I have a main table M (Movies) and other tables L (Location), G (Genre), and S (Sub Genre). Each of the "other" tables are in a one to many relationship to table M, using.
I want to list all the Blu Ray titles and pull in their Location, Length (Time), Comments, Genre, and Sub Genre.
My query is:
SELECT L.Location, M.Title, M.Length, M.Comments, G.Genre, S.SubGenre
FROM ((L
INNER JOIN M ON M.Location = L.ID)
INNER JOIN G ON M.Genre = G.ID)
INNER JOIN SubGenre ON M.SubGenre = SubGenre.ID
ORDER BY M.ID
WHERE M.Type is "BluRay"
ORDER BY M.ID;
It gives me a subset of what the subset (26) of what the total number of records should be (447.)
1. Do I have the proper table relationships?
2. Do I really need the parentheses? (error without them)
3. How do I change my query to give me all the Location records, with the appropriate movie-related information?
4. What if I want to add additional tables?
The DB schema:
-- Note that Type and Length are in between square brackets, because those are reserved words.
-- Avoid use of reserved words with MovieType and MovieLength
SELECT
L.LocationName
, M.Title
, M.[Length]
, M.Comments
, G.GenreName
, S.SubGenreName
FROM Movies M
INNER JOIN Location L ON L.LocationID = M.LocationID
INNER JOIN Genre G ON G.GenreID = M.GenreID
INNER JOIN SubGenre S ON S.SubGenreID = M.SubGenreID
WHERE M.[Type] = 'BluRay'
ORDER BY M.MovieID
You need to JOIN on shared table columns.
For "How to change your query to give all Location records, with appropriate movie-related information" that depends on what you think is appropriate.
You should not need the parentheses. Unless you are using a SQL database I am not familiar with.
You do not need to put the INNER in because the default JOIN is INNER JOIN in all flavors of SQL databases. You also have 2 ORDER BY M.ID you only want the one after the WHERE.
I am not sure what you mean by more tables do you mean you tables to the JOIN or actually more tables?
Related
I don't quite understand my why code works but how does SQL do the joining process with 3 tables?
Heres the code with the table diagram below:
select category.category_id, name, count(film_id)
from films
right join film_category using (film_id)
right join category using (category_id)
group by category.category_id, name
order by count desc, name
I selected films in the 2nd line and right joined it to film_category, can someone confirm that keeps the 'film_category' information if 'films' doesn't contain the same id? Does SQL just magically know it should join 'film_category' with 'category'? Does that mean I can shuffle the order of joins around then?
Thanks
Most people don't recommend using RIGHT [OUTER] JOIN, for they are usually less readable than LEFT [OUTER] JOIN, as you must read the FROM clause backwards. I, too, advise not to use them.
You select from category, outer join film_category and then outer join films. Thus you select all categories with their films keeping categories in the result that have no associated film.
The query is wrong in only two regards:
Semantical: With USING(film_id) the film_id is never null, so COUNT(film_id) will not count zero when there is no film, but one.
Syntactical: The COUNT in ORDER BY lacks its parameter.
The query would be more readable with left outer joins as mentioned. With ON instead of USING and all columns qualified with their table names and with alias names for readability:
select c.category_id, c.name, count(f.film_id)
from category c
left join film_category fc on fc.category_id = c.category_id
left join films f on f.film_id = fc.film_id
group by c.category_id, c.name
order by count(f.film_id) desc, c.name;
i was trying to apply joins on 4 tables. but i could not find proper result for that.
i have 4 tables like 1.students,2.college,3.locations,4.departments. so i have same column sid in all tables which can be used to join conditions.
i want all matched rows from four tables as mentioned columns in select statement below and unmatched rows in left table which is left outer join work.
i have tried this syntax.
select
students.sname,
college.cname,
locations.loc,
department.dept
from students, college, locaions, departments
where student.sid=college.sid(+)
and college.sid=locations.sid(+)
and locations.sid=department.sid(+);
is this right ?
This is an old-fashioned way of outer-joining in an Oracle database. For Oracle this statement is correct; in other DBMS it is invalid.
Anyway, nowadays (as of Oracle 9i; in other DBMS much longer) you should use standard SQL joins instead.
select
s.sname,
c.cname,
l.loc,
d.dept
from students s
left outer join college c on c.sid = s.sid
left outer join locations l on l.sid = c.sid
left outer join departments d on d.sid = l.sid;
Given what you've shown it would appear that what you really want is
select s.sname,
c.cname,
l.loc,
d.dept
from students s
LEFT OUTER JOIN college c
ON c.SID = s.SID
LEFT OUTER JOIN locations l
ON l.SID = s.SID
LEFT OUTER JOIN departments d
ON d.SID = s.SID
The issue in your original query is that because an OUTER JOIN is an optional join, you can end up with NULL values being returned in one of the join fields, which then prevents any of the downstream values being joined. I agree with #ThorstenKettner who observes in a comment that SID is apparently a "student ID", but it's not reasonable or appropriate to have a "student ID" field on tables named COLLEGE, LOCATIONS, or DEPARTMENTS. Perhaps you need to update your design to allow any number of students to be associated with one of these entities, perhaps using a "join" table.
Best of luck.
I have four tables :
Applicant (aid, aname)
entrance_test (Etid, etname)
etest_centre (etcid, location)
etest_details (aid, etid, etcid, etest_dt)
I want to select the number of applicants who have appeared for each test, test center wise.
This is my current query:
select
location, etname, count(Aid) as number of applicants
from
applicant as a
inner join
etest_details as d on a.aid = d.aid
inner join
Entrance_Test as t on t.Etid = d.Etid
inner join
Etest_Centre as c on c.Etcid = d.Etcid
group by
Location, Etname
This is the error I am getting :
Ambiguous column name 'Aid'
You have the column aid in multiple tables, and it doesn't know which to pick from. You should specify which table it is from using the aliases you defined.
In this case, since a.Aid is the same as d.Aid (due to the JOIN), I'm using the a alias, but do keep in mind if location and etname also appear in multiple tables, you need to specify which table it should pick from.
Select c.location, t.etname, Count(a.Aid)
From Applicant As a
Inner Join etest_details As d On a.aid = d.aid
Inner Join Entrance_Test As t On t.Etid = d.Etid
Inner Join Etest_Centre As c On c.Etcid = d.Etcid
Group By c.Location, t.Etname
As a rule of thumb, when you have multiple sources in one query, you should always be explicit about which table it should come from. Even if you're sure it only exists in one of them, it's a good habit to get into to avoid issues like this in the future.
You need to mention the alias in the COUNT clause. Since you are using aliases, it would be better if you use them in the SELECT and GROUP BY sections as well. In this case, it should be :
SELECT a.location,
a.etname,
COUNT(d.Aid)
FROM applicant AS a
INNER JOIN etest_details AS d ON a.aid = d.aid
INNER JOIN Entrance_Test AS t ON t.Etid = d.Etid
INNER JOIN Etest_Centre AS c ON c.Etcid = d.Etcid
GROUP BY a.Location,
a.Etname
In Postgres, is there a way to perform a left join between tables linked by a junction table, with some filtering on the linked table?
Say, I have two tables, humans and pets, and I want to perform a query where I have the human ID, and the pet name. If the human ID exists, but they don't have a pet with that name, I still want the human's row to be returned.
If I had a FK relationship from pets to humans, this would work:
select h.*, p.*
from humans as h
left join pets as p on p.human_id = h.id and p.name = 'fluffy'
where h.id = 13
and I'd get a row with human 13's details, and fluffy's values. In addition, if human 13 didn't have a pet named 'fluffy', I'd get a row with human 13's values, and empty values for the pet's columns.
BUT, I don't have a direct FK relationship, I have a junction table between humans and pets, so I'm trying a query like:
select h.*, p.*
from humans as h
left join humans_pets_junction as j on j.human_id = h.id
left join pets as p on j.pet_id = p.id and p.name = 'fluffy'
where h.id = 13
Which returns rows for all of human 13's pets, with empty columns except for fluffy's row.
If I add p.name = 'fluffy' to the WHERE clause, that filters out all the empty rows, but also means I get 0 rows if human 13 doesn't have a pet named fluffy at all.
Is there a way to replicate the behavior of the FK-style left join, but when used with a junction table?
One method is to do the comparison in the where clause:
select h.*, p.*
from humans as h left join
humans_pets_junction as j
on j.human_id = h.id left join
pets as p
on j.pet_id = p.id and p.name = 'fluffy'
where h.id = 13 and (p.name = 'fluffy' or p.id is null);
Alternatively, join the junction table and the pets table as a subquery or CTE:
select h.*, p.*
from humans h left join
(select j.*
from humans_pets_junction j join
pets p
on j.pet_id = p.id and p.name = 'fluffy'
) pj
on pj.human_id = h.id
where h.id = 13;
In Postgres you can use parentheses to prioritize JOIN order. You do not need a subquery:
SELECT h.*, p.id AS p_id, p.name AS pet_name
FROM humans h
LEFT JOIN (pets p
JOIN humans_pets_junction j ON p.name = 'fluffy'
AND j.pet_id = p.id
AND j.human_id = 13) ON TRUE
WHERE h.id = 13;
Per documentation:
Parentheses can be used around JOIN clauses to control the join order.
In the absence of parentheses, JOIN clauses nest left-to-right.
I added the predicate j.human_id = 13 to the join between your junction table and the pets to eliminate irrelevant rows at the earliest opportunity. The outer LEFT JOIN only needs the dummy condition ON TRUE.
SQL Fiddle.
Aside 1: I assume you are aware that you have a textbook implementation of a n:m (many-to-many) relationship?
How to implement a many-to-many relationship in PostgreSQL?
Aside 2: The unfortunate naming convention in the example makes it necessary to deal out column aliases. Don't use "id" and "name" as column names in your actual tables to avoid such conflicts. Use proper names like "pet_id", "human_id" etc.
I'm having problems getting an accurate count of a column after joining others. When a column is joined I would still like to have a DISTINCT count of the table that it is being joined on.
A restaurant has multiple meals, meals have multiple food groups, food groups have multiple ingredients.
Through the restaurants id I want to be able to calculate how many of meals, food groups, and ingrediants the restaurant has.
When I join the food_groups the count for meals increases as well (I understand this is natural behavior I just don't understand how to get what I need due to it.) I have tried DISTINCT and other things I have found, but nothing seems to do the trick. I would like to keep this to one query rather than splitting it up into multiple ones.
SELECT
COUNT(meals.id) AS countMeals,
COUNT(food_groups.id) AS countGroups,
COUNT(ingrediants.id) AS countIngrediants
FROM
restaurants
INNER JOIN
meals ON restaurants.id = meals.restaurant_id
INNER JOIN
food_groups ON meals.id = food_groups.meal_id
INNER JOIN
ingrediants ON food_groups.id = ingrediants.food_group_id
WHERE
restaurants.id='43'
GROUP BY
restaurants.id
Thanks!
The DISTINCT goes inside the count
SELECT
COUNT(DISTINCT meals.id) AS countMeals,
COUNT(DISTINCT food_groups.id) AS countGroups,
COUNT(DISTINCT ingrediants.id) AS countIngrediants
FROM
restaurants
INNER JOIN
meals ON restaurants.id = meals.restaurant_id
INNER JOIN
food_groups ON meals.id = food_groups.meal_id
INNER JOIN
ingrediants ON food_groups.id = ingrediants.food_group_id
WHERE
restaurants.id='43'
GROUP BY
restaurants.id
You're going to have to do subqueries, I think. Something like:
SELECT
(SELECT COUNT(1) FROM meals m WHERE m.restaurant_id = r.id) AS countMeals,
(SELECT COUNT(1) FROM food_groups fg WHERE fg.meal_id = m.id) AS countGroups,
(SELECT COUNT(1) FROM ingrediants i WHERE i.food_group_id = fg.id) AS countGroups
FROM restaurants r
Where were you putting your DISTINCT and on which columns? When using COUNT() you need to do the distinct inside the parentheses and you need to do it over a single column that is distinct for what you're trying to count. For example:
SELECT
COUNT(DISTINCT M.id) AS count_meals,
COUNT(DISTINCT FG.id) AS count_food_groups,
COUNT(DISTINCT I.id) AS count_ingredients
FROM
Restaurants R
INNER JOIN Meals M ON M.restaurant_id = R.id
INNER JOIN Food_Groups FG ON FG.meal_id = M.id
INNER JOIN Ingredients I ON I.food_group_id = FG.id
WHERE
R.id='43'
Since you're selecting for a single restaurant, you shouldn't need the GROUP BY. Also, unless this is in a non-English language, I think you misspelled ingredients.