hobbies into fields retrieved with "Select * from PeopleTBL"
Result desired based on above data:
NameID Name Hobby1 Hobby2
1 joe skiing
2 frank skiing swimming
3 sam
I'm not sure if I understand correctly, but if you want to fetch all the hobbies for a person in one row, the following query might be useful (MySQL):
SELECT NameID, Name, GROUP_CONCAT(HobbyName) AS Hobbies
FROM PeopleTBL
JOIN HobbiesTBL USING NameID
JOIN HobbyTBL USING HobbyID
Hobbies column will contain all hobbies of a person separated by ,.
See documentation for GROUP_CONCAT for details.
I don't know what engine are you using, so I've provided an example with MySQL (I don't know what other sql engines support this).
Select P.NameId, P.Name
, Min( Case When H2.HobbyId = 1 Then H.HobbyName End ) As Hobby1
, Min( Case When H2.HobbyId = 2 Then H.HobbyName End ) As Hobby2
From HobbyTbl As H
Join HobbiesTbl As H2
On H2.HobbyId = H.HobbyId
Join PeopleTbl As P
On P.NameId = H2.NameId
Group By P.NameId, P.Name
What you are seeking is called a crosstab query. As long as the columns are static, you can use the above solution. However, if you want to dynamic build the columns, you need to build the SQL statement in middle-tier code or use a reporting tool.