I got 4 tables, Email_Company_Contact_Ref table is the table which linked with Email, Company, and Contact.
**Email_Company_Contact_Ref**
id = primary key
email_id = reference to Email.`id`
ref_id = it can be Company.id / Contact.id
table = reference from which table name
I try to use left join to get my output, but I got duplicated result. If I try inner join, I will not get any result at all, it is because Company and Contact this two tables does not have any thing common.
This is the output I would like to complete.
I able to use UNION to get the output, but it not really effective. I am thinking it should be a way to get the output result.. Please help.
Thanks!
Here is my mysql answer, hope this can help
SELECT e.email, r.table, c1.name AS company_name, c2.name AS contact_name
FROM email_company_contact_ref r
JOIN email e ON e.id = r.email_id
LEFT JOIN company c1 ON (c1.id = r.ref_id AND r.table = 'company')
LEFT JOIN contact c2 ON (c2.id = r.ref_id AND r.table = 'contact')
GROUP BY r.table, e.email
I don't think it can be done without a UNION. Here's my suggestion.
SELECT email_address, eccr.table table, company_name, contact_name
FROM Email e, Email_Company_Contact_Ref eccr,
(SELECT "Company" table, id, company_name, NULL contact_name
FROM Company
UNION ALL
SELECT "Contact" table, id, NULL company_name, contact_name
FROM Contact) cc
WHERE e.id = eccr.email_id
AND eccr.table = cc.table
AND eccr.email_id = cc.id
I'm not getting the ref_id part... Is it a foreign key? Or is that the primary key for the Email_Company_Contact_Ref table?
I would think you'd want to put the reference for the Email table in the Company and Contact tables. If you need more than one emails for them, then you should create two join tables: Company_Email and Contact_Email. Your current design (with references to table names as values for a column) is bad SQL design -- just because things like RoR promote it, it won't get any better.
With proper design, the equivalent of that complicated query would look something like:
CREATE TABLE Company_Email (company_id integer, email_address varchar(100),
FOREIGN KEY company_id REFERENCES Company (id));
CREATE TABLE Contact_Email (contact_id integer, email_address varchar(100),
FOREIGN KEY contact_id REFERENCES Contact (id));
SELECT email_address, 'Company' AS kind, company_name AS name
FROM Company_Email ce JOIN Company c ON company_id = c.id
UNION
SELECT email_address, 'Contact', contact_name
FROM Contact_Email ce JOIN Contact c ON contact_id = c.id;
If you can't change it, you'll have to do the UNION along the lines Barmar explained it.
Or, you can do a SELECT DISTINCT to get rid of the duplicates from your left joined query.
Related
I need to make a query, where there are columns of client's names and their orders per month.
Some clients don't have orders at some months and there fields must have 0.
The problem is, when i use WHERE and OUTER JOIN (no matter which one) at one query*, nessesary zero`s cutteed by WHERE. So how can i solve that?
Descripton of tables are pinned.
SELECT name
, ordering.id_client
, COUNT(order_date)
FROM ordering
RIGHT
OUTER
JOIN client
ON client.id_client = ordering.id_client
WHERE month(order_date) = 1
GROUP
BY name;
**Descripton**: (https://i.imgur.com/TrUGOLW.png)
**Example of my query** (there are 6 notes about clients at my db, showed only 4 of 6):
(https://i.imgur.com/ABP6pP0.png)
**MRE stuff**
Client: create table client(id_client int primary key auto_increment, name var char(50), passport_code int, addr varchar(70));
insert into client values(null, 'Penny Anderson', 6485, 'New Orlean');
Ordering: create table ordering(id_order int primary key auto_increment, id_client int, order_date date, foreign key(id_client) references client(id_client));
insert into ordering values(null, 1, date('2020-05-01'));
Try a simple left join starting from client's table
SELECT client.name
, client.id_client
, COUNT(order_date)
FROM client
LEFT JOIN ordering ON client.id_client = ordering.id_client
AND month(ordering.order_date) = 1
GROUP BY client.id_client;
If the condition for join is related to the left joined table then add this condition in the related ON clause and not in where otherwise this work as an inner join
I'm new to SQL, how could I answer the following question? Do I use join?
Thank you so much.
What are the names and home cities for people searched for the word "drain"?
.schema is below:
CREATE TABLE `users` (
`id` INTEGER,
`name` VARCHAR,
`email` VARCHAR,
`city` VARCHAR,
`state` VARCHAR,
`last_visit` DATE,
`page_views` INTEGER,
PRIMARY KEY (`id`)
);
CREATE TABLE `search_terms` (
`id` INTEGER,
`word` VARCHAR,
PRIMARY KEY (`id`)
);
select u.name, u.city
from users u
inner join search_terms st on st.id = u.id
where st.word = 'drain'
I hope this helps.
You do need to join search_terms table with users table. I assume the search_terms.id is referencing to users.id.
Linking them both, will give us the results of the search terms for each user.
from there you can add more filters (WHERE conditions) to have more specific results.
So, to get the names and the home cities for each user you need to select name and city from users table, then join search_terms table and link id columns to know what words that each user has used in the search.
The query should be something like this :
SELECT name, city
FROM users
LEFT JOIN search_terms ON search_terms.id = users.id
WHERE
search_terms.word LIKE 'drain';
You could use a JOIN or a sub-query. Here's the basic strategy:
Get the id values that match what you're looking for
Look up any other info for those id values
.
SELECT name, city
FROM users usr
INNER JOIN search_terms stm ON usr.id = stm.id
WHERE stm.word = 'drain';
or ...
SELECT name, city
FROM users
WHERE id IN (SELECT id FROM search_terms WHERE word = 'drain');
I have a project where I have to use nested select and operators like EXISTS, IN, ALL, ANY, to find the name of the owners who own more apartments.
I have the 2 tables owners and apartments
Owner
"ID" NUMBER(5,0),
"NAME" VARCHAR2(20),
"PHONE" NUMBER(10,0),
CONSTRAINT "PROPR" PRIMARY KEY ("ID")
USING INDEX ENABLE;
Apartment
"ID_AP" NUMBER(5,0),
"ADDRESS" VARCHAR2(35),
"SURFACE" NUMBER(10,0),
"ID" NUMBER(5,0),
CONSTRAINT "APART" PRIMARY KEY ("ID_AP")
USING INDEX ENABLE;
In order to find the owners who have more than one apartment, I wrote this
SELECT name, id, count(id)
from apartment join owner
using (id)
group by id, name
having count(id)>1
But how do I use nested Selects and one of the operators EXISTS, IN, ALL, ANY?
For example like this:
select * from owner where id IN (
SELECT id
from apartment
group by id
having count(id)>1)
I am not quite sure that the query you wrote is correct .
The correct query that you provided would count(id_ap) instead of count(id)
SELECT o.id, o.name, count(a.id_ap)
from apartment a
join owner o on o.id = a.id
group by o.id, o.name
having count(a.id_ap)>1
The same query using in clause, but without the number of apartments, you only know that it's 2 or more
select o.id, o.name
from owner o
where o.id in (select distinct(a.id) from apartment a
group by a.id having count(a.id_ap) > 1);
You are missing a foreign key constraint on apartment table referencing owner table as well.
WITH myView(name, id,appcount)
as
SELECT name, id, (select count(id) FROM apartment where id = ow.id)
FROM owner ow
SELECT * from myView
WHERE appcount > 2
Create a inline View using WITH and use to write a query. (select count(id) FROM apartment where id = ow.id) in select of outer query will calculate apartment count.
CREATE TABLE Institutions
(
Institution_ID INTEGER PRIMARY KEY,
Institution_Name VARCHAR(200))
CREATE TABLE Contact_Persons
(
Contact_No INTEGER PRIMARY KEY,
First_Name VARCHAR(60) NOT NULL,
Last_Name VARCHAR(60) NOT NULL,
Institution_ID INTEGER,
FOREIGN KEY (Institution_ID) REFERENCES Institutions(Institution_ID))
How can I write a mysql query that shows the Institutions that have more than one one Contact Person at them? The Query should show Institution_ID and Institution_Name from Table Institutions. The following Query works but It only shows Institution_ID. I want to see Institution_Name too. I think there must be a join between these two tables. Please help.
SELECT Institution_ID
FROM Contact_Persons
GROUP BY Institution_ID
HAVING COUNT(*)>1;
You were right, you just need to join your two tables, and make sure you also group by everything you want to select (unless it is an aggregate), so in this case add Institution_Name to the group by clause:
SELECT i.Institution_ID, i.Institution_Name
FROM Contact_Persons AS cp
INNER JOIN Institutions AS i
ON i.Institution_ID = cp.Institution_ID
GROUP BY i.Institution_ID, i.Institution_Name
HAVING COUNT(*) > 1;
Try this
SELECT I.Institution_ID,I.Institution_name
FROM Contact_Persons as C inner join Institutions as I on
C.Institution_ID=I.Institution_ID
GROUP BY I.Institution_ID,I.Institution_name
HAVING COUNT(*)>1;
I have four schemas:
takes(ID,course_id,sec_id,semester,year)
student(ID,name,dept_name,tot_credit)
course(course_id,title,dept_name,credits)
department(dept_name,building,budget)
I want to create a query that finds the name and id of each Astronomy student whose name begins with the letter âTâ and who has not taken at least 16 Astronomy courses.
What's the easiest way I could do this?
I already wrote this beginning bit
SELECT name, id
FROM student
WHERE dept_name='Astronomy' AND name LIKE '%T%'
I'm not quite sure how to finish this off.
Any help would be greatly appreciated :)
Here's the result
NAME ID CLASS_TAKEN
-------------------- ----- -----------
Tolle 38279 12
Teo 62268 13
Tolle 93223 13
Tsukamoto 17707 5
Titi 11576 9
Teo 91772 12
Toraichi 50387 11
Tewari 80754 14
Tiroz 64091 14
9 rows selected
I need Teo with the id 91772 and Tewari 80754 to be gone
Given my reading of the requirements and the comments it's pretty clear that the question is not very clear. :-) What you're looking for are students where (total # of courses given by the Astronomy department) - (# of Astronomy courses taken by student) >= 16. So, how do we find these values? First, let's start with the total number of courses given by the Astronomy department. This is pretty simple:
SELECT COUNT(*) AS ASTRONOMY_COURSE_COUNT
FROM COURSE
WHERE DEPT_NAME = 'ASTRONOMY'
Now, the second part is to determine how many courses given by the Astronomy department each student has taken. To do this we need to start with the student, join to the courses the student has taken (the TAKES table), then join to the COURSES table to find out which department each course is part of. Something like the following should do it:
SELECT s.ID, s.NAME, COUNT(*) AS STUDENT_ASTRO_COUNT
FROM STUDENT s
INNER JOIN TAKES t
ON t.ID = s.ID
INNER JOIN COURSE c
ON c.COURSE_ID = t.COURSE_ID
WHERE c.DEPT_NAME = 'ASTRONOMY' AND
s.NAME LIKE 'T%'
GROUP BY s.ID, s.NAME;
OK, now we need to put this together. You've tagged this question for both Oracle and MySQL so I'm going to guess you'll accept valid syntax for either database; thus I'll use Oracle Common Table Expression syntax to pull everything together:
WITH ASTRONOMY_COURSES AS (SELECT COUNT(*) AS ASTRONOMY_COURSE_COUNT
FROM COURSE
WHERE DEPT_NAME = 'ASTRONOMY'),
STUDENT_ASTRO_COURSES AS (SELECT s.ID,
s.NAME,
COUNT(*) AS STUDENT_ASTRO_COUNT
FROM STUDENT s
INNER JOIN TAKES t
ON t.ID = s.ID
INNER JOIN COURSE c
ON c.COURSE_ID = t.COURSE_ID
WHERE c.DEPT_NAME = 'ASTRONOMY' AND
s.NAME LIKE 'T%'
GROUP BY ID)
SELECT s.ID,
s.NAME,
s.STUDENT_ASTRO_COUNT,
a.ASTRONOMY_COURSE_COUNT - s.STUDENT_ASTRO_COUNT AS UNTAKEN_COUNT
FROM STUDENT_ASTRO_COURSES s
CROSS JOIN ASTRONOMY_COURSES a
WHERE a.ASTRONOMY_COURSE_COUNT - s.STUDENT_ASTRO_COUNT >= 16;
Note here that a CROSS JOIN is used to put together the subqueries. This means that all the rows of each subquery are joined to all the rows of the other subquery - but since in this case the ASTRONOMY_COURSES subquery will only return a single row what we're doing is appending the ASTRONOMY_COURSE_COUNT value onto each row returned by the STUDENT_ASTRO_COURSES subquery.
That should at least get you pretty close. Amend as needed.
Not tested on animals - you'll be first! :-)
Share and enjoy.
Do you need to use all tables?
Table department has no links with the student,
Table takes has no links with the student,
Table coursehas no links with the student.
If student lists total credits that are all Astronomy I think this can be used:
select name, id, MAX(tot_credit) as credits
from student
where dept_name='Astronomy' and name like 'T%'
group by name, id
having MAX(tot_credit)<=16
PS - you schema is not good; PK-FK references are missing
Your query will need to reference more tables than just the student table.
Your tables seem be missing some important information, which student has taken which course. There's a table named takes, but there doesn't appear to be any relationship between takes and student.
So first, figure out how to list the students along with the Astronomy courses they have taken. Each row will identify the student and a course.
SELECT s.id AS student_id
, s.name AS student_name
, t.???
FROM student s
JOIN ??? t
ON t.student_id = s.id
WHERE ...
You may also need to include another "join" to an additional table, in order to identify which course a student has taken is an Astronomy course.
To also include students that have not take any Astronomy courses, you can use an outer join, rather than an inner join. (That would mean including the LEFT keyword before JOIN, and relocating predicates from the WHERE clause to the ON clause. (A predicate in the WHERE clause that can only be satisfied by non-NULL values will negate the outer-ness of the join.)
Once you have a query that returns that set (students along with any astronomy courses they've taken), you can then add a GROUP BY clause to "collapse" a set of rows into a single row. (Looks like you want the rows "grouped" by student.)
And then an aggregate function like COUNT() or SUM() can be used to get a count of rows for each group. (If you don't want to count any re-takes of a course (a "duplicate" course for a student) you may be able to make use of the COUNT(DISTINCT t.foo) form.
And then a HAVING clause can be added to the query, to compare the value returned from the aggregate expression to a constant value, to return only rows that satisfy a specific condition.
FOLLOWUP
Assuming:
CREATE TABLE course
( id INT UNSIGNED NOT NULL COMMENT 'PK'
, title VARCHAR(30) NOT NULL COMMENT 'course title'
, dept_name VARCHAR(30) NOT NULL COMMENT 'FK ref dept.name'
, credits DECIMAL(5,2) COMMENT 'credit hours'
, PRIMARY KEY (id)
);
CREATE TABLE student
( id INT UNSIGNED NOT NULL COMMENT 'PK'
, name VARCHAR(30) NOT NULL COMMENT 'student name'
, dept_name VARCHAR(30) NOT NULL COMMENT 'FK ref dept.name'
, tot_credit INT COMMENT '?'
, PRIMARY KEY (id)
);
CREATE TABLE takes
( student_id INT UNSIGNED NOT NULL COMMENT 'FK ref student.id'
, course_id INT UNSIGNED NOT NULL COMMENT 'FK ref course.id'
, sec_id INT UNSIGNED NOT NULL COMMENT '?'
, semester INT UNSIGNED NOT NULL COMMENT '?'
, year INT UNSIGNED NOT NULL COMMENT '?'
, PRIMARY KEY (student_id, course_id, sec_id, semester, year)
, CONSTRAINT FK_takes_course FOREIGN KEY (course_id) REFERENCES course (id)
, CONSTRAINT FK_takes_student FOREIGN KEY (student_id) REFERENCES student (id)
);
Query to get a list of students...
SELECT s.id
, s.name
FROM student s
WHERE s.name LIKE 'T%'
AND s.dept_name = 'ASTRONOMY'
Get list of students along with the courses they've taken, returning the id of the ASTRONOMY courses they've taken...
SELECT s.id AS student_id
, s.name AS student_name
, c.id AS course_id
FROM student s
LEFT
JOIN takes t
ON t.student_id = t.id
LEFT
JOIN course c
ON c.id = t.course_id
AND c.dept_name = 'ASTRONOMY'
WHERE s.name LIKE 'T%'
AND s.dept_name = 'ASTRONOMY'
Collapse the rows to one per student using a GROUP BY, and use aggregate functions to get counts or totals.
SELECT s.id AS student_id
, s.name AS student_name
, SUM(c.credits) AS total_astronomy_credits_taken
, COUNT(c.id) AS count_astronomy_courses_taken
, COUNT(DISTINCT c.id) AS count_distinct_astronomy_courses_taken
FROM student s
LEFT
JOIN takes t
ON t.student_id = t.id
LEFT
JOIN course c
ON c.id = t.course_id
AND c.dept_name = 'ASTRONOMY'
WHERE s.name LIKE 'T%'
AND s.dept_name = 'ASTRONOMY'
GROUP
BY s.id
, s.name
To omit rows from this resultset, add a HAVING clause. For example, to exclude rows where total_astronomy_credits_taken is greater than or equal to 12...
HAVING total_astronomy_credits_taken >= 12
If you want the rows returned in a certain sequence, specify that in an ORDER BY clause
ORDER BY s.id
If you want to replace NULL values from the aggregates with zeroes, you can warp the aggregate expression in an IFNULL(foo,0) function, e.g.
, IFNULL(COUNT(c.id),0) AS count_astronomy_courses_taken
Try this :
select a.name, a.id, count(b.ID) as class_taken
from student a inner join takes b
on a.ID = b.ID
inner join course c
on b.course_id = c.course_id
where a.dept_name='Astronomy' and substring(a.name,1,1) = 'T'
group by a.name, a.id
having count(b.ID) < 17